Assembler1

chapter two assembler of system programming

1. ASSEMBLER

2. ASSEMBLER FUNCTION OF AN ASSEMBLER ASSEMBLY LANG. PROGRAM DATABASE ASSEMBLER MACHINE LANG. & OTHER INFORMATION FOR LOADER 2 8/3/2016 jaya

3. ASSEMBLER General format for assembly language statement Example: JOHN START 0 FOUR DC F’4’ A 2,FOUR Opcode ------- 1 digit Register operand ---------- 1 digit Memory operand ---------- 3 digits [ Label *] [ opcode ] [operand1]*, [operand2]* 3 8/3/2016 jaya

4. ASSEMBLER LABEL FIELD when processed by assembler it will store this label and its information. Example: JHON DATA 4 8/3/2016 jaya

5. ASSEMBLER Mnemonic Opcode Field instructions Types of assembly statements Imperative statements Declarative statements Directive statements 5 8/3/2016 jaya

6. ASSEMBLER Imperative statements Specifies the operation to be performed during the execution of program Example: MOVEM AREG , B MOVER AREG , C ADD AREG , ONE COMP AREG , =‘1’ SUB BREG , =‘2’ constant memory register 6 8/3/2016 jaya

7. ASSEMBLER Declarative Statements declare constant or storage variable Types of declarative statements:- DS(Declare Storage) ex: AB DS 1 X DS 200 DC(Declare Constant) ex: ONE DC 1 name 1 word mem. 7 8/3/2016 jaya

8. ASSEMBLER Assembler Directive statements or pseudo ops It specifies the operation to be performed during the process of assembly . Types of pseudo opcodes START END LTORG ORIGIN EQU 8 8/3/2016 jaya

9. ASSEMBLER START Tells the start of program Gives name to program Sets LC ex: TEST START JHON START 484 9 8/3/2016 jaya

10. ASSEMBLER END Tells end of program. Ex: END LTORG Tells the assembler to place the literals at an earlier location Used in very long programs Ex: LTORG 10 8/3/2016 jaya

11. ASSEMBLER ORIGIN used to reset location counter default value of reset is LC=0 Ex: ORIGIN <address space> 11 8/3/2016 jaya

12. ASSEMBLER EQU allows to define variable Syntax: <symbol> EQU <address space> Ex: BASE EQU 15 12 8/3/2016 jaya

13. ASSEMBLER Operand Field with this assembler perform actions Types of operands Register Symbol Literal 13 8/3/2016 jaya

14. ASSEMBLER Advantages of assembler Unambiguous code and compact As close to machine as possible Extremely fast Compiled assembly programs are small Storage can be allocated and deallocated dynamically. 14 8/3/2016 jaya

15. ASSEMBLER Advantages of assembly Language Easy to memorize Mnemonic opcodes are helpful for diagnostics Program modification is easy Manual conversion of constants into machine representation 15 8/3/2016 jaya

16. ASSEMBLER General Design Procedure Specify the problem Specify data structure Define format of data structure Specify algorithm Look for modularity Repeat 1 through 5 on modules 16 8/3/2016 jaya

17. ASSEMBLER Statement of problem Processing of each assembly instruction varies depending upon types of instruction. Processing of each assembly instruction assembler processes each field of instruction. In this labels or symbols can appear before they are defined, so it is convenient to make two passes over the input. Passes of assembler Pass 1 Pass2 Generate instructions (basic requirement) Evaluate the mnemonic in the operation field to produce its machine code Evaluate the subfields-find the value of each symbol, process literals and assign addresses. Process pseudo ops 17 8/3/2016 jaya

18. ASSEMBLER Pass1: purpose - define symbol and literals Determine length of machine instructions(MOTGET1) Keep track of Location Counter(LC) Remember values of symbols until pass 2(STSTO) Process some pseudo ops , e.g. EQU, DS (POTGET1) Remember literals(LITSTO) Pass2: purpose - generate object program Look up value of symbols(STGET) Generate instructions(MOTGET2) Generate data(for DS, DC, and literals) Process pseudo ops(POTGET2) 18 8/3/2016 jaya

19. ASSEMBLER DATA STRUCTURES second step of design procedure for assembler Pass 1 data base: Input source program A Location Counter(LC), The Machine Opcode Table(MOT) The Pseudo Opcode Table(POT) The Symbol Table(ST) The Literal Table(LT) A copy of input used b pass2 19 8/3/2016 jaya

20. ASSEMBLER DATA STRUCTURES second step of design procedure for assembler Pass 2 data base: Copy of source program input to pass 1 A Location Counter(LC), The Machine Opcode Table(MOT)(mnemonic, length, binary m/c opcode and format) The Pseudo Opcode Table(POT) The Symbol Table(ST) The Base Table(BT) A workspace , INST A workspace, PRINT LINE A workspace PUNCH CARD Output deck of assembled instruction 20 8/3/2016 jaya

21. ASSEMBLER MOT(Machine Op-code Table) 21 Mnemonic opcode (4 bytes) (char) Binary opcode (1 byte) (hexadecimal) Instruction Length (2 bits) (binary) Instruction Format (3 bit) (binary) Not used in this design (3 bits) “Abbb” 5A 10 001 “AHbb” 4A 10 001 “ALbb” 5E 10 001 “ALRb” 1E 01 000 “ARbb” 1A 01 000 -------------- ------------ -------------- -------------- “MVCb” D2 11 100 000 RR 001 RX 010 RS 011 SI 100 SS Instruction format 01 1 half words 2 bytes 10 2 half words 4 bytes 11 3 half words 6 bytes Instruction length 6 bytes 8/3/2016 jaya

