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CSW 353 (Assembly Language)

Computer Architecture

Dr. Salma Hamdy s.hamdy@cis.asu.edu.eg

Computer System Architecture

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Chapter 1: Digital Logic Circuits Chapter 2: Digital Components Chapter 3: Data Representation Chapter 4: Register Transfer and Microoperations

Chapter 5: Basic Computer Organization Chapter 6: Programming the Basic Computer Chapter 7: Microporgammed Control

Chapter 8: CPU

Chapter 11: I/O Organization

Chapter 12: Memory Organization

Basic Computer Organization and

Design - I 1. Instruction Codes 2. Computer Registers 3. Computer Instructions 4. Timing and Control

Ch5. Introduces the basic computer and shows its operation can be specified with register transfer statements.

1. Instruction Codes

• The general-purpose computer is capable of executing various microoperations, and can be instructed as to what specific sequence of operations it must perform.

• The user of a computer can control the process by means of a program (set of instructions that specify the operations, operands, and the sequence by which processing has to occur.

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1. Instruction Codes – (cont.)

• Instructions and data are stored in memory.

• Every computer has its own unique instruction set.

• The most important property of a general-purpose computer is the ability to store and execute instructions (stored program concept).

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Control register Memory Read instruction

• Interpret binary code of instruction. • Execute it by issuing a sequence of

microoperations.

1. Instruction Codes – (cont.)

• Instruction: group of bits that instruct the computer to perform a specific operation.

• Its most basic part is the operation part, which is the group of bits that define such operations as add, subtract, shift, … etc.

• Size of operation part depends on the total number of operations available in the computer (𝟐𝒏 distinct operations require an operation part of 𝒏 bits).

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…………………… operation part

1. Instruction Codes – (cont.)

• Executing an instruction by control unit:

– Receives instruction from memory.

– Interprets its operation code (opcode) bits.

– Issues a sequence of control signals to initiate microoperations in internal computer registers.

• Operations vs. microoperation.

• Operation code = macrooperation.

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1. Instruction Codes – (cont.)

• e.g. A computer with 64 distinct operations has an instruction set with a 6-bit operation code. One of them can be ADD operation with code 110010. When the control interprets this code, it issues control signals to read operand from memory and add it to a processor register.

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1. Instruction Codes – (cont.)

• Operation must be performed on some data stored in memory.

• An instruction code therefore must include not only the operation, but also the registers or memory words where the operands are stored, as well as the register or memory word where the result is to be stored.

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instruction

information about data

opcode Specify: - Memory word by address. - Processor register by 𝒌 bits to select one

among 𝟐𝒌.

1. Instruction Codes – (cont.)

• Instruction code formats are invented by the designer who specify the architecture of the computer.

• In our basic computer design, we will adopt the following stored program organization:

– One processor register (accumulator 𝑨𝑪).

– Two-part instruction code.

– Address is where to find operand.

– Operation is performed on operand and content of processor register. 10 10/30/2013

1. Instruction Codes – (cont.)

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1. Instruction Codes – (cont.)

Effective Address

• Address of operand in a computational instruction.

• Target address in a branch-type instruction.

Addressing modes:

• When the second part of instruction code specifies

– An operand immediate.

– Address of operand direct.

– Address of a memory word that stores the address of operand indirect.

• One bit 𝑰 in the instruction code is used to distinguish between direct and indirect.

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1. Instruction Codes – (cont.)

• Indirect access mode bit

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Direct Indirect

𝑰 0 1

Memory references

1 2

Effective address

457 1350

2. Computer Registers

• Instructions are normally stored in consecutive memory locations and executed sequentially one at a time starting from a specific address.

Counter to calculate address of next instruction.

Register for holding memory addresses.

Register to store instruction read from memory.

Register to manipulate data.

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2. Computer Registers – (cont.)

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• Basic computer registers

and memory and their

functions.

branching

2. Computer Registers – (cont.)

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• Basic computer registers

and memory and their

functions.

branching

How to connect all of them?

2. Computer Registers – (cont.)

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Common Bus System • Bus size.

• Selecting output on bus.

• Receiving data from bus.

• Memory read/write.

• Varying register size.

• Controls.

2. Computer Registers – (cont.)

Registers Design

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2. Computer Registers – (cont.)

Registers Design

• Binary counter with parallel load and synchronous clear.

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2. Computer Registers – (cont.)

Registers Design

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2. Computer Registers – (cont.)

Registers Design

• Register with parallel load.

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2. Computer Registers – (cont.)

Memory Design

• Address is specified by 𝑨𝑹, eliminating the need for address bus.

• Content of any register can be specified from memory for a write operation.

