Assembler Language Tutorial for Mainframe Programmers
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Transcript of Assembler Language Tutorial for Mainframe Programmers
Assembler/Session 1
Assembler Language Assembler Language for for Mainframe Mainframe ProgrammersProgrammers
Objectives
• Familiarize with IBM 370 Assembly Language
ObjectivesObjectives
Assembler/Session 1
SESSION 1SESSION 1Day 1Day 1
Introduction
SESSION 2SESSION 2Day 1Day 1
Addressing
SESSION 3SESSION 3Day 2Day 2 Machine Instructions
Assembler/Session 1
COURSE SCHEDULE COURSE SCHEDULE
Writing a complete program
SESSION 4SESSION 4Day 3Day 3
Program Sectioning
SESSION 5SESSION 5Day 3Day 3
Assembler Directives
SESSION 6SESSION 6Day 3Day 3
Assemble and link program SESSION 7SESSION 7Day 4Day 4
COURSE SCHEDULEAssembler/Session 1
Macro Language SESSION 8SESSION 8Day 4 Day 4
Other Topics SESSION 9SESSION 9Day 5Day 5
Assembler/Session 1
COURSE SCHEDULE
Assembler LanguageAssembler Language
SESSION 1
Assembler/Session 1
Objectives• An assembler language is a symbolic form of
machine language • Assembler translates assembler language
program to machine language • An assembler program consists of many
statements• In general, one assembler language statement
corresponds to one machine language instruction
INTRODUCTION
Assembler/Session 1
ObjectivesSTATEMENT FORMAT
1 10 16 30label operation operands comments
e.g..INIT1 LA R5,4 ;INITIALISE REGISTER 5
Rules for choosing labels:Rules for choosing labels:• maximum 8 characters• Alphabets, digits, @, #, $• First character should not be a digit• label should begin in column 1
Assembler/Session 1
Objectives Col1 Col10 Col.16
L 2,A A 2,B ST 2,ANS ….. ….. A DC F’15’ B DC F’20’ ANS DS F
Sample program
Assembler/Session 1
ObjectivesSTATEMENT FORMAT
Operation• One of the 200 M/C instruction mnemonics (eg. MVC)
Operand• can be a register or memory location
Continuing a statement• Place any character in column 72 of the line to be continued• Continue the statement from column 16 of next line• Maximum 2 continuation lines for a statement
Assembler/Session 1
ObjectivesSTATEMENT FORMAT
Comment Statement• * in column 1• Any text in columns 2 - 71
Note : Fields separated by one or more blanks
Assembler/Session 1
ObjectivesTYPES OF INSTRUCTIONS
1. Machine Instructions
2. Assembler Instructions (Directives)
3. Macro Instructions
Assembler/Session 1
ObjectivesREGISTERS
Registers are storage areas inside the processor
Advantages:- No need to retrieve data from main storage
(saves time)- Shared resource that allows inter
communication between programs
Assembler/Session 1
ObjectivesREGISTERS
General purpose registers:* 16 registers available* Numbered 0 - 15* Holds 32 bits (4 bytes) of data (1 Full word)
Floating point registers:* 4 registers available* Numbered 0,2,4,6* Holds 64 bits (8 bytes) of data
Note : The registers 0, 1, 13, 14 and 15 are reserved for special purpose By IBM convention these registers are used for calling
subprograms
Assembler/Session 1
ObjectivesDATA REPRESENTATION
Binary fields- Always fixed in length, either 2 or 4 bytes (Full word or Half word) - Negative numbers stored in 2’s complement form
Examples: A DC H’295’ 01 27
B DC H’-75’ FF 35
Assembler/Session 1
Objectives2’s complement form
Assembler/Session 1
How to identify a negative number?How to identify a negative number?
- - Leading bit contains a 1 (In Hex 8 to F)Leading bit contains a 1 (In Hex 8 to F)
How to convert to a negative numberHow to convert to a negative number??
-First switch the bits (1 to 0 , 0 to 1)First switch the bits (1 to 0 , 0 to 1)
-Finally add 1Finally add 1
ObjectivesBoundary requirements
Assembler/Session 1
Full word – Should begin in a full word boundary
(Achieved by aligning with 0F)
Half word – Should begin in a half word boundary (Achieved by aligning with 0H)
How to find:
The starting address of Full word should end with 0, 4, 8 or C and Half words should end with 0, 2, 4, 6, 8, A, C or E
ObjectivesDATA REPRESENTATION
Characters- One byte (EBCDIC form) - Character representation of decimal digits is called
Zoned Decimal (first nibble is zone and next is digit)
Zone digit Zone Code 0 - 9 + C, A,E,F
- D, B +, - , blank Blank F
Assembler/Session 1
ObjectivesDATA REPRESENTATION
Floating Point Numbers - Always fixed in length, 4, 8 or 16 bytes
(Full word, double word, double double word) - Left most bit represents sign
(0 - positive; 1 - negative)- Next 7 bits represent exponent- Remaining bytes represent the fraction
Assembler/Session 1
ObjectivesDATA REPRESENTATION
Decimal numbers ( Packed Decimal representation)- Each byte but the rightmost has 2 decimal digits (0-9)- The right most byte contains a digit in the left half and a sign indicator in the right
Sign indicator: C- Positive D - Negative
Example: 753 - 7 5 3 C
Assembler/Session 1
ObjectivesAddressing Operands
• Register addressing• Base, displacement addressing• Base, index and displacement addressing
Assembler/Session 1
ObjectivesINSTRUCTION FORMATSRR opcode R1 R2
SI opcode I2 B1 D1
SS opcode L B1 D1 B2 D2
SS opcode L1 L2 B1 D1 B2 D2
RX opcode R1 X2 B2 D2
RS opcode R1 R3 B2 D2
Assembler/Session 1
ObjectivesAddressing RX Operands:Implicit format:L 3,VAR
Explicit format:L 3,100(0,12)
Register Displacement Index reg Base reg
Assembler/Session 1
Assembler LanguageAssembler Language
SESSION 2SESSION 2Addressing
Assembler/Session 2
ObjectivesSTORAGE DEFINITIONS
Two ways to define fields :
1. Define a field and initialize the data in it using the DC assembler directive
2. Define a field without initializing using the DS assembler directive
Assembler/Session 2
ObjectivesSTORAGE DEFINITIONS
Format:label {DS/DC} dtLn’value’
where : label : Label used to name the field (optional)d : Duplication factor (optional)t : Type of data ( required)Ln : The letter ‘L’ followed by the length of the field in bytes (optional)value : Represents the value enclosed in apostrophes
Assembler/Session 2
ObjectivesSTORAGE DEFINITIONS
Examples:ALPHA DC C’ABC EF’FLDS DS 3CL2H1 DC H’29’F2 DC F’-10’F1 DC X’03’F3 DC PL4’-72’
Note : for character constants truncation or padding is to the right and for almost all others it is to the left.
