Assembly language by Puskar Suwal Complete Reference

ASSEMBLY LANGUAGE

BY: PUSKA

ANGUAGE PROGRAMMING

1

8086 Architecture

USKAR SUWAL

INTRODUCTION TO 16 BIT MARCHITECTURE

¢ Intel 8086 – 16 bit microprocessor (arithmetic logic unit, internal registers, most of instructions are designed to work with 16 bit binary words).

¢Data bus : 16 bit (read data from or write data to memory and ports either 16 bits or 8 bits at a time.

¢Address bus : 20 bit (can address any one of 21048576 = 1MB memory locations).� Address range : 00000H to FFFFFH

MICROPROCESSOR

microprocessor (arithmetic logic unit, internal registers, most of instructions are designed to work with 16 bit binary words).

(read data from or write data to memory and ports either 16 bits or 8 bits at a time.

(can address any one of 210 = memory locations).

Address range : 00000H to FFFFFH

2

8086 Architecture

¢16 bit words will be stored in two consecutive memory locations.

¢ If first byte of the data is stored at an address, 8086 can read the entire word in one operation.� For example if the 16 bit data is stored at even address

00520H is 2607,MOV BX, [00520]

8086 reads the first byte and stores the data in BL and reads the second byte and stores the data in BH.

BL ß (00520)BH ß (00521)

16 bit words will be stored in two consecutive

If first byte of the data is stored at an even , 8086 can read the entire word in one

For example if the 16 bit data is stored at even address

8086 reads the first byte and stores the data in BL and reads the second byte and stores the data in BH.

3

8086 Architecture

¢ If the first byte of data is stored at an needs two operation to read the 16 bit data.� For example if the 16 bit data is stored at even address

00521H is F520,MOV BX, [00521]

In first operation, 8086 reads the 16 bit data from the 00520 location and stores the data of 00521 location in register BL and discards the data of 00520 location.

In second operation, 8086 reads the 16 bit data from the 00522 location and stores the data of 00522 location in register BH and discards the data of 00523 location.

If the first byte of data is stored at an odd address, 8086 needs two operation to read the 16 bit data.

For example if the 16 bit data is stored at even address

8086 reads the 16 bit data from the 00520 location and stores the data of 00521 location in register BL and discards the data of 00520 location.

8086 reads the 16 bit data from the 00522 location and stores the data of 00522 location in register BH and discards the data of 00523

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8086 Architecture

BLOCK DIAGRAM OF INTELNTEL 8086

5

8086 Architecture

¢ 8086 microprocessor is divided internally into two separate units:

¢Bus Interface Unit (BIU)¢Execution Unit (EU)

¢ The two units functions independently.¢ The is responsible for decoding and executing instructions. ¢ It contains arithmetic logic unit (ALU), status and control

flags, general-purpose register, and temporaryregisters.

¢ Maintain the microprocessor status and control flags, manipulates the general registers and instruction operands.

8086 microprocessor is divided internally into two separate

The two units functions independently.The is responsible for decoding and executing instructions.

arithmetic logic unit (ALU), status and control register, and temporary-operand

Maintain the microprocessor status and control flags, manipulates the general registers and instruction operands. 6

8086 Architecture

¢ The BIU is responsible for performing all external bus operations, such as instruction fetching, reading and writing of data operands for memory, address generating, and inputting or outputting data for input/output peripherals.

¢ These operations are take place over the system bus. This bus includes 16-bit bidirectional data bus, a 20bus, and the signals needed to control transfer over the bus.

¢ The BIU uses a mechanism known as This queue permits the 8086 to preinstruction code.

The BIU is responsible for performing all external bus operations, such as instruction fetching, reading and writing of data operands for memory, address generating, and inputting or outputting data for input/output

These operations are take place over the system bus. This bit bidirectional data bus, a 20-bit address

bus, and the signals needed to control transfer over the

The BIU uses a mechanism known as instruction queue. the 8086 to pre-fetch up to 6 bytes of

7

8086 Architecture

THE EXECUTION UNIT

¢ The EU decodes and executes the instructions.¢ A decoder in the EU control system translates

instructions.¢ The EU has 16 bit ALU for performing arithmetic and

logic operations.¢ The EU has nine 16 bit registers

BP, SI, DI and Flag registers).� AX, BX, CX, DX (general purpose registers)

eight 8 bit registers (AH, AL, BH, BL, CH, CL, DH, DL).

The EU decodes and executes the instructions.in the EU control system translates

The EU has 16 bit ALU for performing arithmetic and

16 bit registers (AX, BX, CX, DX, SP,

(general purpose registers) can be used as eight 8 bit registers (AH, AL, BH, BL, CH, CL, DH, DL).

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8086 Architecture

GENERAL PURPOSE REGISTERS

¢ general purpose registers (AH, AL, BH, BL, CH, CL, DH, DL).

¢ These registers can be used as 8individually or can be used as 16BX, CX, and DX.

¢ The AL register is also called the accumulatorsome features that the other general purpose registers do not have.

REGISTERS

general purpose registers (AH, AL, BH, BL, CH, CL,

These registers can be used as 8-bit registers individually or can be used as 16-bit in pair to have AX,

the accumulator. It has some features that the other general purpose registers

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8086 Architecture

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8086 Architecture

¢AX� AX à 16 bit accumulator

accumulator� Used for operations involving input/output and

most arithmetic.� For example: multiply, divide, and translate

instructions assume the use of AX.� Also, some instructions generate more efficient

machine code if they reference AX rather than another register.

16 bit accumulator; AL à 8 bit

Used for operations involving input/output and

For example: multiply, divide, and translate instructions assume the use of AX.Also, some instructions generate more efficient machine code if they reference AX rather than

11

8086 Architecture

¢BX� BX is known as the Base register� This is only general purpose register whose

contents can be used for addressing 8086 memory.

� All memory reference utilizing this register content for addressing uses the DS as the default segment register.

� BX can also be combined with DI or SI as a base register for special addressing.

� BX register is similar to the 8085 HL register. (BHàH; BLàL)

the Base register.This is only general purpose register whose contents can be used for addressing 8086

All memory reference utilizing this register content for addressing uses the DS as the default

BX can also be combined with DI or SI as a base register for special addressing.BX register is similar to the 8085 HL register. 12

8086 Architecture

¢CX� CX is known as the counter register� It may contain a value to control the number of

times a loop is repeated or a value to shift bits left or right.

¢DX� DX is known as a data register� Used to hold 16 bit result (data).� Some I/O operations require its use, and multiply

and divide operations that involve large values assume the use of DX and AX together as a pair.

the counter register.It may contain a value to control the number of times a loop is repeated or a value to shift bits

a data register.Used to hold 16 bit result (data).Some I/O operations require its use, and multiply and divide operations that involve large values assume the use of DX and AX together as a pair. 13

8086 Architecture

POINTER & INDEX REGISTERS

¢The 8086 has four other generalregisters, two pointer registers two index registers DI and to store what are called offset addresses

¢An offset address represents the displacement of a storage location in memory from the segment base address in a segment register.

¢Unlike the general-purpose data registers, the pointer and index registers are only accessed as words (16 bits).

EGISTERS

other general-purpose registers, two pointer registers SP and BP, and

and SI. These are used offset addresses.

represents the displacement of a storage location in memory from the segment base address in a segment register.

purpose data registers, the pointer and index registers are only accessed as

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8086 Architecture

POINTER REGISTERS

¢ The two pointer registers (16 bits), and base pointer (BP) are used to access data in the stack segment.

¢ The 16 bit SP register provides an offset value, which, when associated with the SS register (SP:SS), refers to the current word being processed in the stack.

¢

¢ The SP contents are automatically updated during the execution of a POP and PUSH

The two pointer registers (16 bits), stack pointer (SP)are used to access data in the

The 16 bit SP register provides an offset value, which, when associated with the SS register (SP:SS), refers to the current word being processed in the stack.

The SP contents are automatically updated during the instruction.

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8086 Architecture

¢ The 16 bit BP facilitates referencing parameters, which are data and addresses that a program passes via the stack.

¢ The processor combines the address in SS with the offset in BP.

¢ BP can also be combined with DI and with SI as a base register for special addressing.

The 16 bit BP facilitates referencing parameters, which are data and addresses that a program passes via the

The processor combines the address in SS with the

BP can also be combined with DI and with SI as a base register for special addressing.

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8086 Architecture

INDEX REGISTERS

¢ The two 16 bit index registers, Source Index (SI and Destination Index (DI) are used in indexed addressing.

¢ SI register: is required for some string (character) handling operations. In this context, SI is associated with the DS register.

¢ DI register: is also required for some string operations. In this context, DI is associated with the ES register.

two 16 bit index registers, Source Index (SI and are used in indexed addressing.

SI register: is required for some string (character) handling operations. In this context, SI is associated

DI register: is also required for some string operations. In this context, DI is associated with the ES register.

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8086 Architecture

FLAG REGISTERS

¢ A flag is a flip-flop that indicates some condition produced by the execution of an instruction or controls certain operations of the EU.

¢ A 16 bit flag register in the EU contains nine active flags. Six of them are used to indicate some condition produced by an instruction and remaining flags are used to control certain operations of the processor.

flop that indicates some condition produced by the execution of an instruction or controls

A 16 bit flag register in the EU contains nine active flags. Six of them are used to indicate some condition produced by an instruction and remaining flags are used to control certain operations of the processor.

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8086 Architecture

¢ The six conditional flags are Parity flag (PF), Auxiliary carry flag (AF), Zero flag (ZF), Sign flag (SF) and Overflow flag (OF).

¢ The carry flag (CF): CF is set if there is a carry-out or a borrow-in for the most significant bit of the result during the execution of an instruction. Otherwise it is reset.

¢ The parity flag (PF): PF is set if the result produced by the instruction has even paritythat is, if it contains an even number of bits at the 1 logic level. If parity is odd, PF is reset.

The six conditional flags are Carry flag (CF), Parity flag (PF), Auxiliary carry flag (AF), Zero flag (ZF), Sign flag (SF) and Overflow flag (OF).

CF is set if there is a in for the most significant

bit of the result during the execution of an instruction. Otherwise it is reset.