• Any register can receive data from memory after a read operation except 𝑨𝑪. 22 10/30/2013

2. Computer Registers – (cont.)

AC

• Its 16 inputs come from adder logic circuit.

• This circuit has three sets of inputs.

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2. Computer Registers – (cont.)

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Clock

• Single pulse to 𝑫𝑹 ← 𝑨𝑪, 𝑨𝑪 ← 𝑫𝑹

3. Computer Instructions

Basic Computer Instruction Format

• Memory-reference instruction.

• Register-reference instruction.

• Input-output instruction.

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3. Computer Instructions – (cont.)

Basic Computer Instruction Format

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3. Computer Instructions – (cont.)

• Ho many instructions can this computer be designed to perform?

• 3-bit opcode only 7 memory instructions?

• 12 bits in register and input-output more!

e.g.

• AND memory word in address 000000001100 to 𝑨𝑪. (AND opcode: 000)

• Instruction: 0 000 000000001100

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3. Computer Instructions – (cont.)

• Ho many instructions can this computer be designed to perform?

• 3-bit opcode only 7 memory instructions?

• 12 bits in register and input-output more!

e.g.

• AND memory word in address 000000001100 to 𝑨𝑪. (AND opcode: 000)

• Instruction: 0 000 000000001100

• Hexadecimal: 0 0 0 C 28 10/30/2013

3. Computer Instructions – (cont.)

• ISZ opcode: 110

• Instruction: 0 110 0000 0000 0101

• Hexadecimal: 6 0 0 5

• Instruction: 1 110 0000 0000 1100

• Hexadecimal: E 0 0 C

Memory Reference Instructions

• 𝑰=0 first hex digit ranges from 0 to 6.

• 𝑰=1 first hex digit ranges from 8 to E. 29 10/30/2013

3. Computer Instructions – (cont.)

Register Reference Instructions

• Instruction: 0 111 0000 0000 0101

• Hexadecimal: 7 0 0 5

• Instruction: 0 111 0000 0000 1100

• Hexadecimal: 7 0 0 C

First hex digit is always 7. 30 10/30/2013

3. Computer Instructions – (cont.)

Input-Output Instructions

• Instruction: 1 111 0000 0000 0101

• Hexadecimal: F 0 0 5

• Instruction: 1 111 0000 0000 1100

• Hexadecimal: F 0 0 C

First hex digit is always F. 31 10/30/2013

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3. Computer Instructions – (cont.)

Instruction Set

• Completeness – Arithmetic, logic, shift, moving data, control, input

output.

• Efficiency. – e.g. multiplication, XOR must be programmed in the

basic computer (Ch6).

• Commercial computers – Ch8.

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4. Timing and Control

• Timing of all registers and flip-flops is controlled by a master clock generator.

• But the clock doesn’t change the state of a register unless the register is enabled by a control signal generated in a control unit.

• Hardwired control vs. microprogrammed control (ch7).

• For now, we will design a hardwired control unit.

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4. Timing and Control – (cont.)

• The control unit that supervises the operations in a digital system would normally consists of timing signals that determine the time sequence in which the operations are executed must know operation.

• The timing sequence in the control unit can be easily generated by means of counters or shift registers.

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4. Timing and Control – (cont.)

Simple Examples

1. Word-time Generation for Serial Operation

• The control unit in a serial operation must generate a word-time that stays on for a number of pulses equal to the number of bits in the shift register.

• This word-time signal can be generated by means of a counter that counts the required number of pulses.

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4. Timing and Control – (cont.)

1. Word-time Generation for Serial Operation

• If the word-time signal must stay on for eight clock pulses

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To shift register

Notice that the flip-flops are negative-edge-triggered

4. Timing and Control – (cont.)

2. Timing-Signals Generation for Parallel Mode

• The control units in systems the operate in parallel mode must generate timing signals that stay on for only one clock pulse, but these timing signals must be distinguished from each other.

• These signals can be generated by a circular shift register, or a counter with a decoder.

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4. Timing and Control – (cont.)

2. Timing-Signals Generation for Parallel Mode

• Ring Counter: circular shift register with one flip-flop set to one at a time.

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initial value 1000

Notice that the flip-flops are negative-edge-triggered

4. Timing and Control – (cont.)

2. Timing-Signals Generation for Parallel Mode

• Counter that goes through distinct states, with decoder to decode the states.

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4. Timing and Control – (cont.)

2. Timing-Signals Generation for Parallel Mode

• The timing signals when enabled by the clock pulse, can be used to generate multiple-phase clock pulses.

