One Pass Assembler

Translate assembly language programs to object programs or machine code is called an Assembler. •

One-pass assemblers are used when it is necessary or desirable to avoid a second pass over the source program •

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the external storage for the intermediate le between two passes is slow or is inconvenient to use ain problem! forward references to both data and instructions One simple way to eliminate this problem! re"uire that all areas be dened before they are referenced. #t is possible$ although inconvenient$ to do so for data items. %orward jump to instruction items cannot be easily eliminated. • •

&ata structures for assembler!

Op code table 'oo(ed up for the translation of mnemonic code • •

(ey! mnemonic code result!bits

)ashing is usually used • • •

once prepared$ the table is not changed e*cient loo(up is desired since mnemonic code is predened$ the hashing function can be tuned a priori

The table may have the instruction format and length •

to decide where to put op code bits$ operands bits$ o+set bits

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for variable instruction si,e used to calculate the address

Symbol table tored and loo(ed up to assign address to labels • •

e*cient insertion and retrieval is needed deletion does not occur

&i*culties in hashing • roblem •

nonrandom(eys

the si,e varies widely

pass /! loop until the end of the program /. 0ead in a line of assembly code 1. Assign an address to this line •

increment 2 3word addressing or byte addressing4

5. ave address values assigned to labels •

insymboltables

6. rocess assembler directives • •

constant declaration space reservation

Algorithm for Pass 1 assembler: begin if starting address is given 'O77T0 8 starting address9 else 'O77T0 8 :9 while O7O&; <8 ;2& do

99 or ;O%

begin read a line from the code if there is a label if this label is in =TA>$ then error else insert 3label$ 'O77T04 into =TA> search OTA> for the op code if found 'O77T0 ?8 2

99 2 is the length of this instruction 36

for #4 else if this is an assembly directive update 'O77T0 as directed else error write line to intermediate le

end program si,e 8 'O77T0 - starting address9 end

'oad-and-go assembler • 'oad-and-go assembler generates their object code in memory for immediate execution. •

2o object program is written out$ no loader is needed.

• #t is useful in a system oriented toward program development and testing such that the e*ciency of the assembly process is an important consideration

%orward 0eference! o o o

o

o

o

o

'oad-and-go assembler Omits the operand address if the symbol has not yet been dened ;nters this undened symbol into =TA> and indicates that it is undened Adds the address of this operand address to a list of forward references associated with the =TA> entry cans the reference list and inserts the address when the de nition for the symbol is encountered. 0eports the error if there are still =TA> entries indicated undened symbols at the end of the program earch =TA> for the symbol named in the ;2& statement and jumps to this location to begin execution if there is no error