PF is set if the result produced by the instruction has even parity-that is, if it contains an even number of bits at the 1 logic level. If parity is odd, PF is reset. 19

8086 Architecture

¢ The auxiliary flag (AF): AFthe low nibble into the high nibblenibble into the low nibble of theOtherwise, AF is reset.

¢ The zero flag (ZF): ZF is setinstruction is zero. Otherwise, ZF

¢ The sign flag (SF): The MSBThus, SF is set if the result is apositive.

¢ The overflow flag (OF): Whensigned result is out of range. If theremains reset.

is set if there is a carry-out fromnibble or a borrow-in from the high

the lower byte in a 16-bit word.

set if the result produced by anZF is reset.

MSB of the result is copied into SF.a negative number of reset if it is

When OF is set, it indicates that thethe result is not out of range, OF

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8086 Architecture

¢ The three control flags are Trap flag (TF), Interrupt flag (IF)Direction flag (DF).

¢ The trap flag (TF): if TF is set, the 8086 goes into the singlemode of operation. When in the singleinstruction and then jumps to a special service routine that may determine the effect of executing the instruction. This type of operation is very useful for debugging programs.

¢ The interrupt flag (IF): For the 8086 to recognize interrupt requests at its interrupt (INT) input, the IF flag must be set. When IF is reset, requests at INT are ignored and the interrupt interface is disabled.

¢ The direction flag (DF): The logic level of DF determines the direction in which string operations will occur. When set, the string instructions automatically decrement the address; therefore the string data transfers proceed from high address to low address.

Trap flag (TF), Interrupt flag (IF) and

if TF is set, the 8086 goes into the single-step mode of operation. When in the single-step mode, it executes an instruction and then jumps to a special service routine that may determine the effect of executing the instruction. This type of operation is very useful for debugging programs.

For the 8086 to recognize maskableat its interrupt (INT) input, the IF flag must be set.

When IF is reset, requests at INT are ignored and the maskable

The logic level of DF determines the direction in which string operations will occur. When set, the string instructions automatically decrement the address; therefore the string data transfers proceed from high address to low address.

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8086 Architecture

BUS INTERFACE UNIT (BIU)

¢ BIU delivers instruction and data to EU.¢ It manage the bus control unit, segment registers and

instruction queue. BIU controls the buses that transfer data to the EU, to memory, and to I/O devices, whereas the segment registers control memory addressing.

¢ Another function of BIU is to provide access to instructions. Because the instructions for a program that is executing are in memory, the BIU must access instructions form memory and place them in instruction queue.

¢ The Instruction Queue is a FIFO group of register in which upto 6 bytes of instruction code are preinstructions.

(BIU)

BIU delivers instruction and data to EU.It manage the bus control unit, segment registers and instruction queue. BIU controls the buses that transfer data to the EU, to memory, and to I/O devices, whereas the segment registers control memory addressing.Another function of BIU is to provide access to instructions. Because the instructions for a program that is executing are in memory, the BIU must access instructions form memory and place them in an

The Instruction Queue is a FIFO group of register in of instruction code are pre-fetched 22

8086 Architecture

¢ The EU and BIU work in parallel,one step ahead.

¢ When the EU is ready for its nextreads the instruction byte(s) forqueue in the BIU.

¢ The top instruction is the currentlywhile the EU is occupied executingBIU fetches another instructionfetching overlaps with executionprocessing.

¢ Fetching the next instructioninstruction executes is called pipelining

parallel, with the BIU keeping

next instruction, it simplyfor the instruction from the

currently executable one and,executing an instruction, the

instruction from memory. Thisexecution and speeds up

instruction while the currentpipelining. 23

8086 Architecture

SEGMENTS AND ADDRESSING¢ Segments are special areas defined

the code, the data and the stack

¢ A segment begins on a paragraphlocation evenly divisible by 16,

¢ The 8086 BIU sends out 20 bitof 220 bytes (1MB) in memory.8086 works with only four 65,536this 1M range.

¢ Four segment registers in the16 bits of the starting addressesthe 8086 is working with at a particular

ADDRESSINGdefined in a program for containing

stack.

paragraph boundary, that is, at aor hex 10.

bit addresses, so it can address anyHowever, at any given time the

536 byte (64Kbyte) segments with

BIU are used to hold the upperaddresses of four memory segments that

particular time.24

8086 Architecture

¢Three segments are:� Code segment (CS) :

instructions that are to executeexecutable instruction is atCS register addresses the code

� Data segment (DS) : containsdata, constants, and workaddresses the data segment

� Stack segment (SS) :addresses that the programtemporarily or for use by subroutine

contains the machineexecute. Typically, the first

the start of this segment.code segment.

contains a program’s definedwork areas. DS register

.

contains any data andprogram needs to savesubroutine.

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8086 Architecture

4 segments can be separated or overlap

(in small program which do not need 64K)

64K

64K

64K

64K

The registers and segments are not necessarily in the order shown.

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8086 Architecture

SEGMENT BOUNDARIES

¢A segment register is 16 bitscontains the starting address of a segment.

¢A segment begins on a paragraph boundary, which is an address evenly divisible by decimal 16, or hex 10.

¢The BIU inserts zeros (0) for the lowest 4 bits (nibble) of the 20 bit starting address for a segment.

¢ If the code segment register contains 348AH, for example, then the code segment will start at address 348A0H.

16 bits in size and starting address of a segment.

A segment begins on a paragraph boundary, which is an address evenly divisible by decimal

The BIU inserts zeros (0) for the lowest 4 bits (nibble) of the 20 bit starting address for a

If the code segment register contains 348AH, for example, then the code segment will start at

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8086 Architecture

SEGMENT OFFSETS

¢ Within a program, all memory locations within a segment are relative to the segment’s starting address.

¢ The distance in bytes from the segment address to another location within the segment is expressed as offset (or displacement).

¢ 2-byte (16 bit) offset can range from 0000H through FFFFH.

¢ To reference any memory location in a segment, the processor combines the segment address in a segment register with the offset value of that location, that is, its distance in bytes from the start of the segment.

Within a program, all memory locations within a segment are relative to the segment’s starting address.The distance in bytes from the segment address to another location within the segment is expressed as an

byte (16 bit) offset can range from 0000H through

To reference any memory location in a segment, the processor combines the segment address in a segment register with the offset value of that location, that is, its distance in bytes from the start of the segment. 28

8086 Architecture

¢ Consider the data segment that begins at location The DS register contains the segment address of the data segment, 038E[0], and an instruction references a location with an offset of 0032H bytes from the start of the data segment.

¢ To reference the required location, the processor combines the address of the data segment with the offset:

DS segment addressOffsetActual address

Consider the data segment that begins at location 038E0H. The DS register contains the segment address of the data

], and an instruction references a location H bytes from the start of the data

To reference the required location, the processor combines the address of the data segment with the offset:

038E0H+0032H03912H à physical address

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8086 Architecture

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8086 Architecture

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8086 Architecture

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8086 Architecture

SEGMENT REGISTER

¢A segment register provides for addressing an area of memory known as the current segment.

¢ Segment register is used to hold the upper 16 bits of the starting address for each of the segments.

¢The four segment registers are:� Code segment (CS) register� Data segment (DS) register� Stack segment (SS) register� Extra segment (ES) register

A segment register provides for addressing an area of memory known as the current

hold the upper 16 bits of the starting address for each of the segments.

registers are:Code segment (CS) registerData segment (DS) registerStack segment (SS) registerExtra segment (ES) register 33

8086 Architecture

¢CS register: contains theprogram’s code segment. Thisplus an offset value in the(IP) register (CS:IP), indicatesinstruction to be fetched for

¢DS register: contains theprogram’s data segment.address to locate data; thisvalue in an instruction, causesspecific byte location in the

the starting address of aThis segment address,

the Instruction Pointerindicates the address of an

for execution.

the starting address of aInstructions use this

this address, plus an offsetcauses a reference to a

the data segment.34

8086 Architecture

¢ SS register: permits the implementationmemory, which a program usesaddresses and data. The systemaddress of a program’s stackThis segment address, plusPointer (SP) register (SS:SP)in the stack being addressed.

¢ ES register: used by somememory addressing. In thisassociated with the DI register

implementation of a stack inuses for temporary storage ofsystem stores the starting

stack segment in SS register.an offset value in the Stack

SP), indicates the current word.

string operations to handlethis context, ES register is

register.35

8086 Architecture

INTRODUCTION TO PROGRAMMING

8086

¢There are three language levelsused to write a program for a microcomputer.

¢Machine language¢Assembly language¢High level language

PROGRAMMING THE

three language levels that can be used to write a program for a microcomputer.

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8086 Architecture

MACHINE LANGUAGE

¢ Binary form of the program is referred to as machine language because it is the form required by the machine.

¢ It is difficult for a programmer to memorize the thousands of binary instruction codes.

¢ Very easy for an error to occur when working with long series of 1’s and 0’s.

¢ Using hexadecimal representation for the binary codes might help some, but there are still thousands of instruction codes to cope with.

Binary form of the program is referred to as machine language because it is the form required by the

It is difficult for a programmer to memorize the thousands of binary instruction codes.

Very easy for an error to occur when working with long

Using hexadecimal representation for the binary codes might help some, but there are still thousands of

37

8086 Architecture

B82301 MOV AX, 0123

Machine codeMachine instructions may be one, two, or three bytes in length.

First byte is the actual operation, and any other bytes that are present are operands - reference to an immediate value, a register, or a memory location.

MOV AX, 0123

Machine instructions may be one, two, or three bytes in length.

First byte is the actual operation, and any other bytes that are present

reference to an immediate value, a register, or a memory

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8086 Architecture

ASSEMBLY LANGUAGE

¢Much more readable form of machine language, called assembly language, uses mnemonic codesmachine code instructions, rather than simply using the instructions’ numeric values.

¢Translate to machine language so that it can be loaded into memory and run.

¢Assembly language uses two, three or four letter mnemonics to represent each instruction type.

¢Assembly language statements are usually written in a standard form that has four fields.

