Instruction set

Instruction set

• Instruction set

• of SIC

TEST START 1003

• TEST START 1003

• FIRST LDA FIVE

• STA ALPHA

• ALPHA RESW 1 /*symbolic variable*/

• FIVE WORD 5 /*symbolic constant, Literal */

• END FIRST

TEST START 1003

• TEST START 1003

• FIRST LDA FIVE 001012

• STA ALPHA 0C1009

• ALPHA RESW 1 ******

• FIVE WORD 5 000005

• END FIRST

LDA ALPHA

• LDA ALPHA

• ADD INCR

• SUB ONE

• STA BETA

• ONE WORD 1

• ALPHA RESW 1

• BETA RESW 1

• INCR RESW 1

TEST START 1003

• TEST START 1003

• FIRST LDA FIVE 001012

• STA ALPHA 0C1009

• ALPHA RESW 1 ******

• FIVE WORD 5 000005

• END FIRST

Data structures

• Data structures

Operation Code Table (OPTAB)

• Operation Code Table (OPTAB)

• Used to look up mnemonic operation codes and translate them into machine language equivalents
• Contains the mnemonic operation code and its machine language equivalent
• In more complex assemblers, contains information like instruction format and length

Symbol Table

• Symbol Table

• Used to store values (addresses) assigned to labels
• Includes the name and value for each label
• Flags to indicate error conditions, e.g. duplicate definition of labels
• May contain other info like type or length about the data area or instruction labeled

LOCCTR

• LOCCTR

• Used to help in the assignment of addresses
• Initialized to the beginning address specified in the START statement
• After each source statement is processed, the length of the assembled instruction or data area to be generated is added
• Gives the address of a label

Two pass algorithm

• Two pass algorithm

• Pass 1 (Define symbols):

• (a)Assign addresses to all statements (LOC)

• (b) Save the addresses assigned to all labels in symbol table

• (c) Perform some processing for assembler directives

Two pass algorithm

• Two pass algorithm

• Pass 2 (Generate object code):

• Translate opcode and operands

• Generate data values for WORD

• Write object program

TEST START 1003

• TEST START 1003

• FIRST LDA FIVE 001012

• STA ALPHA 0C1009

• ALPHA RESW 1 ******

• FIVE WORD 5 000005

• END FIRST

H | COPY | 001000 | 00107A

• H | COPY | 001000 | 00107A

• T | 001000 | 1E | 141033 | 482039 | 001036 | …

• T | 00101E | 15 | 0C1036 | 482061 | 081033 | …

• …

• T | 002073 | 07 | 382064 | 4C0000 | 05

• E | 001000

begin

• begin

• read first input line

• if OPCODE = 'START' then

• begin

• save #[OPERAND] as starting address

• initialized LOCCTR to starting address

• write line to intermediate file

• read next input line

• end {if START}

• else

• initialized LOCCTR to 0

while OPCODE != 'END' do

• while OPCODE != 'END' do

• begin

• if this is not a comment line then

• begin

• if there is a symbol in the LABEL field then

• begin

• search SYMTAB for LABEL

• if found then

• set error flag (duplicate symbol)

• else

• insert (LABEL, LOCCTR) into SYMTAB

• end {if symbol}

• search OPTAB for OPCODE

• if found then

• add 3 {instruction lengh} to LOCCTR

• else if OPCODE = 'WORD' then

• add 3 to LOCCTR

• else if OPCODE = 'RESW' then

• add 3 * #[OPERAND] to LOCCTR

• else if OPCODE = 'RESB' then

• add #[OPERAND] to LOCCTR

• else if OPCODE = 'BYTE' then

• begin

• find length of constant in bytes

• add length to LOCCTR

• end {if BYTE}

• else

• set error flag (invalid operation code)

• end {if not a comment}

• write line to intermediate file

• read next input line

• end {while not END}

• write last line to intermediate file

• save (LOCCTR - starting address) as program length

• end

begin

• begin

• read first input file {from intermediate file}

• if OPCODE = 'START' then

• begin

• write listing line

• read next input line

• end {if START}

• write header record to object program

• initialized first Text record

while OPCODE != 'END' do

• while OPCODE != 'END' do

• begin

• if this is not a comment line then

• begin

• search OPTAB for OPCODE

• if found then

• begin

• if there is a symbol in OPERAND field then

• begin

• search SYMTAB for OPERAND

• if found then

• store symbol value as operand address

• else

• begin

• store 0 as operand address

• set error flag (undefined symbol)

• end

• end {if symbol}

• else

• store 0 as operand address

• assemble the object code instruction

• end {if opcode found}

• else if OPCODE = 'BYTE' or 'WORD' then

• convert constant to object code

• if object code not fit into the current Text record then

• begin

• write Text record to object program

• initialized new Text record

• end

• add object code to Text record

• end {if not comment}

• write listing line

• read next input line

• end {while not END}

write last Text record to object program

• write last Text record to object program

• write End record to object program

• write last listing line

• end

H | COPY | 001000 | 00107A

• H | COPY | 001000 | 00107A

• T | 001000 | 1E | 141033 | 482039 | 001036 | …

• T | 00101E | 15 | 0C1036 | 482061 | 081033 | …

• …

• T | 002073 | 07 | 382064 | 4C0000 | 05

• E | 001000