Assembler/Session 2
ObjectivesSTORAGE DEFINITIONS
DC TYPES Type Implied Alignment Data Representation Length C - None Character X - None Hex digits B - None Binary digits F 4 Full word Binary H 2 Half word Binary E 4 Full word Floating point D 8 Double word Floating point L 16 Double word Floating point
P - None Packed decimal
Assembler/Session 2
ObjectivesSTORAGE DEFINITIONS
Data Representation in other languages: Assembler FORTRAN COBOL PASCAL BASICLanguageDC TypeC Character Display String StringF, H Integer COMP Integer IntegerE Real COMP-1 Real Single precisionD Double COMP-2 Real Double Precision PrecisionX, B Logical N/A Boolean HexP N/A COMP-3 N/A N/A
Assembler/Session 2
ObjectivesSTORAGE DEFINITIONS
Literals• A literal is a constant preceded by an equals sign ‘=‘.• Can be used as a main-storage operand but not as a
destination field of an instruction• Causes assembler to define a field that is initialized with
the data specified• All constants defined by literals are put by the assembler
in a literal pool, usually at the very end of the program (Unless changed by LTORG instruction)
L R4,=F’1’
Assembler/Session 2
ObjectivesExercise 1 Q 1 and Q2.
2.What will happen in the following casesDC CL5’123’DC CL5’123456’DC X’A1245’DC XL2’A1245’DC XL5’A1245’DC F’19’DC FL1’513’
Assembler/Session 2
ObjectivesEQU (Assembler directive)
• The EQU statement is used to associate a fixed value with a symbol
R4 EQU 4DRBACK EQU OUT+25
Assembler/Session 2
ObjectivesESTABLISHING ADDRESSABILITY
• By establishing the addressability of a coding section, you can refer to the symbolic addresses defined in it in the operands of machine instruction
• Assembler will convert the implicit addresses into explicit addresses (base - displacement form)
Assembler/Session 2
ObjectivesESTABLISHING ADDRESSABILITY
To establish the address of a coding section :• Specify a base address from which the
assembler can compute displacements• Assign a base register to contain this base
address• Write the instruction that loads the base
register with the base addressNote: The base address should remain in the base
register throughout the execution of the program
Assembler/Session 2
ObjectivesESTABLISHING ADDRESSABILITY
Establishing Base Register
The USING and DROP assembler instructions enable one to use expressions representing implicit addresses as operands of machine instruction statements, leaving the assignment of base registers and the calculation of displacements to the assembler
USING - Use Base Address Register- allows one to specify a base address and assign
one or more base registers
Assembler/Session 2
ObjectivesESTABLISHING ADDRESSABILITY
To use the USING instruction correctly, one should know :• which locations in a coding section are made addressable
by the USING statement• where in a source module you can use these established
addresses as implicit addresses in instruction operands
Format:symbol USING base address,basereg1| basereg2|,..e.g. USING BASE,9,10,11
USING *,12
Assembler/Session 2
ObjectivesESTABLISHING ADDRESSABILITY
Range of a USING instruction:• The range of a USING instruction is the 4096
bytes beginning at the base address specified in the USING instruction
Domain of a USING instruction• The domain of a USING instruction begins
where the USING instruction appears in a source module to the end of the source module
Assembler/Session 2
ObjectivesESTABLISHING ADDRESSABILITY
The assembler converts implicit address references into their explicit form:
• if the address reference appears in the domain of a USING instruction
• if the addresses referred to lie within the range of the same USING instruction
Guideline:• Specify all USING instructions at the beginning of the
source module• Specify a base address in each USING instruction that lies
at the beginning of each control section
Assembler/Session 2
ObjectivesRELATIVE ADDRESSING
• Relative addressing is the technique of addressing instructions and data areas by designating their location in relation to the location counter or to some symbolic location
ALPHA LR 3,4CR 4,6 ALPHA+2 or
BETA-4BCR 1,14
BETA AR 2,3
Note : Always avoid using relative addressing
Assembler/Session 2
Assembler LanguageAssembler Language
SESSION 3 & 4Machine Instructions
Assembler/Session 3 & 4
ObjectivesHANDLING CHARACTER DATA
Move Character Instruction (MVC)
• Copy data from one place in memory to another
Format : MVC operand1,operand2
S1(L), S2 - implicit
D1(L,B1),D2(B2) - explicit
e.g...
MVC INPUT(5),OUTPUT
Assembler/Session 3 & 4
ObjectivesHANDLING CHARACTER DATA
Move Immediate Instruction (MVI)
• Can move only one byte of constant data to a field
Format : MVI operand1,operand2
S1,I2 - implicit
D1(B1),I2 - explicit
e.g..