Label Op-code OperandNEXT: ADD AL, 07H

Much more readable form of machine language, called mnemonic codes to refer to

machine code instructions, rather than simply using the instructions’ numeric values.Translate to machine language so that it can be loaded

Assembly language uses two, three or four letter mnemonics to represent each instruction type.Assembly language statements are usually written in a standard form that has four fields.

comment;Add 07 and content of AL

¢ A label is a symbol or grouprepresent an address whichat the time the statementusually followed by a colon.

¢ The op-code field of themnemonic for the instructionInstruction mnemonics are(op-code).

¢ The operand field of the statementmemory address, port address,register on which the instruction

¢ Comment field starts withinformation about the instructionthe program.

group of symbols used towhich is not specifically known

statement is written. Labels are

instruction contains theinstruction to be performed.

are also called operation code

statement contains the data,address, or the name of the

instruction is to be performed.semicolon and contain the

instruction but are not part of 40

8086 Architecture

HIGH LEVEL LANGUAGE

¢High level language use program statements which are even more Englishassembly language.

¢Compiler translate high-level language statement to machine code which can be loaded into memory and executed.

¢Programs can usually be written faster than in assembly language because it works with bigger building blocks.

¢Execute slowly and require more memory than the same program written in assembly language.

High level language use program statements which are even more English-like than those of

level language statement to machine code which can be loaded into memory

Programs can usually be written faster than in assembly language because it works with bigger

Execute slowly and require more memory than the same program written in assembly language.

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8086 Architecture

TRANSLATION TO MACHINE

¢Microprocessor only understand the binary numbers and hence a translator must be used to convert assembly/high-level language programs into binary machine language so that the microprocessor can execute the program.

¢An assembler translates the program written in assembly language into machine language program (object code).

¢Assembly language program ¢Machine language program

MACHINE CODE

Microprocessor only understand the binary numbers and hence a translator must be used to

level language programs into binary machine language so that the microprocessor can execute the program.

translates the program written in assembly language into machine language program

Assembly language program à source codesMachine language program à object codes.

42

8086 Architecture

Assembly language

(Source code)

*.ASM

Object code *.OBJ

Assembler

o Translator converts source codes to object codes and then into executable formats.

o Source code à object code …………. o Object code à executable format …….

Object code *.OBJ

Executable file

*.EXE or

*.COM

Linker

43

8086 Architecture

converts source codes to object codes and then into

object code …………. Assemblerexecutable format ……. linker

¢ There are two ways of converting an assembly language program into machine language:� Manual assembly� Using assembly

¢ With manual assembly, the programmer is the assembler; programmer translates each mnemonic into its numerical machine language representation by looking up a table of the microprocessor’s instruction set.

¢ Manual assembly is acceptable for short programs but becomes very inconvenient for large programs

ways of converting an assembly language program into machine language:

With manual assembly, the programmer is the assembler; programmer translates each mnemonic into its numerical machine language representation by looking up a table of the microprocessor’s instruction set.

Manual assembly is acceptable for short programs but becomes very inconvenient for large programs

44

8086 Architecture

¢ When an assembler is used, the assembler reads each assembly instruction of a program as ASCII characters and translates them into respective binary op

¢ Address computation is the advantage of the assembler. (assembler computes the actual address for the programmer and fills it in automatically).

When an assembler is used, the assembler reads each assembly instruction of a program as ASCII characters and translates them into respective binary op-codes.

Address computation is the advantage of the assembler. (assembler computes the actual address for the programmer and fills it in automatically).

45

8086 Architecture

TYPES OF ASSEMBLER

¢ One pass assembler� Assembler goes through

program once and translatesprogram.

� Can not resolve the forward

� Either all labels used indefined in the source programreferenced, or forward referencesprohibited.

the assembly languagetranslates the assembly language

forward referencing.

in forward references areprogram before they are

references to data items are46

8086 Architecture

¢ Two pass assembler

� More efficient & easy to use.� Performs two sequential scans over the source code.� Pass 1:

¢Scans the code.¢Validates the tokens.¢Creates a symbol table.

� Pass 2:¢Solves forward referencing.¢Converts the code to the machine code.

More efficient & easy to use.Performs two sequential scans over the source code.

Solves forward referencing.Converts the code to the machine code.

47

8086 Architecture

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8086 Architecture

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8086 Architecture

ASSEMBLY LANGUAGE FEATURES

¢ Program comment:� The use of comments throughout a program can

improve its clarity, especially in assembly language, where the purpose of a set of instructions is often unclear.

� A comment begins with a semicolon (;), and wherever it is coded, the assembler assumes that all characters on the line to its right are comments.

� A comment may contain any printable character, including blank.

MOV AX, BX ; move the content of BX to AX.

FEATURES

The use of comments throughout a program can improve its clarity, especially in assembly language, where the purpose of a set of instructions is often

A comment begins with a semicolon (;), and wherever it is coded, the assembler assumes that all characters on the line to its right are comments.

A comment may contain any printable character,

; move the content of BX to AX.

50

8086 Architecture

¢ Reserved words� Certain names in assembly language are reserved for

their own purposes, to be used only under special conditions.

� Reserved words, by category, include:¢Instructions, such as MOV and ADD, which are

operations that the computer can execute;¢Directives, such as END or SEGMENT, which is used to

provide information to the assembler.¢Operators, such as FAR and SIZE, which is used in

expressions.¢Predefined symbols, such as @Data and @Model, which

return information to the program during the assembly.

Certain names in assembly language are reserved for their own purposes, to be used only under special

Reserved words, by category, include:, such as MOV and ADD, which are

operations that the computer can execute;, such as END or SEGMENT, which is used to

provide information to the assembler., such as FAR and SIZE, which is used in

, such as @Data and @Model, which return information to the program during the assembly.

51

8086 Architecture

¢ Identifiers:� An identifier (or symbol) is a name apply to an item in the

program for reference.

� Two types of identifier:¢Name: refers to the address of a data item, such as

COUNTER in COUNTER DB 0

¢Label: refers to the address of an instruction, procedure, or segment, such as MAIN and B30

MAIN PROC FARB30: ADD BL, 25

An identifier (or symbol) is a name apply to an item in the

: refers to the address of a data item, such as

: refers to the address of an instruction, procedure, or 30 in the following statements.

52

8086 Architecture

¢ Identifier can use the following characters:¢CATEGORY ALLOWABLE CHARACTER

Alphabetic letters: A – Z and a Digit: 0 – 9 (not the first character)Special characters: question mark(?)

underline( _ )dollar ($)at (@)dot ( . ) not first character

¢ The maximum length of an identifier is 31 character up to MASM 6.0 and 247 since.

Identifier can use the following characters:ALLOWABLE CHARACTER

Z and a – z9 (not the first character)

question mark(?)underline( _ )dollar ($)at (@)dot ( . ) not first character

The maximum length of an identifier is 31 character up 53

8086 Architecture

¢Statements� An assembly program consists of a set of

statements.� Two types of statements:¢Instructions: such as MOV and ADD, which the

assembler translates to object code; and¢Directives: which tell the assembler to perform a

specific action, such as define a data item.

� Format of a statement:

[identifier] operation [operand

An assembly program consists of a set of

Two types of statements:such as MOV and ADD, which the

assembler translates to object code; andwhich tell the assembler to perform a

specific action, such as define a data item.

[operand (s)] [;comment]54

8086 Architecture

¢ An identifier (if any), operation, and operand (if any) are separated by at least one blank or tab character.

¢ There is maximum of 132 characters on a line up to MASM and 512 since.

¢ Examples:IDENTIFIER OPERATION OPERAND COMMENT

Directive: COUNT DBInstruction: L30: MOV

An identifier (if any), operation, and operand (if any) are separated by at least one blank or tab character.

characters on a line up to MASM 6.0

IDENTIFIER OPERATION OPERAND COMMENT1 ;Name, operation, operand

AX,0 ;label, operation, operand

55

8086 Architecture

¢ Directives:� Assembly language support

that enable to control theprogram assembles and lists

� Describe the way accordingmicroprocessor is directed to

� Act only during the assemblygenerate no machine executable

support a number of statementsthe way in which a sourcelists.

according to which theto perform a specific task.

assembly of a program andexecutable code.

56

8086 Architecture

MOST COMMON DIRECTIVES

¢ PAGE and TITLE Listing Directives:� The PAGE and TITLE directives help to control the

format of a listing of an assembled program.� They have no effect on subsequent execution of the

program.

� At the start of the program, the designates the maximum number of lines to list on a page and the maximum number of characters on a line.

� Its format isPAGE [length] [, width]

DIRECTIVES

PAGE and TITLE Listing Directives:The PAGE and TITLE directives help to control the format of a listing of an assembled program.They have no effect on subsequent execution of the

At the start of the program, the PAGE directive designates the maximum number of lines to list on a page and the maximum number of characters on a

PAGE [length] [, width]57

8086 Architecture

¢ PAGE 60, 132 à length is 60 lines per page and width is 132 character per line.

¢ The number of lines per page may range from 10 through 255, and the number of characters per line may range from 60 through 132.

¢ Omission of a PAGE statement causes the assembler to default to PAGE 50, 80.

length is 60 lines per page and width

The number of lines per page may range from 10 through 255, and the number of characters per line may

Omission of a PAGE statement causes the assembler to

58

8086 Architecture

¢ The TITLE directive to cause a title for a program to print on line 2 of each page of the program listing.

¢ Format of TITLE directive isTITLE text [comment]

¢ For text, a common practice is to use the name of the program as cataloged on disk.

TITLE ASMSORT Assembly program to sort CD titles

Directive text Comment ( ‘;’ is not required)

directive to cause a title for a program to of each page of the program listing.

TITLE text [comment]For text, a common practice is to use the name of the

TITLE ASMSORT Assembly program to sort CD titles

59

8086 Architecture

Comment ( ‘;’ is not required)

¢ SEGMENT and ENDS Directives:� An assembly language program in .EXE format consist of one

or more segments.� The directives for defining a segment,

ENDS, have the following format:

segment_name SEGMENTMOV AX, BXADD AX, BX……………..

segment_name ENDS

SEGMENT and ENDS Directives:An assembly language program in .EXE format consist of one

The directives for defining a segment, SEGMENT and , have the following format:

SEGMENTMOV AX, BXADD AX, BX……………..