22. ASSEMBLER MOT example 22 NAME MNEMONIC BINARY OPCODE FORMAT OPERANDS Add(c) AR 1A RR R1,R2 Add (c ) A 5A RX R1,D2(X2,B2) Branch & link BALR 05 RR R1,R2 Start I/O SIO 9C SI D1(B1) Move MVC D2 SS D1(L,B1),D2(B2) 8/3/2016 jaya

23. ASSEMBLER POT (pseudo opcode table) 23 PSEUDO OPCODE (5 BYTES CHAR) ADDRESS OF ROUTINES TO PROCESS PSEUDO OP (3 BYTES=24 BIT ADDRESS ”DROPb” P1 DROP “ENDbb” P1 END “EQUbb” P1 EQU “START” P1 START “ORIGIN” P1 USING 8/3/2016 jaya

24. ASSEMBLER ST(symbol table) 14 bytes per entry 24 SYMBOL (8 BYTES CHAR) VALUE (4 BYTE) HEXADECIMAL LENGTH (1 BYTE) HEXADECIMAL RELOCATION (1 BYTE) CHAR “SSSSbbbb” 0000 01 “R” “FOURbbbb” 0000 04 “R” “FIVEbbbb” 0010 04 “R” “TEMPbbbb” 0014 04 “R” 14 bytes per entry 8/3/2016 jaya

25. ASSEMBLER LT(Literal table) 14 bytes per entry 25 LITERAL (8 BYTES CHAR) VALUE (4 BYTE) HEXADECIMAL LENGTH (1 BYTE) HEXADECIMAL RELOCATION (1 BYTE) CHAR A(DATA1) 0048 04 “R” F’800’ 0060 04 “R” 8/3/2016 jaya

26. ASSEMBLER BT(Base Table) 15 entries 26 AVAILABILITY INDICATOR (1 BYTE) CHAR DESIGNED RELATIVE ADDRESS CONTENTS OF BASE REGISTER (3 BYTES = 24 BIT ADDRESS) HEXADECIMAL “N” ------------------ “N” ----------------- ----------------------- ----------------- “N” ---------- “Y” 00 00 00 N =register specified in USING Y= never specified 8/3/2016 jaya

27. 27 Pass 1 LC  0 Read card from file copy READ1 Assign value to symbol in label field STSTO L  length Process any literals, enter into literal table LTSTO Write copy of card on file for use by pass2 WRITE1 Search pseudo-op table POTGET1 Which one ? Adjust LC to proper alignment L  length of data field DLENGTH Assign current value of LC to symbol STSTO Evaluate operand field EVAL Search machine-op table MOTGET1 Assign storage locations to literals LITASS Rewind and reset copy file On to pass 2 found DS DC NO EQU USING , DROP END Not found YES LC  LC + L Is there symbol in label field 8/3/2016 jaya

28. 28 Pass 2 LC  0 Read card from file copy READ2 Enter base reg. no. and value into base table BTSTO Get op-code byte and format code L  length Type of instruction Evaluate both register expressions and insert into 2 nd byte EVAL “punch” Assembled instruction PUNCH “print” assembly listing line PRINT LC  LC + L Search pseudo-op table POTGET2 Which one ? Adjust LC to proper alignment L  length of data field DLENGTH DC or DS Evaluate register and index expressions and insert into 2 nd byte EVAL Calculate effective address(EA) of operand EVAL Determine appropriate displacement and base register D + C(B ) =CA BTGET Put B & D into bytes 3 and 4 From constant and insert in assembled program DCGEN Evaluate operand EVAL Search pseudo-op table MOTGET2 Print listing PRINT Indicate base reg. no. unavailable BTDROP Generate literals for entries in literal table LTGEN STOP a b a b found DS DC DC DS EQU START USING DROP END Not found RX RR 8/3/2016 jaya

29. ASSEMBLER Look for modularity PASS1 29 function description READ1 Read the next assembly source card. POTGET1 Search the pass 1 pseudo-op table(POT) for a match with the operation field of the current source card. MOTGET1 Search the machine –op table (MOT) for a match with the operation of the current source card. STSTO Store a label and its associated value into the symbol table (ST) LTSTO Store a literal into the literal table(LT) WRITE1 Write a copy of the assembly source card on a storage device for use by pass 2 DLENGTH Scan operand field of DS or DC pseudo-op to determine the amount of storage required. EVAL Evaluate an arithmetic expression consisting of constants and symbols. STGET Search the symbol table(ST) for the entry corresponding to a specific symbol. 8/3/2016 jaya

30. ASSEMBLER Look for modularity PASS2 30 function description READ2 Read the next assembly source card from the file copy. POTGET2 Search the pass 2 pseudo-op table(POT) for a match with the operation field of the current source card. MOTGET2 Search the machine –op table (MOT) for a match with the operation of the current source card. EVAL Evaluate an arithmetic expression consisting of constants and symbols. PUNCH Convert generated instruction to card format. PRINT Convert relative location and generated code to character format; print the line along with copy of the source card. DCGEN Process the fields of the DC pseudo-op to generate object code. DLENGTH Scan operand field of DS or DC pseudo-op to determine the amount of storage required. BTSTO Insert data into appropriate entry of Base Table(BT). BTDROP Insert “unavailable” indicator into appropriate entry of BT. BTGET Convert affective address into base and displacement by searching BASE Table (BT) for available base registers. LTGEN Generate code for literals(uses DCGEN) 8/3/2016 jaya