MVI CTL,C’B’
Assembler/Session 3 & 4
ObjectivesHANDLING CHARACTER DATA
Advanced Techniques
1. Explicit lengths and relative addressingMVC PAD+6(4),=CL4’ ‘
PAD DS CL10
2. Overlapping fields and the MVC instructionMVC FLDB,FLDA
FLDA DC C’A’
FLDB DS CL3
Limitation of MVC : Can only move 256 bytes
Assembler/Session 3 & 4
ObjectivesHANDLING CHARACTER DATA
Moving more than 256 characters: MVCL instruction Uses 2 pairs of even-odd pair of registers
Format : MVCL R1,R2 (Both are even registers)
Reg R1 – Address of destination R1+1 – Length
Reg R2 - Source R2+1 – Padding character (1st 8 bits) and LengthEg: LA 2,Q LA 3,2000 LA 4,P LA 5,1500 MVCL 2,4
Assembler/Session 3 & 4
ObjectivesHANDLING CHARACTER DATA
Comparison Instructions
• Compares 2 values - the values are found in fields, in registers or in immediate data
CLC - Compare logical character
e.g. CLC FLDA,FLDB
CLI - Compare logical immediate
e.g. CLI FLDA,C’K’
Assembler/Session 3 & 4
ObjectivesExercise 2 Q1 and Q2
2. What will be the effect of the following instructions :
MVI OUTAREA,C’ ‘
MVCOUTAREA+1(132),OUTAREA
OUTAREA DS 133C
Assembler/Session 3 & 4
ObjectivesBINARY INSTRUCTIONS
Three types of binary instructions
•Full word
•Half word
•Register
The Binary Move Instructions
L, LH, LR ,ST, STH
Type : R,X Register and indexed storage
e.g... L 5,FULL LR 5,7
STH 7,HALF
Assembler/Session 3 & 4
ObjectivesBINARY INSTRUCTIONS
Note : Do not mix up the instruction types and field types
e.g.
LH 5,FULL - right half of Reg 5 gets 1st 2 bytes at FULL
L 6,HALF - Reg 6 gets 4 bytes starting from HALF
ST 3,RES - 4 bytes of reg 3 are stored starting from RES
RES DS H
HALF DC H’15’
FULL DC F’8’
Assembler/Session 3 & 4
ObjectivesBINARY INSTRUCTIONS
Binary Addition (A, AH and AR)
• Fixed-point overflow occurs when the sum will not fit in the receiving register
• Type R-X
e.g.
A 5,FULL
AH 6,HALF
AR 7,3
Assembler/Session 3 & 4
ObjectivesBINARY INSTRUCTIONS
Binary Subtraction (S, SH and SR)
• Type R-X
e.g.
S 5,FULL
SH 6,HALF
SR 7,3
Assembler/Session 3 & 4
ObjectivesBINARY INSTRUCTIONS
Binary comparisons (C, CH and CR)
e.g.
C 5,FULL
CH 6,HALF
CR 7,3
Condition code set as HIGH, LOW or EQUAL
Assembler/Session 3 & 4
ObjectivesBinary Multiplication (M, MR, MH)
Format : M op1,op2
op1 : An even numbered register; refers to an even-odd pair of registers
(any register in case of half word format)
op2 : storage area (full word/half word/register)
Assembler/Session 3 & 4
Binary Multiplication (M, MR, MH) ...
Function : The value in OP2 is multiplied by the value in the odd register of the even-odd pair and the result placed in even-odd registers
(For half word format : The half word specified in OP2 is multiplied by the value in OP1 and result stored in OP1.)
ObjectivesBINARY INSTRUCTIONS
Binary Division (D, DR)
Format: D op1,op2
Type : R-X / R-R
Op1 : An even numbered register. It refers to an even-odd pair of registers. The pair holds the double word to be divided. The even register receives the remainder; the odd register receives the quotient.
e.g. D 4,FULL
Assembler/Session 3 & 4
ObjectivesBC and BCR Instructions
• instructions that do or do not branch depending on the value of the condition code
Format : BC M1,S2
BCR M1,R2
e.g. BC B’1001’,BRPTA
will cause a branch to the instruction named BRPTA, if at the time the instruction is executed, the condition code is 0 or 3.
Assembler/Session 3 & 4
ObjectivesBRANCHING
A branch causes execution to continue at some other instruction in the program
• Branch conditions : Arithmatic B, BZ,BP,BM, BNZ,BNP,BNM,BO,BNO
• Comparison BH, BL, BE, BNH, BNL,BNE
e.g : CLI FLDA,C’K’BNL GOOD
Assembler/Session 3 & 4
ObjectivesCONDITION CODE PROCESSING
• condition code occupies 2 bits of PSW• condition code is set by each of a number of instructions• condition code is an extremely important intermediary
between arithmetic instructions and conditional branch instructions
• very important in implementing control structures CC Arithmetic Comparison
0 Zero First operand = Second operand
1 < Zero First operand < Second operand
2 >Zero First operand > second operand
3 Overflow Not set
Assembler/Session 3 & 4
ObjectivesLPR, LNR and LCR Instructions
Format: LPR,LNR or LCR R1,R2
LPR - Load positive register (Loads into R1 the absolute value of R2)
LNR Load Negative register (Loads into R1 the negative of absolute value of R2)
LCR Load complement register (Loads opposite sign of the value in R2)
Note: R1 and R2 can be the same
Assembler/Session 3 & 4
ObjectivesBIT MANIPULATIONS
Operation S-I S-S R-R R-X
OR OI OC OR O
AND NI NC NR N
Exclusive OR XI XC XR X
e.g... OI FLDA,X’0F’
NR 5,7
X 9,FULL
Assembler/Session 3 & 4
ObjectivesBIT MANIPULATIONS
OR Second 0 1 AND Second 0 1
First 0 0 1 First 0 0 0
1 1 1 1 0 1
Exclusive OR Second 0 1
First 0 0 1
1 1 0
Assembler/Session 3 & 4
ObjectivesBIT MANIPULATIONS
Testing individual bits - Test under mask (TM)
TM S1,I2
Function : The bits of S1 ( a single byte) are tested under the control of the mask in I2 and condition code is set as ‘all zeroes’, all ones’ or ‘mixed’
e.g. TM EMP,B’00000101’BNM NEXT
Assembler/Session 3 & 4
ObjectivesBIT MANIPULATIONS
Bit Shifting InstructionsSLL, SLDL Left logicalSRL, SRDL Right logical
(No condition code set)SLA, SLDA Left arithmeticSRA, SRDA Right arithmetic
(Sign bit not affected and condition code set)
e.g. SLL 5,1SRDA 4,5
Assembler/Session 3 & 4
ObjectivesBIT MANIPULATIONS
Bit Shifting InstructionsCondition code setting for arithmetic shift instructions 0- Result is zero
1- Result is negative 2- Result is positive 3- Overflow generatedOverflow is generated when a bit other than the sign
bit is shifted out
Assembler/Session 3 & 4
ObjectivesBIT MANIPULATIONS
Translations• To translate from one bit combination to
anotherFormat : TR S1(L),S2 or S1,S2
S1 : The field whose data is to be translated
S2 : A 256-byte translation tableFunction : The value of the original byte is used as a
displacement into the translation table. The byte found there replaces the original byte.
e.g. TR WORK,XTABLE If the source byte is x’40’ (Space), then the displacement into
the table is 64. The value in the table at displacement 64 will be replacing the source.