60

8086 Architecture

� The SEGMENT statement defines the start of a segment.� The segment_name must be present, must be unique, and

must follow assembly naming conventions.� The ENDS statement indicates the end of the segment and

contains the same name as the SEGMENT statement.� The maximum size of a segment is 64K.

ARRAY1 SEGMENTMOV AX, BXADD AX, BX

ARRAY1 ENDS

statement defines the start of a segment.must be present, must be unique, and

must follow assembly naming conventions.statement indicates the end of the segment and

contains the same name as the SEGMENT statement.The maximum size of a segment is 64K.

61

8086 Architecture

� Segment_nameà ARRAY1� The assembler will assign a numeric value to ARRAY1

corresponding to the base value of the Data segment.� The programmer can load ARRAY1 into the DS using the

following instruction:MOV AX, @ARRAY1MOV DS, AX

� The segment register like DS, CS etc must be loaded via 16 bit register such as AX or by the contents of a memory location.

� A data array or an instruction sequence between the SEGMENT and ENDS directives is called a

The assembler will assign a numeric value to ARRAY1 corresponding to the base value of the Data segment.The programmer can load ARRAY1 into the DS using the

The segment register like DS, CS etc must be loaded via 16 bit register such as AX or by the contents of a memory

A data array or an instruction sequence between the SEGMENT and ENDS directives is called a logical segment.

62

8086 Architecture

¢ ASSUME Directive:� An 8086 program may have several logical segments that

contain code and several that contain data.� However, at ay given time the 8086 works directly with only

four physical segments: CS, DS, SS and ES.� The ASSUME directive tells the assembler which logical

segment to use for each of these physical segments at a given time.

� The format is:ASSUME ss:stackname, DS:datasegname, CS:codesegname

An 8086 program may have several logical segments that contain code and several that contain data.However, at ay given time the 8086 works directly with only four physical segments: CS, DS, SS and ES.The ASSUME directive tells the assembler which logical segment to use for each of these physical segments at a given

ASSUME ss:stackname, DS:datasegname, CS:codesegname

63

8086 Architecture

� The above statement tells the assembler that the logical segment named codesegname contains the instruction statements for the program and should be treated as a code segment. It also tells the assembler that it should treat the logical segment datasegname as the data segment. In order words, the DS:datasegnament part of the statement tells the assembler that for any instruction which refers to data in the data segment, data will be found in the logical segment datasegname.

� ASSUME may also contain an entry for the ES register, such as ES:datasegname; if the program does not use ES, its reference is omitted or code ES:NOTHING.

The above statement tells the assembler that the logical segment named codesegname contains the instruction statements for the program and should be treated as a code segment. It also tells the assembler that it should treat the logical segment datasegname as the data segment. In order words, the DS:datasegnament part of the statement tells the assembler that for any instruction which refers to data in the data segment, data will be found in the logical segment

ASSUME may also contain an entry for the ES register, such as ES:datasegname; if the program does not use ES, its reference is omitted or code ES:NOTHING.

64

8086 Architecture

¢ PROC Directive� The code segment contains the executable code for a program

which consists of one or more procedures, defined initially with the PROC directive and ended with the ENDP directive.

� The format is

NAME OPERATIONprocedure_name PROC

……………………procedure_name ENDP

The code segment contains the executable code for a program which consists of one or more procedures, defined initially with the PROC directive and ended with the ENDP directive.

OPERAND COMMENTFAR ; Begin proc

……………………; End proc

65

8086 Architecture

� The procedure_name must be present, must be unique, and must follow assembly language naming conventions.

� The operand FAR in this case, is related to program execution.

� The ENDP directive indicates the end of a procedure and contains the same name as the PROC statement to enable the assembler to relate the end to the start.

� Because a procedure must be fully contained within a segment, ENDP defines the end of the procedure before ENDS defines the end of the segment.

� The code segment may contain any number of procedures used as subroutines, each with its own set of matching PROC and ENDP statements.

� Each additional PROC is usually coded with (or default to) the NEAR operad.

The procedure_name must be present, must be unique, and must follow assembly language naming conventions.The operand FAR in this case, is related to program

The ENDP directive indicates the end of a procedure and contains the same name as the PROC statement to enable the assembler to relate the end to the start.Because a procedure must be fully contained within a segment, ENDP defines the end of the procedure before ENDS defines the end of the segment.The code segment may contain any number of procedures used as subroutines, each with its own set of matching PROC

Each additional PROC is usually coded with (or default to) 66

8086 Architecture

¢ END Directive� An END directive ends the entire program and appears as

the last statement.

� Its format is:

END [entry-point]

� Entry-point (procedure_name) tells the assembler and linker where the program will begin execution.

An END directive ends the entire program and appears as

point]

point (procedure_name) tells the assembler and linker where the program will begin execution.

67

8086 Architecture

¢ MODEL Directive� The MODEL directive selects a standard memory model for

the program. � It determines the way segments are linked together, as well

as the maximum size of each segment.� Its format is

.MODEL memory_model

� The memory_model may be Tiny, Medium, Compact, Large, Huge, or Flat.

The MODEL directive selects a standard memory model for

It determines the way segments are linked together, as well as the maximum size of each segment.

.MODEL memory_model

The memory_model may be Tiny, Medium, Compact, Large,

68

8086 Architecture

MODEL Description

Tiny Code & data together may not be greater than 64K

Small Neither code nor data may be greater

Medium Only the code may be greater than 64K

Compact Only the data may be greater than 64K

Large Both code & data may be greater than 64K

Huge All available memory may be used for code & data

69

8086 Architecture

Description

& data together may not be greater than 64K

Neither code nor data may be greater than 64K

Only the code may be greater than 64K

Only the data may be greater than 64K

code & data may be greater than 64K

be used for code & data

¢ The formats (including the leading dot) for the directives that define the stack, data, and code segments are:

.STACK

.DATA

.CODE¢ Each of these directives causes the assembler to generate the

required SEGMENT statement and its matching ENDS.¢ The default stack size is 1024 bytes, which ca be override.¢ The instruction used to initialize the address of the data segment

in DS are:MOV AX,@dataMOV DS,AX

The formats (including the leading dot) for the directives that define the stack, data, and code segments are:

[size]

[segment_name]Each of these directives causes the assembler to generate the required SEGMENT statement and its matching ENDS.The default stack size is 1024 bytes, which ca be override.The instruction used to initialize the address of the data segment

;initialize DS with;address of data segment

70

8086 Architecture

DEFINING TYPE OF DATA

¢ The data segment in an .EXE program contains constants, work areas, and input/output areas.

¢ The assembler provides a set of directives that permits definitions of items by various types and lengths; for example, DB defines byte and DW defines word.

¢ A data item may contain an undefined (uninitialized) value, or it may contain an initialized constant, defined either as a character string or as a numeric value.

¢ Format for data defining:[name] Dn

The data segment in an .EXE program contains constants, work areas, and input/output areas.The assembler provides a set of directives that permits definitions of items by various types and lengths; for example, DB defines byte and DW defines word.A data item may contain an undefined (uninitialized) value, or it may contain an initialized constant, defined either as a character string or as a numeric value.

expression

71

8086 Architecture

¢ Name: � a program that reference a data item does so by means of a

name, as indicated by the square brackets.

¢ Directive (Dn): � the directives that define data items are DB (byte), DW

(word), DD (doubleword), DF (farword), DQ (quadword), and DT (tenbytes), each of which explicitly indicates the length of the defined item.

a program that reference a data item does so by means of a name, as indicated by the square brackets.

the directives that define data items are DB (byte), DW (word), DD (doubleword), DF (farword), DQ (quadword), and DT (tenbytes), each of which explicitly indicates the length of

72

8086 Architecture

¢ Expression:� the expression in an operand may specify an uninitialized

value or a constant value. To indicate an uninitialized item, define the operand with a question mark, such as

DATAX DB ? ;uninitialized item

� When program begins execution, the initial value of DATAX is unknown.

� The operand can be used to define a constant, such asDATAY DB 25

� Use this initialized value 25 throughout the program and can even change the value.

the expression in an operand may specify an uninitialized value or a constant value. To indicate an uninitialized item, define the operand with a question mark, such as

;uninitialized item

When program begins execution, the initial value of DATAX

The operand can be used to define a constant, such as;initialized item

Use this initialized value 25 throughout the program and can 73

8086 Architecture

� An expression may contain multiple constants separated by commas and limited only by the length of the line, as follows:

DATAZ DB 21, 22, 23, 24, 35, 26, ……

� The assembler defines these constants in adjacent bytes, from left to right.

DATAZ + 0 à 21DATAZ + 1 à 22DATAZ + 2 à 23 …..

� The instruction MOV AL, DATAZ+3 loads the value 24 (18H) into the AL register.

An expression may contain multiple constants separated by commas and limited only by the length of the line, as follows:

DATAZ DB 21, 22, 23, 24, 35, 26, ……

The assembler defines these constants in adjacent bytes,

The instruction MOV AL, DATAZ+3 loads the value 24 (18H)

74

8086 Architecture

� The expression also permits duplication of constants in a statement of the format

[name] Dn repeat-count DUP (expression) ……

� The following examples illustrate duplicationDW 10 DUP(?) ; ten words, uninitializedDB 5 DUP(12) ;five bytes containing hex ocococococDB 3 DUP(5 CUP(4)) ;fifteen 4s

� An expression may define and initialized a character string or a numeric constant.

The expression also permits duplication of constants in a

count DUP (expression) ……

The following examples illustrate duplication; ten words, uninitialized;five bytes containing hex ocococococ;fifteen 4s

An expression may define and initialized a character string 75

8086 Architecture

¢ Character string� Character string are used for descriptive data such as

people’s names and product descriptions.� The string is defined within single quotes, such as ‘PC’, or

within double quotes, such as “PC”.� The assembler stores the contents of the quotes as object

code in normal ASCII format, without the apostrophes.� DB is the only format that defines a character string

exceeding two characters with the characters stored as left adjusted ad in normal left-to-right sequences.

