ASSEMBLER PASS - 1
Let me clear what a Two-Pass Assembler does in Pass-1 and Pass-2 in nutshell:
Pass 1:
- Separate the labels,mnemonic op-codes and operand fields.
- Calculate the size of each assembly instruction.
- Update Symbol Table with addresses of labels and variables.
- Update Literal Table with addresses of literals.
Pass 2:
- Replace variable with its address by referring to Symbol Table.
- Replace literal with its address by referring to Literal Table.
- Generate the machine code.
Here is the code which illustrates the above concept:
#include<stdio.h>
#include<string.h>
#include<conio.h>
#include<stdlib.h>
struct MOT
{
char mname[10];
int nmopc,nmsize;
};
struct RT
{
char rname[10];
int rtopc;
};
struct POT
{
char pname[10];
int popc;
};
struct ST
{
char sname[10];
int len,val,addr;
};
struct LT
{
char lname[10];
int addr;
};
struct MOT m[5]=
{
{"mov",41,4},
{"add",42,4},
{"sub",43,4},
{"div",44,4}
};
struct POT p[5]=
{
{"start",0},
{"end",1},
{"origin",2},
{"ltorg",3}
};
struct RT r[5]=
{
{"areg",1},
{"breg",2},
{"creg",3},
{"dreg",4}
};
struct ST s[10];
struct LT l[5];
void main()
{
FILE *fptr;
int flag=0,flag1=0,i,j,f,num,x,z,lc=0,cnt=0,cnt1=0,cnt2=0,k,q,val1,y,flag2=0;
float scnt;
char ch,a[10],b[10],c[10],d[10],e[10],filename[10],ch1;
clrscr();
printf("\n Enter Filename: ");
scanf("%s",&filename);
fptr=fopen(filename,"r");
if(fptr==NULL)
{
printf("\n no file found");
exit(0);
}
ch=fgetc(fptr);
i=0;
while(ch!=EOF)
{
if(flag==0)
{
if(ch==' ' || ch==',' || ch=='\n')
{
a[i]='\0';
flag=1;
}
else
{
a[i]=ch;
i++;
//ch=fgetc(fptr);
}
}
if(flag==1)
{
for(j=0;j<4;j++)
{
x=strcmp(p[j].pname,a);
if(x==0)
{
if(j==0 || j==2)
{
ch=fgetc(fptr);
z=0;
while(ch!='\n')
{
b[z]=ch;
z++;
ch=fgetc(fptr);
}
b[z]='\0';
lc=atoi(b);
}
if(j==1 || j==3)
{
while(cnt<cnt1)
{
l[cnt].addr=lc;
printf("%d)",lc);
printf("%s\n",l[cnt].lname);
lc=lc+4;
cnt++;
}
}
flag1=1;
}
}
for(j=0;j<3;j++)
{
x=strcmp(a,r[j].rname);
if(x==0)
{
printf("%d",r[j].rtopc);
flag1=1;
}
}
for(j=0;j<3;j++)
{
x=strcmp(a,m[j].mname);
if(x==0)
{
printf("%d) ",lc);
printf(" %d ",m[j].nmopc);
lc=lc+m[j].nmsize;
flag1=1;
}
}
if(a[0] =='=')
{ //ch=fgetc(fptr);
//if(ch=='=')
//{
printf("%s",a);
strcpy(l[cnt1].lname,a);
l[cnt1].addr=0;
cnt1++;
flag1=1;
// }
}
if(flag1!=1)
{
k=0;
if(ch=='\n')
{ printf("%s",a);
flag2=1;
}
if(flag2==0)
{
num=0;
for(f=0;a[f]!='\0';f++)
{
num=num+(f*a[f]);
}
scnt=num%10;
strcpy(s[scnt].sname,a);
ch=fgetc(fptr);
while(ch!=' ')
{
b[k]=ch;
k++;
ch=fgetc(fptr);
}
b[k]='\0';
x=strcmp(b,"dc");
if(x==0)
{
y=0;
ch=fgetc(fptr);
while(ch!='\n')
{
c[y]=ch;
y++;
ch=fgetc(fptr);
}
c[y]='\0';
for(f=0;c[f]!='\0';f++)
{
if(c[f]=='\'')
c[f]=' ';
}
val1=atoi(c);
s[scnt].val=val1;
s[scnt].len=4;
s[scnt].addr=lc;
lc=lc+4;
//cnt2++;
printf("%d)",lc);
printf("%s",a);
}
else
{
q=0;
ch=fgetc(fptr);
while(ch!='\n')
{
c[q]=ch;
q++;
ch=fgetc(fptr);
}
c[q]='\0';
val1=atoi(c);
val1=val1*4;
lc=lc+val1;
s[scnt].val=0;
s[scnt].addr=lc;
s[scnt].len=val1;
//cnt2++;
printf("%d)",lc);
printf("%s",a);
}
}
}
if(ch==',')
printf(" ");
if(ch=='\n')
printf("\n");
if(ch==' ')
printf(" ");
flag=0;
flag1=0;
i=0;
flag2=0;
}
ch=fgetc(fptr);
}
printf("%d)",lc);
for(i=0;i<cnt1;i++)
{
printf("%s\t%d",l[i].lname,l[i].addr);
printf("\n");
}
for(i=0;i<10;i++)
{
printf("%s\t%d\t%d\t%d",s[i].sname,s[i].addr,s[i].len,s[i].val);
printf("\n");
}
fclose(fptr);
getch();
}
*********************************************************************************
To check the output of the above code, you can use the following input or you can try your own inputs.Please copy and paste the input in a text file.
NOTE: Please paste this input in the input file that you are opening in your program.
INPUT 1:
start 200
add creg,sum
mov areg,='7'
add areg,creg
ltorg
mov creg,='5'
origin 400
add areg,breg
ltorg
add areg,='2'
mov creg,='1'
mov areg,creg
sum dc '08'
xyz ds 30
end
INPUT 2:
start 100
add areg,='1'
sub breg,as
as dc '02'
df dc '01'
cd dc '03'
mn ds 04
br dc '07'
end
INPUT 3:
start 200
mov areg,='10'
sum dc '20'
add breg,creg
ltorg
mov areg,='2'
mov creg,sum
end
*********************************************************************************
Flow-Chart for Pass-1:
 |
| Flow-Chart: Assembler Pass-1 |
THANKS
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