Assembler/Session 3 & 4
ObjectivesBIT MANIPULATIONS
Assembler/Session 3 & 4
Translations
1 byte - 256 possible combinations
x’00’,x’01’, x’02’, x’03’,…………..x’0F’
x’10’,x’11’,x’12’,…………………..x’1F’
…………………………………………..
x’F1’,x’F2’,x’F3’,…………………x’FF’
The table should start with replacement byte for x’00’ and end with replacement for x’FF’
ObjectivesBIT MANIPULATIONS (TRT)
Assembler/Session 3 & 4
Translations - TRT (Translate and test register)-Similar to TR but the source is not changed-Table is searched similar to TR taking the displacement into the table-Usually employed for editing purposes-The characters we need to search will have non zeros (x’00’) but other characters will be x’00’.-Source is searched one character at a time from left to right-The first nonzero match in the table halts the instruction-Condition code is set to 1 if match found before last byte, 2 if found at the last and 0 if not found-Loads address of source operand if found in last 24 bits of register 1, value from the table into last bit of register 2. No bits are changed in both the registers
ObjectivesBIT MANIPULATIONS (TRT continued)
Assembler/Session 3 & 4
Translations - TRT (Translate and test register)
This example searches for a period X’4B’The period 4B is decimal 75. So the X’4B’ is placed at the 76th position in the table. (Any non zero character may be placed in the tableTable should be declared as follows:
TABLE DC 75X’00’ DC X’4B’ DC 180X’00’
ObjectivesNumeric Conversions
1. Conversion to binary (CVB)
Format: CVB operand1,operand2
operand1 : Register
operand2 : a double word (containing
valid packed decimal number)
e.g. CVB 5,DOUBLE
Use : Character data -(PACK)->Packed decimal-(CVB)-> binary
Assembler/Session 3 & 4
ObjectivesNumeric Conversions
2. Conversion from binary (CVD)
Format: CVD operand1,operand2
operand1 : Register
operand2 : a double word
e.g. CVD 5,DOUBLE
Use : Binary-(CVD)->Packed decimal-(UNPK)-> Character data
Assembler/Session 3 & 4
ObjectivesNumeric Conversions
3. Conversion from Zoned decimal to packed (PACK) (SS instruction)
Format: PACK operand1,operand2
operand1 : Packed decimal
operand2 : Zoned Decimal
e.g. PACK PACKED(3),ZONED(5)
Assembler/Session 3 & 4
ObjectivesNumeric Conversions
4 Packed decimal to Zoned decimal (UNPACK)
Format: UNPACK operand1,operand2
operand1 : Zoned decimal
operand2 : Packed decimal
e.g. UNPACK ZD(5),PACKED(2)
Assembler/Session 3 & 4
ObjectivesRelation between CVD,CVB,PACK and UNPACK
Assembler/Session 3 & 4
Binary in Binary in RegisterRegister
PackedPackedDecimalDecimal
Zoned Zoned DecimalDecimal
CVBCVB PACKPACK InputInput
CVDCVD UNPKUNPK OutputOutput
ObjectivesExample code for Different conversions
Assembler/Session 3 & 4
PACK PNUM(8),START(3)PACK PNUM(8),START(3)
CVB 7,PNUMCVB 7,PNUM
A 7,=F’1’A 7,=F’1’
CVD 7,PNUMCVD 7,PNUM
UNPK ANS(3),PNUM(8)UNPK ANS(3),PNUM(8)
……
……
START DC C’125’START DC C’125’
ANS DS CL3ANS DS CL3
PNUM DS DPNUM DS D
ObjectivesPacked decimal operations
Assembler/Session 3 & 4
SS format - OPCODE D1(L1,B1),D2(L2,B2)
AP - Add packed
SP - Subtract packed
ZAP - Zero and add packed
MP - Multiply packed
DP - Divide packed
CP - Compare packed
Note: All these operations ignore the decimal places. You have to track the decimal places and edit it with ED and EDMK instructions
ObjectivesPacked decimal operations
Assembler/Session 3 & 4
Advanced instructions:SRP - Shift and Round packed OPCODE D1(L,B1),D2(B2),I3
First operand - Memory location including length
Second operand - Direction and number of places to shift
Third operand - Whether to round or not
-------------------------------------------------------------------------
Second operand, <= 32, left shift is done and 33 to 64 right shift is done.
Number for right shift = ( 64 - number of digits to be shifted)
(No rounding is involved in left shift
ObjectivesPacked decimal operations
Assembler/Session 3 & 4
Advanced instructions: (SRP continued)NUM is a 5 byte packed decimal number and contains 001234567C.
What is the value in number after each of these instructions?