DB ‘Computer city’DB “crazy sam’s CD emporium”

Character string are used for descriptive data such as people’s names and product descriptions.The string is defined within single quotes, such as ‘PC’, or within double quotes, such as “PC”.The assembler stores the contents of the quotes as object code in normal ASCII format, without the apostrophes.DB is the only format that defines a character string exceeding two characters with the characters stored as left

right sequences.

DB “crazy sam’s CD emporium”76

8086 Architecture

DIRECTIVE FOR DEFINING DATA

¢ DB or BYTE: Define Byte� A DB (or BYTE) numeric expression may define one or more

1 byte constants, each consisting of two hex digits.� For unsigned data, the range of values is

data, the range of values is -128 � The assembler converts numeric constants to binary object

code (represented in hex).

BYTE1 DBBYTE2 DBBYTE3 DBBYTE4 DBBYTE5 DB

DIRECTIVE FOR DEFINING DATA

A DB (or BYTE) numeric expression may define one or more byte constants, each consisting of two hex digits.

For unsigned data, the range of values is 0 to 255; for signed 128 to +127.

The assembler converts numeric constants to binary object

?4830H01111010B10 DUP(0) 77

8086 Architecture

¢ DW or WORD : Define Word� The DW directive defines items that are one word (two bytes)

in length. � A DW numeric expression may define one or more one word

constants.� For unsigned numeric data, the range of values is 0 to 65535;

for signed data, the range of value is � The assembler converts DW numeric constants to binary

object code (represented in hex), but stores the bytes in reverse sequence.

� Consequently, a decimal value defined as 12345 converts to hex 3039, but is stored as 3930.

The DW directive defines items that are one word (two bytes)

A DW numeric expression may define one or more one word

For unsigned numeric data, the range of values is 0 to 65535; for signed data, the range of value is -32768 to +32767.The assembler converts DW numeric constants to binary object code (represented in hex), but stores the bytes in

Consequently, a decimal value defined as 12345 converts to

78

8086 Architecture

WORD1 DW 0FFF0HWORD2 DW 01111010BWORD3 DW 2, 4, 6, 7, 8WORD4 DW 8 DUP(0)

0FFF0H01111010B2, 4, 6, 7, 88 DUP(0)

79

8086 Architecture

¢ DD or DWORD: Define Doubleword� The DD directive defines items that are a doubleword (four

byte) in length.� A DD numeric expression may define one or more constants,

each with a maximum of four bytes ( 8 hex digit).� For unsigned numeric data, the range of values is 0 to

4294967295; for signed data, the range is +2147483647.

� The assembler converts DD numeric constants to binary object code (represented in hex), but stores the bytes in reverse sequence.

� Consequently, the assembler converts a decimal value defined as 12345678 to 00BC614EH and stores it as 4E61BC00H.

DD or DWORD: Define DoublewordThe DD directive defines items that are a doubleword (four

A DD numeric expression may define one or more constants, each with a maximum of four bytes ( 8 hex digit).For unsigned numeric data, the range of values is 0 to 4294967295; for signed data, the range is -2147483648 to

The assembler converts DD numeric constants to binary object code (represented in hex), but stores the bytes in

Consequently, the assembler converts a decimal value defined as 12345678 to 00BC614EH and stores it as 80

8086 Architecture

DWORD1 DD ?DWORD2 DD 41562DWORD3 DD 24, 48DWORD4 DD BYTE3 3 - BYTE2

81

8086 Architecture

¢ EQU Directive� The EQU directive (short form of equivalent) an be used to

assign a name to constant.� PROD EQU 55H directs the assembler to assign the value

55H every time it finds PROD in the program.� MOV BX, PROD moves 55H in BX.

The EQU directive (short form of equivalent) an be used to

H directs the assembler to assign the value H every time it finds PROD in the program.

H in BX.

82

8086 Architecture

SAMPLE ASSEMBLY LANGUAGE PROGRAM

SAMPLE ASSEMBLY LANGUAGE PROGRAM

83

8086 Architecture

1 Page 60, 132

2 TITLE A05ASM1

3 ; -----------------------------------------------------------------------------

4 0000 STACK SEGMENT

5 0000 0020[0000] DW

6 0040 STACK ENDS

7 ; ……………………………………………………………………………………………………….

8 0000 DATASEG SEGMENT

9 0000 00D7 FLDD DW

10 0002 007D FLDE DW

11 0004 0000 FLDF DW

12 0006 DATASEG ENDS

13 ; …………………………………………………………………………………………………………

14 0000 CODESEG SEGMENT

15 0000 MAIN PROC

16 ASSUME

17 0000 B8 ---- R MOV

18 0003 8E D8 MOV

19 0005 A1 0000 R MOV

20 0008 03 06 0002 R ADD AX, FLDE

21 000C A3 0004 R MOV FLDF, AX

22 000F B8 4C00 MOV AX, 4C

23 0012 CD 21 INT

24 0014 MAIN ENDP

25 0014 CODESEG ENDS

26 END MAIN

move and add operations

---------------------------------------------------------------------------------------------------------------------

32 DUP (0)

; ……………………………………………………………………………………………………….

SEGMENT

215

125

?

; …………………………………………………………………………………………………………

SEGMENT

FAR

SS:STACK, DS:DATASEG, CS:CODESEG

AX, DATASEG ;set address of data segment

DS, AX ; in DS

AX, FLDD ;move 0215 to AX

;add 0125 to AX

MOV FLDF, AX ;store sum in FLDF

C00H ;end processing

21H

;end of procedure

;end of segment

84

8086 Architecture

1 PAGE 60,132

2 TITLE A05ASM3

3 ; ------------------------------------------------------------------------------------

4 .MODEL

5 .STACK

6 .DATA

7 0000 00D7 FLDD DW

8 0002 007D FLDE DW

9 0004 0000 FLDF DW

10 ; ---------------------------------------------------------------------------------------

11 .CODE

12 0000 MAIN PROC

13 0000 B8 ---- R MOV AX, @data

14 0003 8E D8 MOV DS, AX

15

16 0005 A1 0000 R MOV AX, FLDD

17 0008 03 06 0002 R ADD AX, FLDE

18 000C A3 0004 R MOV FLDF, AX

19

20 000F B8 4C00 MOV AX, 4C00H

21 0012 CD 21 INT 21H

22 0014 MAIN ENDP

23 END MAIN

Move and add operation

------------------------------------------------------------------------------------

SMALL

64 ;define stack

;define data

215

125

?

---------------------------------------------------------------------------------------

;define code segment

FAR

MOV AX, @data ;set address of data segment in DS

MOV DS, AX

MOV AX, FLDD ;move 0215 to AX

ADD AX, FLDE ;add 0125 to AX

MOV FLDF, AX ;store sum in FLDF

MOV AX, 4C00H ;End processing

;End of procedure

END MAIN ;End of program

85

8086 Architecture

MACRO ASSEMBLER

¢ Translate a program written in macro language into the machine language.

¢ A macro language is the one in which all the instruction sequence can be defined using macros.

¢ A macro is an instruction sequence that appears repeatedly in a program assigned with a specific name.

¢ The macro assembler replaces a macro name with the appropriate instruction sequence each time it encounters a macro name.

¢ The main difference between a macro and a procedure is that in the macro the passage of parameters is possible and in the procedure it is not.

Translate a program written in macro language into the

A macro language is the one in which all the instruction sequence can be defined using macros.A macro is an instruction sequence that appears repeatedly in a program assigned with a specific name.The macro assembler replaces a macro name with the appropriate instruction sequence each time it

The main difference between a macro and a procedure is that in the macro the passage of parameters is possible

86

8086 Architecture

¢ Syntax of macro:� Declaration of the macro� Code of the macro� Macro termination directive

¢ The declaration of the macro is done the following way:NameMacro MACRO [parameter1, parameter2...]

¢ The directive for the termination of the macro is: ENDM

The declaration of the macro is done the following way:NameMacro MACRO [parameter1, parameter2...]

The directive for the termination of the macro is: ENDM

87

8086 Architecture

Addition MACROIN AX, PORTADD AX, BXOUT PORT, AXENDM

¢ When above instruction sequence is to be executed repeatedly macro assembler allow the macro name only to be typed instead of all instructions, provided the macro is defined.

OUT PORT, AX

When above instruction sequence is to be executed repeatedly macro assembler allow the macro name only to be typed instead of all instructions, provided the

88

8086 Architecture

¢ There exist difference between a macro program and a subroutine program.

¢ A specific subroutine occurs once in a program. A subroutine is executed by calling it from a main program. The program execution jumps out of the main program and then executes the subroutine. At the end of the subroutine, a RET instruction is used to resume program execution following the CALL SUBROUTINE instruction in the main program

There exist difference between a macro program and a

A specific subroutine occurs once in a program. A subroutine is executed by calling it from a main program. The program execution jumps out of the main program and then executes the subroutine. At the end of the subroutine, a RET instruction is used to resume program execution following the CALL SUBROUTINE instruction in the main program

89

8086 Architecture

¢ A macro does not cause the program execution to branch out of the main program. Each time a macro occurs, it is replaced with the appropriate sequence in the main program. The advantages of using macros are that the source programs become shorter and program documentation becomes better.

¢ Conditional macro assembler is very useful in determining whether or not an instruction sequence shall be included in the assembly depending on a condition that is true or false.

¢ Based on each condition, a particular program is assembled.

A macro does not cause the program execution to branch out of the main program. Each time a macro occurs, it is replaced with the appropriate sequence in the main program. The advantages of using macros are that the source programs become shorter and program documentation becomes better.Conditional macro assembler is very useful in determining whether or not an instruction sequence shall be included in the assembly depending on a

Based on each condition, a particular program is 90

8086 Architecture

DESCRIPTION OF ASSEMBLY PROCESS IN MACRO ASSEMBLER (MASM)

¢ MASM is two pass assembler.¢ The complete process of assembling, linking, and

executing an assembly language program using a macro assembler is similar as mentioned previous.

Assembly language

(Source code)

*.ASM

Object code *.OBJ

Assembler

DESCRIPTION OF ASSEMBLY PROCESS IN MACRO ASSEMBLER (MASM)

The complete process of assembling, linking, and executing an assembly language program using a macro assembler is similar as mentioned previous.

