Raj Kumar Give the algorithm for pass1 of and 2 pass assembler
Explanation
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The Algorithm for Pass 1:
Begin
read first input line
if OPCODE = ‘START’ then begin
save #[Operand] as starting addr
initialize LOCCTR to starting address
write line to intermediate file
read next line
end( if START)
Else
initialize LOCCTR to 0
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
(if symbol)
search OPTAB for OPCODE
if found then
add 3 (instr length) 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
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 {pass 1}
The Algorithm for Pass 2:
Begin
read 1st input line
if OPCODE = ‘START’ then
begin
write listing line
read next input line
end
write Header record to object program
initialize 1st Text record
while OPCODE != ‘END’ do
begin
if this is not 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 field then
if found then
Begin
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
else if OPCODE = ‘BYTE’ or ‘WORD” then
convert constant to object code
if object code doesn’t fit into current Text record then
Begin
Write text record to object code
initialize new Text record
end
add object code to Text record
end {if not comment}
write listing line
read next input line
end
write listing line
read next input line
write last listing line
End {Pass 2}
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Raj Kumar
Computer Science And Engineering
Raj Kumar