1. SRP NUM(5),2,0
2. SRP NUM(5),62,0
3. SRP NUM(5),62,5
4. SRP NUM(5),60,5
ObjectivesPacked decimal operations
Assembler/Session 3 & 4
Advanced instructions:MVZ - Move Zone (Moves the first half of each byte)
MVN - Move numeric (Moves the second half of each byte)
MVO - Move with offset
EG: Multiply A by 100 where value of A is 123
MVC TEMP(3),A
MVN TEMP+2(1),=X’00’
MVZ TEMP+3(1),=X’00’
MVN TEMP+3(1),A+2
A DC PL3’123’
TEMP DS PL4
ObjectivesEditing the output for printing
ED and EDMK instructions ( D1(L,B1), D2(B2)) (Pattern and PD number)Patterns:x’20’ - Digit selector x’21’ - Significance selectorx’22’ - Field separator x’60’ - Sign indicatorPattern and the packed decimal number processed from left 1 byte at a time X 0 1 2 3 4 5 6 C (Instruction: ED P(12),X)Fill Character P 40 20 20 6B 20 21 20 4B 20 20 60 40 (Before execution) P 40 40 F1 6B F2 F3 F4 4B F5 F6 40 40 (After execution)
1 , 2 3 4 . 5 6 (Last 2 bytes spaces since number is positive)
Assembler/Session 3 & 4
……
……
ObjectivesEditing the output for printing
Assembler/Session 3 & 4
Values beingexamined
Action taken
Patternbyte
PD digit New pattern New state ofSI
Digitselector
0
1-9
Fill characterdigit inEBCIDIC
Off
OnSignificance starter
01-9
Fill character
digit inEBCIDIC
On
OnFieldseperator
None Fill character Off
When thesignificantindicator is off
Any otherbyte
None Fill character Off
Digitselector
0-9 digit inEBCIDIC
On
Significance starter
0-9 digit inEBCIDIC
On
Fieldseperator
None Fill character Off
When thesignificantindicator is on
Any otherbyte
None Pattern bytenot changed
On
ObjectivesEditing the output for printing
Assembler/Session 3 & 4
-ED and EDMK can detect the difference between significant and non significant digits ie between leading and non leading zeros
- Significance starter forces all subsequent digits to be considered significant
-When significance indicator is off and detection of a significant digit turns it on, the address of that significant digit placed in 8-31 of register 1 by EDMK
-EDMK allows a floating currency and/or algebraic sign but ED does not allow
ObjectivesTABLE PROCESSING
A table is a named storage structure consisting of subunits or entries
e.g. RATE DS 6F
L 4,RATE+8
Accessing table elements with indexed storage operands:
e.g. LH 9,=F8’
L 5,RATE(9) (9 - index register)
Assembler/Session 3 & 4
ObjectivesMulti-purpose branching instructions
Convenient when counted repetition structure (table processing) is needed
• Branch on count (BCT and BCTR)
Format: BCT op1,op2 (R-X)
Function: First the op1 value is decremented by 1. Second the branch is taken to the address specified in op2 only if the value in op1 is not 0.
e.g. LH 9,=H’12’
REPEAT EQU *
..
BCT 9,REPEAT
Assembler/Session 3 & 4
Objectives• Branch on index high and branch on index low or equal (BXH and BXLE)
Format: BXLE op1,op2,op3
BXH
op1 : A register known as the index register
op2 : A even-odd pair of registers
Even register - increment register
Odd register - Limit register
op3 : A storage operand. This is the branch address.
Assembler/Session 3 & 4
ObjectivesFunction : First, the value in the increment register is added to the indexed register. Second, the branch is taken only when the value in the index register is ‘lower than or equal to’ / ‘higher than’ the value in the limit register
Useful when the same register is to be used as the count and index register
Assembler/Session 3 & 4
ObjectivesBXLE - ‘DO UNTIL’ repetitions
BXH- ‘DO WHILE’ repetitions
e.g... LH 7,=H’0’ index
LH 2,=H’2’ increment amount
LH 3,=H’18 the limit
---
REPEAT ...
LH 6,TABLE(7)
...
BXLE 7,2,REPEAT
Assembler/Session 3 & 4
ObjectivesLoad instructions with additional features
• Load and Test (LTR)
e.g... LTR 15,15
BNZ ERROR
• Load Address (LA)
LA R1,D2(X2,B2)
Assembler/Session 3 & 4
ObjectivesUSING EQUATES
• To associate a fixed value with a symbol
• Useful for length and relative address calculation
e.g. TABLE DS 0H
DC C’01
DC C’02’
...
TBLEND EQU *
TBLSIZE EQU TBLEND-TABLE
Assembler/Session 3 & 4
ObjectivesUSING EQUATES
Can be used for the following purposes:
1. To assign single absolute values to symbols.
2. To assign the values of previously defined symbols or expressions to new symbols, thus allowing you to use different mnemonics for different purposes.
3. To compute expressions whose values are unknown at coding time or difficult to calculate. The value of the expressions is then assigned to a symbol.
Assembler/Session 3 & 4
Assembler LanguageAssembler Language
SESSION 5Program Sectioning
Assembler/Session 5
ObjectivesBeginning and End of Source Modules
• Code a CSECT segment before any statement that affects the location counter
• END statement is required as the last statement in the assembly
Assembler/Session 5
ObjectivesCONTROL SECTIONS
•A source module can be divided into one or more control sections
•A control section is the smallest subdivision of a program that can be relocated as a unit
Assembler/Session 5
• At coding time, establish the addressability of each control section within the source module, and provide any symbolic linkages between control sections that lie in different source modules.
• Initiated by using the START or CSECT instruction
CONTROL SECTIONS
ObjectivesCONTROL SECTIONS
• Any instruction that affects the location counter, or uses its current value, establishes the beginning of the first control section.
Assembler/Session 5
Format of CSECT:
Name Operation Operand
Any symbol CSECT Not required
or blank
Note: The end of a control section or portion of a control section is marked by (a) any instruction that defines a new or continued control section, or (b) the END instruction.
CONTROL SECTIONS
ObjectivesDUMMY SECTIONS
• A dummy control section is a reference control section that allows you to describe the layout of data in a storage area without actually reserving any virtual storage.
Assembler/Session 5
• Use the DSECT instruction to initiate a dummy control section or to indicate its continuation.
Format of DSECT:
Name Operation Operand
Any symbol DSECT Not required
or blank
DUMMY SECTIONS
ObjectivesDUMMY SECTIONS
To use a dummy section :
• Reserve a storage area for the unformatted data
• Ensure that this data is loaded into the area at execution time
Analogy: Cobol copybook
Assembler/Session 5
• Ensure that the locations of the symbols in the dummy section actually correspond to the locations of the data being described
• Establish the addressability of the dummy section in combination with the storage area
You can then refer to the unformatted data symbolically by using the symbols defined in the dummy section.