• ANDing 𝑻𝒐with 𝑪𝑷 produces a clock pulse at 1/4th the frequency of the master-clock pulses.

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4. Timing and Control – (cont.)

Basic Computer Control Unit

• Two decoders

• Sequence counter

• Control logic gates

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4. Timing and Control – (cont.)

Basic Computer Control Unit

• Counter can be incremented or cleared.

Example

Suppose we want the timing sequence 𝑻𝒐, 𝑻𝟏, 𝑻𝟐, 𝑻𝟑, then at 𝑻𝟒, 𝑺𝑪 is cleared if 𝑫𝟑 is active. In register-transfer logic statement:

𝑫𝟑𝑻𝟒: 𝑺𝑪 ← 𝟎

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4. Timing and Control – (cont.)

Basic Computer Control Unit

• Initially, 𝑺𝑪 is cleared triggering only registers whose control inputs are connected to 𝑻𝒐.

• 𝑺𝑪 increments with every cycle resulting the sequence:

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4. Timing and Control – (cont.)

Basic Computer Control Unit

• 𝑫𝟑𝑻𝟒: 𝑺𝑪 ← 𝟎 How?

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4. Timing and Control – (cont.)

Basic Computer Control Unit

• A memory read/write cycle will be initiated with the rising edge of a timing signal.

• It will be assumed that the memory cycle time is less than the clock cycle time (so that read/write will finish before the next rising edge, and that edge will be used to load memory word

into a register).

• Most commercial computer provide “wait cycles” in the processor. 46 10/30/2013

5. Instruction Cycle

• A program (sequence of instructions) residing in memory is executed by going through a cycle for each instruction, which in turn, is divided into a sequence of subcycles or phases.

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5. Instruction Cycle – (cont.)

• In the basic computer, each instruction goes through these phases:

1. Fetch instruction from memory.

2. Decode the instruction.

3. Read the effective address if needed.

4. Execute the instruction.

• Upon completing step 4, control goes back to step 1 and so on until a HALT instruction is encountered.

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5. Instruction Cycle – (cont.)

Fetch and Decode

• What happens?

• The microoperations can be specified by the following register-transfer statements:

• How are these implemented? 49 10/30/2013

Fetch and Decode

5. Instruction Cycle – (cont.)

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Remember, SC increments after each clock pulse. To produce the sequence 𝑻𝟎, 𝑻𝟏, 𝑻𝟐,…

5. Instruction Cycle – (cont.)

Determining the Type of Instruction

• During 𝑻𝟑.

• Control unit examines opcode and Decoder output 𝑫𝟕 distinguished memory-reference instructions from the other two types.

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5. Instruction Cycle – (cont.)

Instruction Cycle

(Initial configuration)

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5. Instruction Cycle – (cont.)

Instruction Cycle

(Initial configuration)

• ?

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5. Instruction Cycle – (cont.)

Instruction Cycle

(Initial configuration)

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5. Instruction Cycle – (cont.)

Register-Reference Instructions

• 𝑫𝟕 = 𝟏 and 𝑰 = 𝟎, at clock transition 𝑻𝟑

• 𝑰𝑹(0-11) specify one of 12 instructions.

• Let the bits of 𝑰𝑹 be 𝑩𝟏𝟏𝑩𝟏𝟎…. 𝑩𝟎.

• Let the Boolean relation 𝑫𝟕𝑰′𝑻𝟑 = 𝐫.

• Since the control function is distinguished by one of the bits in 𝑰𝑹(0-11), then all control functions can be simply denoted by 𝒓𝑩𝒊.

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5. Instruction Cycle – (cont.)

Register-Reference Instructions

• Example: CLA

Hex. 7 8 0 0

Binary 0 111 1000 000 000

Control function that initiates the microoperations for this instruction is: 𝑫𝟕𝑰′𝑻𝟑𝑩𝟏𝟏 = 𝒓𝑩𝟏𝟏

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𝑰’ 𝑫𝟕 𝑩𝟏𝟏

5. Instruction Cycle – (cont.)

Register-Reference Instructions

• ??

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5. Instruction Cycle – (cont.)

Register-Reference Instructions

• 𝑺𝑪 is cleared after the execution of each instruction initiating 𝑻𝟎 (fetch) that causes a new cycle.

• After a HALT, the start flip-flip must be set manually.

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Selected Problems

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• To be selected!

Next Lecture

Continuation of Basic Computer Organization and Design.

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Assignment

- Reading: Chapter 5: sections 1-5.

References

- Digital Design, 4th ed, M. Morris Mano, Prentice Hall, 2006.

-http://microcom.kut.ac.kr/ ch05

- God bless Google and Wiki!

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