Object code *.OBJ

Executable file

*.EXE or

*.COM

Linker

91

8086 Architecture

¢ The assembly step involves translating the source code into object code and generating an intermediate .OBJ file, or module. One of the assembler’s tasks is to calculate the offsets for every data item in the data segment and for every instruction in the code segment.

¢ The link step involves converting the .OBJ module to an .EXE machine code module. The linker’s tasks include completing any address left open by the assembler and combining separately assembled programs into one executable module.

¢ The last step is to load the program for execution.

The assembly step involves translating the source code into object code and generating an intermediate .OBJ file, or module. One of the assembler’s tasks is to calculate the offsets for every data item in the data segment and for every instruction in the code segment.The link step involves converting the .OBJ module to an .EXE machine code module. The linker’s tasks include completing any address left open by the assembler and combining separately assembled programs into one

The last step is to load the program for execution. 92

8086 Architecture

ASSEMBLING THE SOURCE PROGRAM

¢ The assembler converts the source statements into machine code and displays any error messages on the screen.

¢ Typical errors include a name that violates naming conventions, an operation that is spelled incorrectly (such as MOVE instead of MOV), and an operand containing a name that is not defined.

¢ The assembler attempts to correct some errors but, in any event, reload the editor, correct the .ASM source program, and reassemble it.

¢ Optional output files from the assembly step are object (.OBJ), listing (.LST) and cross reference (.CRF or .SBR).

ASSEMBLING THE SOURCE PROGRAM

The assembler converts the source statements into machine code and displays any error messages on the

Typical errors include a name that violates naming conventions, an operation that is spelled incorrectly (such as MOVE instead of MOV), and an operand containing a name that is not defined.The assembler attempts to correct some errors but, in any event, reload the editor, correct the .ASM source

Optional output files from the assembly step are object (.OBJ), listing (.LST) and cross reference (.CRF or

93

8086 Architecture

LINKING AN OBJECT PROGRAM

¢ When the program is free of error messages, the next step is to link the object module that was produced by the assembler and that contains only machine code.

¢ The linker performs the following functions:� Combines, if requested, more than one separately assembled

module into one executable program, such as two or more assembly programs or an assembly program with a C program.

� Generates an .EXE module and initialize it with special instruction to facilitate its subsequent loading for execution.

¢ Once one or more .OBJ modules are linked into an .EXE module, .EXE module can execute any number of times.

¢ But the source program needs correction: correct source program, assemble again into an .OBJ module, and link .OBJ module into an .EXE module.

LINKING AN OBJECT PROGRAM

When the program is free of error messages, the next step is to link the object module that was produced by the assembler and that

The linker performs the following functions:Combines, if requested, more than one separately assembled module into one executable program, such as two or more assembly programs or an assembly program with a C program.Generates an .EXE module and initialize it with special instruction to facilitate its subsequent loading for execution.

Once one or more .OBJ modules are linked into an .EXE module, .EXE module can execute any number of times.But the source program needs correction: correct source program, assemble again into an .OBJ module, and link .OBJ module into an

94

8086 Architecture

EXECUTING A PROGRAM

¢ Having assembled and linked a program, the program can now execute.

¢ If the .EXE file is in the default drive, ask the loader to read it into memory for execution by typing

A05ASM1.EXE or A05ASM¢ However, since this program produces no visible output,

it is suggested that you run it under DEBUG and use Trace commands to step through its execution. DEBUG load the .EXE program module and displays its hyphen prompt.

EXECUTING A PROGRAM

Having assembled and linked a program, the program

If the .EXE file is in the default drive, ask the loader to read it into memory for execution by typing

ASM1 (without .EXE extension)

However, since this program produces no visible output, it is suggested that you run it under DEBUG and use Trace commands to step through its execution. DEBUG load the .EXE program module and displays its hyphen

95

8086 Architecture

16 BIT MICROPROCESSOR ADDRESSING MODE

¢ The 8086 provides various addressing modes to access instruction operands.

¢ Operands may be contained in registers, in memory or in I/O ports.¢ The three basic modes of addressing are register, immediate, and

memory; memory addressing consists of six types, for eight modes in all.� Register addressing� Immediate addressing� Direct memory addressing� Direct-offset addressing� Indirect memory addressing� Base displacement addressing� Base index addressing� Base-index with displacement addressing

16 BIT MICROPROCESSOR ADDRESSING

provides various addressing modes to access instruction

Operands may be contained in registers, in memory or in I/O ports.The three basic modes of addressing are register, immediate, and memory; memory addressing consists of six types, for eight modes

index with displacement addressing

96

8086 Architecture

REGISTER ADDRESSING

¢ For this mode, a register provides the name of any of the 8, or16 bit register. Depending on the instruction, the register may appear in the first operand, the second operand or both, as the following examples illustrate:

MOV DX, WORD_MEMMOV WORD_MEM, CXMOV DX, BX

REGISTER ADDRESSING

For this mode, a register provides the name of any of the 8, or16 bit register. Depending on the instruction, the register may appear in the first operand, the second operand or both, as the following examples illustrate:

MOV DX, WORD_MEMMOV WORD_MEM, CX

97

8086 Architecture

IMMEDIATE ADDRESSING

¢ An immediate operand contains a constant value or an expression.

¢ For many instructions with two operands, the first operand may be a register or memory location, and the second may be an immediate constant. The destination field (first operand) defines the length of the data.

byte_val DB 150word_val DW 300

MOV word_val, 40HMOV AX, 0245H

IMMEDIATE ADDRESSING

An immediate operand contains a constant value or an

For many instructions with two operands, the first operand may be a register or memory location, and the second may be an immediate constant. The destination field (first operand) defines the length of the data.

;define byte;define word

MOV word_val, 40H 98

8086 Architecture

DIRECT MEMORY ADDRESSING

¢ In this format, one of the operands references a memory location and the other operand references a register.

ADD BYTE_VAL, DLMOV BX, WORD_VAL

DIRECT MEMORY ADDRESSING

In this format, one of the operands references a memory location and the other operand references a register.

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8086 Architecture

DIRECT OFFSET ADDRESSING

¢ This addressing mode, a variation of direct addressing, uses arithmetic operators to modify an address.

¢ The following examples use these definitions of tables:

BYTE_TBL DB 12, 15, 16WORD_TBL DB 163, 227DBWD_TBL DB 465, 563

¢ Byte operations: these instructions access bytes from BYTE_TBL:MOV CL, BYTE_TBL[2]MOV CL, BYTE_TBL+2

¢ Word operation: these instruction access words from WORD_TBL:MOV CX, WORD_TBL[4]MOV CX, WORD_TBL+4

DIRECT OFFSET ADDRESSING

This addressing mode, a variation of direct addressing, uses arithmetic operators to modify an address.The following examples use these definitions of tables:

16, 22, ……..227, 435, ……..563, 897, ……..

Byte operations: these instructions access bytes from BYTE_TBL:

Word operation: these instruction access words from WORD_TBL:

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8086 Architecture

INDIRECT MEMORY ADDRESSING

¢ Indirect addressing takes advantage of the computer’s capability for segment:offset addressing.

¢ The registers used for this purpose are base registers (BX and BP) and index registers (DI and SI), coded within square brackets, which indicate a reference to memory.

¢ An indirect address such as [DI] tells the assembler that the memory address to use will be in DI when the program subsequently executes.

¢ BX, DI, and SI are associated with DS as DS:BX, DS:DI, and DS:SI, for processing data in the data segment.

¢ BP is associated with SS as SS:BP, for handling data in the stack.¢ When the first operand contains an indirect address, the second

operand reference a register or immediate value; when the second operand contains an indirect address, the first operand references a register.

INDIRECT MEMORY ADDRESSING

Indirect addressing takes advantage of the computer’s capability

The registers used for this purpose are base registers (BX and BP) and index registers (DI and SI), coded within square brackets, which indicate a reference to memory.An indirect address such as [DI] tells the assembler that the memory address to use will be in DI when the program

BX, DI, and SI are associated with DS as DS:BX, DS:DI, and DS:SI, for processing data in the data segment.BP is associated with SS as SS:BP, for handling data in the stack.When the first operand contains an indirect address, the second operand reference a register or immediate value; when the second operand contains an indirect address, the first operand references a

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8086 Architecture

¢ A reference in square brackets to BP, BX, DI or SI implies an indirect operand, and the processor treats the contents of the register as an offset address when the program is executing.

� DATA_VAL DB

……..

LEA BX, DATA_VAL

MOV [BX], CL

� ADD CL, [BX]

ADD [BP], CL

A reference in square brackets to BP, BX, DI or SI implies an indirect operand, and the processor treats the contents of the register as an offset address when the program is executing.

50

LEA BX, DATA_VAL

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8086 Architecture

BASE DISPLACEMENT ADDRESSING

¢ This addressing mode also uses base register (BX and BP) and index registers (DI and SI), but combined with a displacement (a number or offset value) to form an effective address.

¢ The following MOV instruction moves zero to a location two bytes immediately following the start of DATA_TBL;

DATA_TBL DB 365 DUP(?)………ADD CL, [DI+12]SUB DATA_TBL, DLMOV DATA_TBL[DI], DL

BASE DISPLACEMENT ADDRESSING

This addressing mode also uses base register (BX and BP) and index registers (DI and SI), but combined with a displacement (a number or offset value) to form an

The following MOV instruction moves zero to a location two bytes immediately following the start of

365 DUP(?)

ADD CL, [DI+12]SUB DATA_TBL, DLMOV DATA_TBL[DI], DL

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8086 Architecture

BASE INDEX ADDRESSING

¢ This addressing mode combines a base register (BX or BP) with an index register (DI or SI) to form an effective address; for example, [BX+DI] means the address in BX plus the address in DI.

¢ A common use for this mode is in addressing a 2dimensional array, where, say, BX references the row and SI the column.

MOV AX, [BX+SI]ADD [BX+DI], CL

BASE INDEX ADDRESSING

This addressing mode combines a base register (BX or BP) with an index register (DI or SI) to form an effective address; for example, [BX+DI] means the address in BX

A common use for this mode is in addressing a 2-dimensional array, where, say, BX references the row

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8086 Architecture

BASE INDEX WITH DISPLACEMENT ADDRESSING

¢ This addressing mode, a variation on base index, combines a base register, an index register, and a displacement to form an effective address.