DUMMY SECTIONS
ObjectivesASMBLY2 CSECTBEGIN BALR 2,0 USING *,2 ... Reg 3 points to data
area LA 3,INPUT
USING INAREA,3 CLI INCODE,C'A' BE ATYPE ...ATYPE MVC WORKA,INPUTA MVC WORKB,INPUTB
. .
Assembler/Session 5
WORKA DS CL20
WORKB DS CL18
INPUT DS CL39
...
INAREA DSECT
INCODE DS CL1
INPUTA DS CL20
INPUTB DS CL18
...
END
ObjectivesAssembler Directives
TITLE : To provide headings for each page of the assembly listing of the source modules.
EJECT : To stop the printing of the assembler listing on the current page, and continue the printing on the next page.
ORG : To reset the location counter
Assembler/Session 5
LTORG : A literal pool is created immediately after a LTORG instruction or, if no LTORG instruction is specified, at the end of the first control section.
PRINT : To control the amount of detail to be printed in the listing of programs.
PRINT NOGEN / GEN
Assembler Directives
Assembler LanguageAssembler Language
SESSION 6Writing a complete program
Assembler/Session 6
ObjectivesProgram Entry and Exit Logic
Program entry - Preserve register contents
Program Exit - Restore register contents
Register save area
Always calling program provides a save area of 18 Full words long used for storage of registers
Save area address passed through register 13 by IBM convention
Assembler/Session 6
ObjectivesA register save area (18 consecutive full words) Word Address Contents1 SAV2 SAV+4 Address of calling program’s save area3 SAV+8 Address of called program’s save area4 SAV+12 Contents of Register 145 SAV+16 Contents of Register 156 SAV+20 Contents of Register 0
...18 SAV+68 Contents of Register 12
Assembler/Session 6
ObjectivesResponsibilities of called program
Program entry conventions
1.Save contents of registers 0-12,14 & 15 in calling program’s save area
2.Establish base register
3.Store calling program’s save area in the 2nd word of its own save area
Assembler/Session 6
ObjectivesProgram entry conventions (contd..)
4. Store the address of its register save area in the third word of the calling program’s register save area
(The addresses in the 3d word of save area establish a chain of register save areas. This will be useful in reading the dump when program crashes).
Assembler/Session 6
ObjectivesResponsibilities of called program (contd..)
Program Entry
STM R14,R12,12(R13)
BALR R12,0
USING *,R12
ST R13,SAVOWN+4 store calling programs save area
LR R14,R13
LA R13,SAVOWN Reg 13 contains current prog’s SA ...
ST R13,8(R14)
Assembler/Session 6
ObjectivesResponsibilities of called program (contd..)
Program Exit conventions
1. Restore registers 0-12 and 14
2. Place the address of the save area provided by the calling program in Reg 13
3. Place a return code in the low order byte of register 15 if one is required. Otherwise restore register 15.
Assembler/Session 6
ObjectivesResponsibilities of called program (contd..)
Program Exit
L R13,4(R13)
LM R14,R12,12(R13)
BR R14
Assembler/Session 6
ObjectivesResponsibilities of calling program
1. Register 13 must contain the address of a register save area.
2. Register 15 should be set to the beginning address of the subroutine
L R15,=V(SUBENTRY)
where SUBENTRY is the entry address (usually the CSECT name) of the subroutine
Assembler/Session 6
ObjectivesResponsibilities of calling program (contd...)
3. Register 14 should have the return address
4. Register 1 should have the address of the parameter list
A BALR instruction stores the address of the next instruction in the calling program into register 14 and transfers control to the called subroutine
BALR R14,R15
Assembler/Session 6
ObjectivesPassing parameters to a subroutine
• The standard interface requires that addresses of parameters be placed in a block of storage, and the address of the block be loaded into register 1 as the subroutine is called
• Both input and output parameters are treated the same way
e.g... ADDS DC A(T)
DC A(U)
DC A(V)
LA R1,ADDS
Assembler/Session 6
ObjectivesR1 Main storage
Addr of parmlist Parmlist parm3
Addr of parm1
Addr of parm2 parm1
Addr of parm3 parm2
Assembler/Session 6
ObjectivesCalled subroutine B may get the second parameter by
L R3,4(,R1)
L R8,0(,R3)
Assembler/Session 6
ObjectivesRegisters with special use
R0 : Contains single word output of a subroutine
R1 : contains the address of an area of main storage that contains addresses of parameters
Assembler/Session 6
ObjectivesRegisters with special use (contd...)
R14 : Contains the return address, the address in the calling routine to which a subroutine should return control when finished
R15 : contains the address of the entry point in the subroutine
R13 : contains the address of an area in which register contents can be stored by a subroutine
Assembler/Session 6
ObjectivesThe subroutine RANDOM
RANDOM STM R14,R12,12(R13)
BALR R12,0
USING *,R12
L R7,RN
M R6,=F’65541’
ST R7,RN
LR R0,R7
LM R1,R12,24(R13)
BR R14
RN DC F’8193’
Assembler/Session 6
ObjectivesSubroutine RDIGIT
RDIGIT STM R14,R12,12(R13)
BALR R12,0
USING *,R12
ST R13,SAV+4
LA R13,SAV
...
L R15,RANDAD
BALR R14,R15
...
L R13,SAV+4
LM R14,R15,12(R13)
LM R1,R12,24(R13)
BR R14
SAV DS 18F
RANDAD DC A(RANDOM)
Assembler/Session 6
ObjectivesLinkage Conventions
•Program divided into 2 or more source modules
•Source module divided into 2 or more control sections
•For link-editing, a complete object module or any individual control section of the object module can be specified
Assembler/Session 6
ObjectivesCommunicating between program parts
• To communicate between 2 or more source modules, symbolically link them together
• To communicate between 2 or more control sections within a source module, establish proper addressability
Assembler/Session 6
ObjectivesEstablishing symbolic linkage
• Identify external symbols in the EXTRN or WXTRN instruction or the V-type address constant
• provide A-type or V-type address constants to reserve storage for addresses represented by external symbols
• In the external source modules, identify these symbols with the ENTRY instruction
(name entry of a START or CSECT instruction is automatically identified as an entry symbol)
External symbol dictionary
Assembler/Session 6
ObjectivesEstablishing symbolic linkage (contd...)
e.g. program A
EXTRN TABLEB
WXTRN TABLEB
TABADR DS V(TABLEB)
program B
ENTRY TABLEB
TABLEB DS ...
