Two pass Assembler

WALCHAND INSTITUTE OF TECHNOLOGY, SOLAPUR
PRESENTING A SEMINAR ON
TWO PASS ASSEMBLERS

TWO PASS ASSEMBLER
• Processing the source program into two passes.
• The internal tables and subroutines that are used only during Pass 1.
• The SYMTAB, LITTAB, and OPTAB are used by both passes.
• The main problems to assemble a program in one pass involves
forward references.
Pass 1
Forward references
table String storage
buffer Partially
configured object file
Assembly
Language
Pass 2 Machine
Language

• PASS 1
• Assign addresses to all statements in the program.
• Addresses of symbolic labels are stored.
• Some assemble directives will be processed.
• PASS 2
• Translate opcode and symbolic operands.
• Generate data values defined by BYTE,WORD etc.
• Assemble directives will be processed.
• Write the object program and assembly listing.

DATA STRUCTURES
• Two major data structures:
• Operation Code Table (OPTAB): is used to look up mnemonic operation
codes and translate them to their machine language equivalents
• Symbol Table (SYMTAB): is used to store values (addresses) assigned to
labels
• Variable:
• Location Counter (LOCCTR) is used to help the assignment of addresses
• LOCCTR is initialized to the beginning address specified in the START
statement
• The length of the assembled instruction or data area to be generated is
added to LOCCTR

Algorithm for Pass 1 of Assembler(3/1)
read first input line
if OPCODE=‘START’ then
begin
save #[OPERAND] as starting address
initialize LOCCTR to starting address
write line to intermediate file
read next input line
end
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
insert (LABEL, LOCCTR) into SYMTAB
end {if symbol}
search OPTAB for OPCODE
if found then
add 3 {instruction 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 {if BYTE}
else
set error flag (invalid operation code)
end {if not a comment}
write line to intermediate file
end {while not END}
Write last line to intermediate file
Save (LOCCTR-starting address) as program length

read first input line (from intermediate file)
If OPCODE=‘START’ then
begin
write listing line
end {if START}
Write Header record to object program
Initialize first Text record
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 will not fit into the current Text record then
begin
write Text record to object program
initialize new Text record
end
add object code to Text record
end {if not comment}
write listing line
end {while not END}
write last Text record to object program
Write End record to object program
Write last listing line

#include<stdio.h>
#include<string.h>
#include<conio.h>
void main()
{
char *code[9][4]={
{"PRG1","START","",""},
{"","USING","*","15"},
{"","L","",""},
{"","A","",""},
{"","ST","",""},
{"FOUR","DC","F",""},
{"FIVE","DC","F",""},
{"TEMP","DS","1F",""},
{"","END","",""}
};
char av[2],avail[15]={'N','N','N','N','N','N','N','N','N','N','N','N','N','N','N'};
int i,j,k,count[3],lc[9]={0,0,0,0,0,0,0,0,0},loc=0;
clrscr();
Program

printf("----------------------------------------------------n");
printf("LABELttOPCODEn");
printf("----------------------------------------------------nn");
for(i=0;i<=8;i++)
{
for(j=0;j<=3;j++)
{
printf("%stt",code[i][j]);
}
j=0;
printf("n");
}
getch();
printf("-----------------------------------------------------");
printf("nVALUES FOR LC : nn");
for(j=0;j<=8;j++)
{
if((strcmp(code[j][1],"START")!=0)&&(strcmp(code[j][1],"USING")!=0)&&(strcmp(code[j][1],"L")!=0))
lc[j]=lc[j-1]+4;
printf("%dt",lc[j]);
}

printf("nnSYMBOL TABLE:n----------------------------------------------------n");
printf("SYMBOLttVALUEttLENGTHttR/A");
printf("n----------------------------------------------------n");
for(i=0;i< 9;i++)
{
if(strcmp(code[i][1],"START")==0)
{
printf("%stt%dtt%dtt%cn",code[i][0],loc,4,'R');
}
else
if(strcmp(code[i][0],"")!=0)
{
printf("%stt%dtt%dtt%cn",code[i][0],loc,4,'R');
loc=4+loc;
}
else
if(strcmp(code[i][1],"USING")==0)
{
}
else
{
loc=4+loc;

printf("----------------------------------------------------");
printf("nnBASE TABLE:n-------------------------------------------------------n");
printf("REG NOttAVAILIBILITYtCONTENTS OF BASE TABLE");
printf("n-------------------------------------------------------n");
for(j=0;j<=8;j++)
{
if(strcmp(code[j][1],"USING")!=0)
{
}
else
{
strcpy(av,code[j][3]);
}
}
count[0]=(int)av[0]-48;
count[1]=(int)av[1]-48;
count[2]=count[0]*10+count[1];
avail[count[2]-1]='Y';

for(k=0;k< 16;k++)
{
printf(" %dtt %cn",k,avail[k-1]);
}
printf("-------------------------------------------------------n");
printf("Continue..??");
getchar();
printf("PASS2 TABLE:nn");
printf("LABELttOP1ttLCtt");
printf("n----------------------------------------------------n");
loc=0;
for(i=0;i<=8;i++)
{
for(j=0;j<=3;j++)
{
printf("%stt",code[i][j]);
}
j=0;
printf("n");
}
printf("-----------------------------------------------------");
getch();

----------------------------------------------------
LABEL OPCODE
----------------------------------------------------
PRG1 START
USING * 15
L
A
ST
FOUR DC F
FIVE DC F
TEMP DS 1F
END
-----------------------------------------------------
VALUES FOR LC :
0 0 0 4 8 12 16 20 24
OUTPUT

SYMBOL TABLE:
----------------------------------------------------
SYMBOL VALUE LENGTH R/A
----------------------------------------------------
PRG1 0 4 R
FOUR 12 4 R
FIVE 16 4 R
TEMP 20 4 R
----------------------------------------------------

BASE TABLE:
-------------------------------------------------------
REG NO AVAILIBILITY CONTENTS OF BASE TABLE
-------------------------------------------------------
0
1 N
2 N
3 N
4 N
5 N
6 N
7 N
8 N
9 N
10 N
11 N
12 N
13 N

Continue..??
PASS2 TABLE:
LABEL OP1 LC
----------------------------------------------------
PRG1 START
USING * 15
L
A
ST
FOUR DC F
FIVE DC F
TEMP DS 1F
END
-----------------------------------------------------

Two pass Assembler

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