MOV AX, [BX+DI+10]MOV CL, DATA_TBL[BX+DI]

BASE INDEX WITH DISPLACEMENT ADDRESSING

This addressing mode, a variation on base index, combines a base register, an index register, and a displacement to form an effective address.

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8086 Architecture

INSTRUCTIONS

� Data Transfer Instructions� Arithmetic Instructions� Bit Manipulation Instructions� String Instructions� Program Execution Transfer Instructions� Processor Control Instructions

Program Execution Transfer Instructions

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8086 Architecture

DATA TRANSFER INSTRUCTIONS¢ General purpose byte or word transfer instructions

¢ Simple input and output port transfer instructions

MOV copy byte or word from specified source to specified destination

PUSH Copy specified word to top of stack

POP Copy word from top of stack to specified location

XCHG Exchange bytes or exchange words

XLAT Translate a byte in AL using a table in memory

IN Copy a byte or word from specified port to accumulator

OUT Copy a byte or word from accumulator to specified port

DATA TRANSFER INSTRUCTIONSGeneral purpose byte or word transfer instructions:

Simple input and output port transfer instructions:

copy byte or word from specified source to specified destination

Copy specified word to top of stack

Copy word from top of stack to specified location

Exchange bytes or exchange words

Translate a byte in AL using a table in memory

Copy a byte or word from specified port to accumulator

Copy a byte or word from accumulator to specified port 107

8086 Architecture

¢ Special address transfer instructions:

¢ Flag transfer instructions

LEA Load effective address of operand into specified register

LDS Load DS register and other specified register from memory

LES Load ES register and other specified register from memory

LAHF Load (copy to ) AH with the low byte of the flag register

SAHF Store (copy) AH register to low byte of flag register

PUSHF Copy flag register to top of stack

POPF Copy word at top of stack to flag register

Special address transfer instructions:

Load effective address of operand into specified register

Load DS register and other specified register from memory

Load ES register and other specified register from memory

Load (copy to ) AH with the low byte of the flag register

Store (copy) AH register to low byte of flag register

Copy flag register to top of stack

Copy word at top of stack to flag register 108

8086 Architecture

ARITHMETIC INSTRUCTIONS

¢ Addition instructions:

¢ Multiplication instructions

ADD Add specified byte to byte or specified word to wordADC Add byte + byte + carry flag or word + word + carry flag

INC Increment specified byte or specified word by 1

AAA ASCII adjust after additionDAA Decimal (BCD) adjust after addition

MUL Multiply unsigned byte by byte or unsigned word by wordIMUL Multiply signed byte by byte or signed word by word

AAM ASCII adjust after multiplication

ARITHMETIC INSTRUCTIONS

Add specified byte to byte or specified word to wordAdd byte + byte + carry flag or word + word + carry flag

Increment specified byte or specified word by 1

ASCII adjust after additionDecimal (BCD) adjust after addition

Multiply unsigned byte by byte or unsigned word by wordMultiply signed byte by byte or signed word by word

ASCII adjust after multiplication 109

8086 Architecture

¢ Subtraction instructions:

¢ Division instructions:

SUB Subtract byte from byte or word for wordSBB Subtract byte and carry flag from byte or word ad carry

flag from word.

DEC Decrement specified byte or specified word by 1

NEG Negate – invert each bit of a specified byte or word and add 1. (form 2’s complement)

CMP Compare two specified bytes or two specified words

AAS ASCII adjust after subtractionDAS Decimal (BCD) adjust after subtraction

DIV Divide unsigned word by byte or unsigned DW by word

IDIV Divide signed word by byte or signed DW by word

AAD ASCII adjust before divisionCBW Fill upper byte of word with copies of sign bit of lower byte

CWD Fill upper word of DW with sign bit of lower word

Subtract byte from byte or word for wordSubtract byte and carry flag from byte or word ad carry

Decrement specified byte or specified word by 1

invert each bit of a specified byte or word and add 1. (form 2’s complement)Compare two specified bytes or two specified words

ASCII adjust after subtractionDecimal (BCD) adjust after subtraction

Divide unsigned word by byte or unsigned DW by word

Divide signed word by byte or signed DW by word

ASCII adjust before divisionFill upper byte of word with copies of sign bit of lower byte

Fill upper word of DW with sign bit of lower word

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8086 Architecture

BIT MANIPULATION INSTRUCTION

¢ Logical instructions:

NOT Invert each bit of a byte or word

AND AND each bit in a byte or word with the corresponding bit in another byte or word

OR OR each bit in a byte or word with the corresponding bit in another byte or word

XOR Exclusive OR each bit in a byte or word with the corresponding bit in another byte or word

TEST AND operands to update flags, but don’t change operands

BIT MANIPULATION INSTRUCTION

Invert each bit of a byte or word

AND each bit in a byte or word with the corresponding bit in another byte or word

OR each bit in a byte or word with the corresponding bit in another byte or wordExclusive OR each bit in a byte or word with the corresponding bit in another byte or word

AND operands to update flags, but don’t change operands

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8086 Architecture

¢ Shift instruction

¢ Rotate instructions:

SHL/SAL Shift bits of byte or word left, put zero(s) in LSB(s).

SHR Shift bits of byte or word right, put zero(s) in MSB(s).

SAR Shift bits of word or byte right, copy old MSB into new MSB

ROL Rotate bits of byte or word left, MSB to LSB and to CFROR Rotate bits of byte or word right, LSB to MSB and to CF

RCL Rotate bits of byte or word left, MSB to CF and CF to LSB

RCR Rotate bits of byte or word right, LSB to CF and CF to MSB

Shift bits of byte or word left, put zero(s) in LSB(s).

Shift bits of byte or word right, put zero(s) in MSB(s).

Shift bits of word or byte right, copy old MSB into new MSB

Rotate bits of byte or word left, MSB to LSB and to CFRotate bits of byte or word right, LSB to MSB and to CF

Rotate bits of byte or word left, MSB to CF and CF to LSB

Rotate bits of byte or word right, LSB to CF and CF to MSB

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8086 Architecture

STRING INSTRUCTIONS

REP An instruction prefix Repeat following instruction until CX=0

REPE/REPZ Repeat while equal/zero

REPNE/REPNZ Repeat while not equal/zero

MOVX/MOVSB/MOVSW Move byte or word from one string to another

COMPS/COMPSB/COMPSW Compare two string bytes or two string words

SCAS/SCASB/SCASW Scan a string. Compare a string byte with a byte in AL or a string word with a word in AX

LODS/LODSB/LODSW Load string byte into AL or string word into AX

STOS/STOSB/STOSW Store byte from AL or word from AX into string

STRING INSTRUCTIONS

An instruction prefix Repeat following instruction until CX=0Repeat while equal/zero

Repeat while not equal/zero

Move byte or word from one string to another

Compare two string bytes or two string words

Scan a string. Compare a string byte with a byte in AL or a string word with a word in AX

Load string byte into AL or string word into AX

Store byte from AL or word from AX into string113

8086 Architecture

PROGRAM EXECUTION TRANSFER INSTRUCTIONS

¢ Unconditional transfer instructions:

¢ Conditional transfer instructions:

CALL Call a procedure (subprogram), save return address on stackRET Return from procedure to calling program

JMP Go to specified address to get next instruction

JA/JNBE Jump if above/ jump if not below or equalJAE/JNB Jump if above or equal/ jump if not below

JB/JNAE Jump if below/ jump if not above or equal

JBE/JNA Jump if below or equal/ jump if not aboveJC Jump if carry =1

JE/JZ Jump if equal/ jump if zeroJG/JNLE Jump if greater/ jump if not less than or equal

PROGRAM EXECUTION TRANSFER

Call a procedure (subprogram), save return address on stackReturn from procedure to calling program

Go to specified address to get next instruction

Jump if above/ jump if not below or equalJump if above or equal/ jump if not below

Jump if below/ jump if not above or equal

Jump if below or equal/ jump if not above

Jump if equal/ jump if zeroJump if greater/ jump if not less than or equal

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8086 Architecture

JGE/JNL Jump if greater than or equal/ jump if not less than

JL/JNGE Jump if less than/ jump if not greater than or equal

JLE/JNG Jump if less than or equal/ jump if not greater than

JNC Jump if no carry

JNE/JNZ Jump if not equal/ jump if not zero

JNO Jump if no overflow

JNP/JPO Jump if not parity/ jump if parity odd (PF = 0)

JNS Jump if not sign

JO Jump if overflow

JP/JPE Jump if parity/ jump if parity even

JS Jump if sign

Jump if greater than or equal/ jump if not less than

Jump if less than/ jump if not greater than or equal

Jump if less than or equal/ jump if not greater than

Jump if not equal/ jump if not zero

Jump if not parity/ jump if parity odd (PF = 0)

Jump if parity/ jump if parity even115

8086 Architecture

¢ Iteration control instructions:

¢ Interrupt instructions:

LOOP Loop through a sequence of instructions until CX=0

LOOPE/LOOPZ Loop through a sequence of instruction while ZF=1 and CX !=0

LOOPNE/LOOPNZ Loop through a sequence of instruction while ZF=0 and CX!=0.

JCXZ Jump to specified address if CX=0.

INT Interrupt program execution, call service procedure

INTO Interrupt program execution if OF=1

IRET Return from interrupt service procedure to main program

Loop through a sequence of instructions

Loop through a sequence of instruction while ZF=1 and CX !=0

Loop through a sequence of instruction while ZF=0 and CX!=0.

Jump to specified address if CX=0.