Assembler/Session 6
ObjectivesAddress Constants (A and V)
• An address constant is a main storage address contained in a constant
• A V-type constant is the value of an external symbol - a relocatable symbol that is external to the current control section.
Used for branching to locations in other control sections
e.g L 5,ADCON
ADCON DC A(SOMWHERE)GSUBADDC V(READATA)
Assembler/Session 6
Assembler LanguageAssembler Language
SESSION 7Assemble and Link Program
Assembler/Session 7
ObjectivesProcessing of InstructionsTime/ M/C Assembler ENTRY Macro
Activity instructions. EXTRN Instr.
Code source m/c DC,DS
instruc.
Preassembly Refer to macro
instruc.
Assembly object code
LKED
Prog fetch
Execution data area form data
area in load mod
Assembler/Session 7
ObjectivesJCL ‘ parm’ processing
EXEC PGM=pgmname,PARM=
When program gets control :
•Register 1 contains the address of a full word on a full word boundary in program’s address space
•the high order bit of this full word is set to 1
(this convention is to indicate the last word in a variable length parameter list)
Assembler/Session 7
JCL ‘ parm’ processing ...
• Bits 1-31 of the full word contain the address of a 2-byte length field on a half word boundary
• The length field contains a binary count of the no. of bytes in the PARM field which immediately follows the length field
ObjectivesCOBOL to Assembler
CALL asmpgm USING COMM-AREA
PL/I to Assembler DCL ASMSUB ENTRY OPTIONS(ASSEMBLER)
CHARSTRING CHAR(25);
CALL ASMSUB(CHARSTRING);
Ref : PL/I Programming Guide, COBOL programming Guide
Assembler/Session 7
Assembler LanguageAssembler Language
SESSION 8Macro Language
Assembler/Session 8
ObjectivesMacros
• Short source routines written and stored in libraries
•Assembler inserts the source statements in the program where the macro appears
Assembler/Session 8
Macro Definition
Format :
•A header statement
•A prototype
•Model statements
•A trailer statement
ObjectivesHeader statement:
MACRO
Prototype:
&name MOVE &TO,&FROM,&LENGTH
Model statements:
A set of machine and assembler instructions
Trailer statement:
&name MEND
Assembler/Session 8
ObjectivesMacro Instruction:• A statement containing the name of a macro
• when expanded, the symbolic parameters in the model statements are replaced by corresponding parameters from the macro instructions
• symbolic parameters may be positional or keyword
Assembler/Session 8
Macro Instruction ...MACRO
&LABEL HALFSWAP ®,&SV
&LABEL ST ®,&SV
SLL ®,8
IC ®,&SV
SLL ®,8
IC ®,&SV+1
MEND
ObjectivesSET Symbols (global or local)
3 types :
• arithmetic (SETA)
• binary (SETB)
• character (SETC)
• SET symbols are declared using,
LCLA LCLB LCLC
GCLA GCLB GCLC
Assembler/Session 8
ObjectivesFormat:Label operation operandssymbol-name SETA An expression
SETBSETC
e.g.LCLA &A1GCLA &A2
&A1 SETA 1&A2 SETA &A1+3
Assembler/Session 8
ObjectivesAttributes
There are 6 attributes of a symbol or symbolic parameter :
type, length, scaling, integer, count and number
System variable symbols&SYSINDX, &SYSDATE, &SYSTIME, &SYSECT, &SYSPARM, &SYSLOC
Assembler/Session 8
ObjectivesConditional Assembly
The assembler can be made to branch and loop among assembler language statements using sequence symbols and the assembler instructions AIF and AGO
Sequence symbol : Period followed by 1 to 7 alphabets or digits of which the first is a letter
e.g. .Z23Ab
Assembler/Session 8
ObjectivesFormat:
Label Operation Operand
seq symbol AGO seq. symbol or blank
-do- AIF A logical expression
enclosed in parenthesis,
followed by seq symbol
Assembler/Session 8
A logical expression is composed of one or more relations or values of SETB symbols connected by logical connects AND, OR, AND NOT, OR NOT
A relation consists of 2 arithmetic expressions or 2 character expressions connected by a relational operator EQ, NE, LT, LE, GT, GE
Objectives e.g.
MACRO
PSRCH &PARAMS,&STRING
GBLB &FOUND
LCLA &I
&FOUND SETB 0
.LP AIF ((&I GE N’&PARAMS) OR &FOUND) .E
&I SETA &I+1
&FOUND SETB (‘&PARAMS(&I)’ EQ ‘&STRING’)
AGO .LP
.E MEND
Assembler/Session 8
Objectives Accessing QSAM files:
Keywords in DCB parameter: DSORG PS Physical sequential
RECFM F,FA,FB,FBA,V,VBA BLKSIZE Block length
LRECL Record Length
DDNAME Dataset name in JCL
MACRF Macro GM - Get Move GL - Get Locate
PM - Put Move PL - Put locate
Move parameter directly puts the record in the storage area specified while Locate mode Loads the address of the record in Register 1
Assembler/Session 8
Objectives Accessing VSAM files: ACB macro AM - VSAM (For documentation)
BUFND - No. of I/O buffers for data control intervals
BUFNI - No. of I/O buffers for index control intervals
BUFSP - Size of an area for data and Index I/O buffers
DDNAME - Filename used in the DD statement. If omitted refers to the ACB macro name
EXLST - Address to the EXLST macro. Generates a list of addresses for user routines
MACRF - Types of processing the file will do
Assembler/Session 8
Objectives Accessing VSAM files: ACB macro (Continued) EXLST options: AM - VSAM EODAD = (Address, A/N, L) (Load module)
EXCPAD = (Address, A/N, L) (Load module)JRNAD = (Address, A/N, L) (Load module)LERAD = (Address, A/N, L) (Load module)SYNAD = (Address, A/N, L) (Load module)
Active/No, Stored in load module
Assembler/Session 8
Objectives Accessing VSAM files: RPL macro (Request parameter list) ACB - Address of the ACB macro AREA - Address of the work area to be used
AREALEN - Length of the work area (Should be large enough to hold largest record in Move mode and at least 4 bytes in the Locate mode)RECLEN -Length of the records in the file (For VB you have to put the length before writing using MODCB)ARG - Label containing the key for the search (Key for KSDS, RRN for RRDS and RBA for ESDS)OPTCD - 5 sets of groups of parameters
Assembler/Session 8
Objectives Accessing VSAM files: RPL macro (Continued) Options for OPTCD: KEY/CNV/ADR - Access by key,Control interval or Relative byte address SEQ/DIR/SKP - Sequential processing,Direct, Skip sequential FWD/BWD - Forward sequential processing,Backward ARD/LRD -Start seq.processing with ARG specified/ Backward processing from the last record NUP/NSP/UPD - No updating(Next rec not ready),No updating Next rec ready(DA only), Record updating) MVE/LOC - Move mode/ Locate mode
Assembler/Session 8
Objectives Accessing VSAM files: OPEN - Open the file CLOSE - Close the file GET - Read a record PUT - Store a record ERASE - Delete a record POINT - Position for access
Advanced macros: SHOWCB, TESTCB, MODCB
Assembler/Session 8
Assembler LanguageAssembler Language
SESSION 9Other Topics
Assembler/Session 9
ObjectivesCharacteristics of good assembler program
• has simple, easy to understand logic
• uses mostly simple instructions
• has no relative addressing
• uses subroutines
Assembler/Session 8
Characteristics of good assembler program ...
• uses DSECTs
• has efficient code (LA R10, 4(0,R10 - A R10,=F’4)
• does not abnormally terminate due to user error
• requests and check feedback from macro instructions
• provides meaningful error messages
ObjectivesCharacteristics of good assembler program (contd..)
• lets the assembler determine lengths
• has opcodes, operand and comments aligned
• contains meaningful comments
• uses meaningful labels
Assembler/Session 8
ObjectivesStructured Programming
• To improve design and understandability of a program
• made up of building blocks of subroutines
Conventions for general purpose registers
• Base registers
• Link registers
Assembler/Session 8
ObjectivesThe EXecute Instruction
• the EX instruction is a R-X type instruction that directs the execution of an instruction called the subject instruction, which is addressed by the second operand
• the subject instruction is in effect a one-instruction subroutine
Assembler/Session 9
•The subject instruction is modified before execution (though not altered at its main storage location) : bits 8-15 of the instruction ORed with bits 24-31 of register R1 to form the second byte of the instruction actually executed
e.g. Let reg 9 have the length of string to be moved
EX R9,VARMVC
VARMVC MVC A(0),B
The EXecute Instruction (contd...)
ObjectivesDEBUGGING
Exceptions and Interrupts
Interrupts that result directly from attempts at invalid program execution are called program-check interrupts; identified by a code
Interruption code 1 : Operation
Interruption code 2 : Privileged operation
Interruption code 4 : Protection
Interruption code 5 :Addressing
Interruption code 6 :Specification
Assembler/Session 9
ObjectivesDEBUGGING
Exceptions and Interrupts (contd..)
Interruption code 7 : Data
Interruption code 8 : Fixed-Point Overflow
Interruption code 9 : Fixed-Point Divide
Other Interruption codes ( 3, 10, 11, 12, 13, 14, 15)
Assembler/Session 9
ObjectivesDEBUGGING
Reading dumps
• whenever a program abends an indicative dump is generated
• The completion code is a code furnished by the O/S to designate the reason for the termination of the job step
• In case of program check interruption, the first 2 digits of the completion code is 0C
Assembler/Session 9
• Locate the entry point of your program
Reading dumps ...
DEBUGGING
ObjectivesDEBUGGING
Reading dumps (contd...)
• The register contents are the contents at the point of interruption (the instruction that caused the interrupt is usually the one just before the interrupt address given)
• use address at interrupt and entry address to locate the instruction that caused the program-check interruption
Assembler/Session 9
ObjectivesDEBUGGING
Full and Partial dumps
• //SYSUDUMP DD SYSOUT=A
• SNAP macro
Assembler/Session 9
Reading the dump
• SAVE AREA trace
• P/P Storage
• Examine register contents, PSW and listed entry point to find the portion of program being executed
• Look at main storage dump to determine the data being used
DEBUGGING
ObjectivesSYSTEM MACROS
Data Management Macros
DCB - Construct a data control block
OPEN - Logically connect a dataset
CLOSE - Logically disconnect a dataset
GET - Obtain next logical record (queued access)
PUT - Write next logical record (queued access)
READ - Read a block (basic access)
WRITE - Write a block (basic access)
Assembler/Session 9
ObjectivesSYSTEM MACROS
Supervisor Services Macros
ABEND - Abnormally terminate a task
CALL - Pass control to a control section
GETMAIN - Allocate virtual storage
FREEMAIN - Free virtual storage
LOAD - Bring a load module into virtual storage
RETURN - return control to the calling program
SAVE - Save register contents
Assembler/Session 9
ObjectivesSYSTEM MACROS
Supervisor Services Macros (contd)
SNAP - Dump virtual storage and continue
LINK - Pass control to a Program in Another load module
WTO - Write to operator
Assembler/Session 9
ObjectivesSYSTEM MACROS
e.g. File I/O
OPEN (INFILE,INPUT)
GET INFILE,RECAREA
PUT OUTFILE,RECAREA
CLOSE (INFILE)
INFILE DCBDSORG=PS,MACRF=GM,DDNAME=IFILE
OUTFILE DCBDSORG=PS,MACRF=PM,DDNAME=OFILE
(RECFM=,LRECL=,BLKSIZE=,)
Assembler/Session 9
ObjectivesSYSTEM MACROS
Three forms :
Standard form : Results in instructions that store into an inline parameter list and pass control to the required program
List form : Provides as out-of-line parameter list
Execute form : Provides the executable instructions required to modify the out-of-line parameter list and pass control to the required program
Assembler/Session 9
Thank youThank you