Interrupt program execution, call service procedure

Interrupt program execution if OF=1

Return from interrupt service procedure to main program116

8086 Architecture

PROCESSOR CONTROL INSTRUCTIONS

¢ Flag set/clear instruction:

STC Set carry flagCLC Clear carry flag

CMC Complement the status of carry flag

STD Set direction flagCLD Clear direction flag

STI Set interrupt enable flag (enable INTR)CLI Clear interrupt enable flag (disable INTR)

PROCESSOR CONTROL INSTRUCTIONS

Complement the status of carry flag

Set interrupt enable flag (enable INTR)Clear interrupt enable flag (disable INTR)

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8086 Architecture

¢ External hardware synchronization instructions:

¢ No operation instructions:NOP No action except fetch and decode

HLT Halt until interrupt or resetWAIT Wait until signal on the TEST pin is low

LOCK An instruction prefix. Prevents another processor from taking the bus while the adjacent instruction executes

External hardware synchronization instructions:

No action except fetch and decode

Halt until interrupt or resetWait until signal on the TEST pin is low

An instruction prefix. Prevents another processor from taking the bus while the adjacent instruction executes

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8086 Architecture

THE MOV INSTRUCTION

¢ MOV transfer (copy) data referenced by the address of the second operand to the address of the first operand. The sending field is unchanged.

¢ The operands that reference memory or registers must agree in size.

MOV destination, source

DestinationMemory

Accumulator

RegisterRegister

MemoryRegister

Memory

SegSeg

Reg_16Memory_16

THE MOV INSTRUCTION

Destination SourceMemory accumulator

Accumulator Memory

Register RegisterRegister Memory

Memory RegisterRegister Immediate

Memory Immediate

Seg-reg Reg_16Seg-reg Memory_16

Reg_16 Seg_-regMemory_16 Seg-reg

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8086 Architecture

¢ MOV SP, BX � Copy a word from the BX register to the SP register

¢ MOV CL, [BX]� Copy a byte to CL from the memory location whose effective

address is contained in BX. The effective address will be added to the data segment base in DS to produce the physical address.

¢ MOV 43H[SI], DH� Copy a byte from the DH register to a memory location. The BIU

will compute the effective address of the memory location by adding the indicated displacement of 43H to the contents of the SI register. The BIU then produces the actual physical address by adding this effective address to the data segment base represented by the 16 bit number in the DS register.

Copy a word from the BX register to the SP register

Copy a byte to CL from the memory location whose effective address is contained in BX. The effective address will be added to the data segment base in DS to produce the physical address.

Copy a byte from the DH register to a memory location. The BIU will compute the effective address of the memory location by adding the indicated displacement of 43H to the contents of the SI register. The BIU then produces the actual physical address by adding this effective address to the data segment base represented by the 16 bit

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8086 Architecture

¢ MOV CX, [434AH]� Copy the contents of two memory locations into the CX register.

The direct address or displacement of the first memory location from the start of the data segment is 437AH. The BIU will produce the physical memory address by adding this displacement to the data segment base represented by the 16 bit number in the DS register.

¢ MOV CS:[BX], DL� Copy a byte from the DL register to a memory location. The

effective address for the memory location is contained in the BX register. Normally an effective address in BX will be added to the data segment base in DS to produce the physical memory address. In this instruction, the CS: in front of [BX] indicates that we want the BIU to add the effective address to the code segment base in CS to produce the physical address. This CS: is called a override prefix.

Copy the contents of two memory locations into the CX register. The direct address or displacement of the first memory location from the start of the data segment is 437AH. The BIU will produce the physical memory address by adding this displacement to the data segment base represented by the 16 bit number in the DS

Copy a byte from the DL register to a memory location. The effective address for the memory location is contained in the BX register. Normally an effective address in BX will be added to the data segment base in DS to produce the physical memory address. In this instruction, the CS: in front of [BX] indicates that we want the BIU to add the effective address to the code segment base in CS to produce the physical address. This CS: is called a segment 121

8086 Architecture

¢ MOV AX, 0010H� Load the immediate word 0010H into the AX register.

¢ MOV [0000], AL� Copy the contents of the AL register to a memory location.

The direct address or displacement of the memory location from the start of the data segment is 0000H.

Load the immediate word 0010H into the AX register.

Copy the contents of the AL register to a memory location. The direct address or displacement of the memory location from the start of the data segment is 0000H.

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8086 Architecture

THE LEA, LDS, LES INSTRUCTION

¢ useful for initializing a register with an offset address.¢ A common use for LEA is to initialize an offset in BX,

DI, or SI for indexing an address in memory.

DATATBL DB 25 DUP (?)BYTEFLD DB ?

…………LEA BX, DATATBLMOV BYTEFLD, [BX]

THE LEA, LDS, LES INSTRUCTION

useful for initializing a register with an offset address.A common use for LEA is to initialize an offset in BX, DI, or SI for indexing an address in memory.

25 DUP (?)

LEA BX, DATATBLMOV BYTEFLD, [BX]

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8086 Architecture

¢ For the figure below, what is the result of executing the following instruction?

LEA SI, [DI + BX + 2H]

DS 0100 DSSI F002 SIDI 0020 DIAX 0003 AXBX 0040 BX

before after

For the figure below, what is the result of executing the

LEA SI, [DI + BX + 2H]

01000042002000030040

after124

8086 Architecture

¢ For these three instructions (LEA, LDS, LES) the effective address could be formed of all or any various combinations of the three elements:

¢ What is the result of executing the following instruction?LDS SI, [DI + BX + 2H]

For these three instructions (LEA, LDS, LES) the effective address could be formed of all or any various combinations

What is the result of executing the following instruction?

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8086 Architecture

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8086 Architecture

THE ADD INTRUCTION

¢ Add a number from some source to a number from some destination and put the result in the specified destination.

¢ The source may be an immediate number, a register, or a memory location.

¢ The destination may be a register, or a memory location.

¢ The source and the destination in an instruction cannot both be memory locations.

¢ The source and the destination must be of the same type.

¢ Flags affected: AF, CF, OF, PF, SF, ZF.

Add a number from some source to a number from some destination and put the result in the specified

The source may be an immediate number, a register, or

The destination may be a register, or a memory

The source and the destination in an instruction cannot

The source and the destination must be of the same

Flags affected: AF, CF, OF, PF, SF, ZF.127

8086 Architecture

¢ EXAMPLES:

� ADD AL, 74H� ADC CL, BL� ADD DX, BX� ADD DX, [SI]� ADD PRICES[BX], AL� ADC AL, PRICES[BX]� ADD AX, [SI + DI + 2H]

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8086 Architecture

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8086 Architecture

THE SUB INSTRUCTION

¢ Subtract the number in the indicated source from the number in the indicated destination and put the result in the indicated destination.

¢ For subtraction, the carry flag (CF) functions as a borrow flags.

¢ The carry flag will be set after a subtraction if the number in the specified source is larger than the number in the specified destination.

¢ Source/destination à same as addition.¢ Flags affected: AF, CF, OF, PF, SF and ZF

THE SUB INSTRUCTION

Subtract the number in the indicated source from the number in the indicated destination and put the result

For subtraction, the carry flag (CF) functions as a

The carry flag will be set after a subtraction if the number in the specified source is larger than the number in the specified destination.

same as addition.Flags affected: AF, CF, OF, PF, SF and ZF

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8086 Architecture

¢ EXAMPLES:� SUB CX, BX� SBB CH, AL� SUB AX, 3481H� SBB BX, [3427H]� SUB PRICES[BX], 04H ; subtract 04 from byte at effective

address PRICE[BX] if PRICES declared with DB. Subtract 04 from word at effective address PRICES[BX} if PRICES declared with DW

� SBB CX, TABLE[BX]� SBB TABLE[BX], CX

; subtract 04 from byte at effective address PRICE[BX] if PRICES declared with DB. Subtract 04 from word at effective address PRICES[BX} if PRICES declared with DW

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8086 Architecture

THE MUL INSTRUCTION

¢ Multiplies an unsigned byte from some source times an unsigned byte in the AL register or an unsigned word from some source times an unsigned word in the AX register.

¢ The source can be a register or a memory location.¢ When a byte is multiplied by the content of AL, the

result is put in AX.¢ A 16 bit destination is required because the result of

multiplying an 8 bit number by an 8 bit number can be as large as 16 bit.

¢ The most significant byte of the result is put in AH, and the least significant byte of the result is put in AL.

THE MUL INSTRUCTION

Multiplies an unsigned byte from some source times an unsigned byte in the AL register or an unsigned word from some source times an unsigned word in the AX

The source can be a register or a memory location.When a byte is multiplied by the content of AL, the

A 16 bit destination is required because the result of multiplying an 8 bit number by an 8 bit number can be

The most significant byte of the result is put in AH, and the least significant byte of the result is put in AL.

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8086 Architecture

¢ When a word is multiplied by the contents of AX, the product can be as large as 32 bits.

¢ The most significant word of the result is put in DX register, and the least significant word of the result is put in the AX register.

¢ EXAMPLES:� MUL BH� MUL CX� MUL CONVERSION[BX]

When a word is multiplied by the contents of AX, the product can be as large as 32 bits.The most significant word of the result is put in DX register, and the least significant word of the result is

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8086 Architecture

THE IMUL INSTRUCTION

¢ Multiplies a signed byte from some source times a signed byte in AL or a signed word from some source times a signed word in AX.

¢ The source can be a register or a memory location.¢ When a byte is multiplied by the content of AL, the

result is put in AX.¢ A 16 bit destination is required because the result of

multiplying an 8 bit number by an 8 bit number can be as large as 16 bit.

¢ The most significant byte of the result is put in AH, and the least significant byte of the result is put in AL.

THE IMUL INSTRUCTION

Multiplies a signed byte from some source times a signed byte in AL or a signed word from some source

The source can be a register or a memory location.When a byte is multiplied by the content of AL, the

A 16 bit destination is required because the result of multiplying an 8 bit number by an 8 bit number can be

The most significant byte of the result is put in AH, and the least significant byte of the result is put in AL.

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8086 Architecture

¢ When a word is multiplied by the contents of AX, the product can be as large as 32 bits.

¢ The most significant word of the result is put in DX register, and the least significant word of the result is put in the AX register.

¢ If the magnitude of the product does not require all the bits of the destination, the unused bits will be filled with copies of the sign bit.

¢ EXAMPLES:� IMUL BH� IMUL AX

When a word is multiplied by the contents of AX, the product can be as large as 32 bits.The most significant word of the result is put in DX register, and the least significant word of the result is

If the magnitude of the product does not require all the bits of the destination, the unused bits will be filled

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8086 Architecture

Assembly language by Puskar Suwal Complete Reference

Engineering

Transcript of Assembly language by Puskar Suwal Complete Reference