ONE PASS ASSEMBLER

AIM:

To write a C program to implement one pass assembler.

ALGORITHM:

Step 1: Start the program.

Step 2: Create the structure for symbol table, opcode, instruction, input and output.

Step 3: In the main function assign the opcode values.

Step 4: Get the label, opcode, operand from the user and compare the label. If it is start, execute the program.

Step 5: Then convert the operand to integer and store counter value.

Step 6: Create the symbol table from the instruction.

Step 7: Do necessary operations and display the result.

Step 8: Stop the program.

SOURCE CODE:

/* ONE PASS ASSEMBLER */

#include<stdio.h>

#include<conio.h>

#include<string.h>

struct input

{

char label[20],opc[20],opr[20];

}ip[20];

struct symbol

{

char sym[20],val[20];

int addr;

}s[20];

struct obj

{

int addr;

char obj[20];

}ob[20];

struct table

{

int addr;

char sym[20];

}t[20];

struct optab

{

char opc[20],val[20];

}op[20];

void main()

{

int n=0,t1=0,o1=0,in=0,i,j,temp,lc,flag=0,k;

char str[20];

clrscr();

strcpy(op[1].opc,"ADD");

strcpy(op[1].val,"10");

strcpy(op[2].opc,"SUB");

strcpy(op[2].val,"20");

strcpy(op[3].opc,"MUL");

strcpy(op[3].val,"30");

strcpy(op[4].opc,"LDA");

strcpy(op[4].val,"40");

strcpy(op[5].opc,"LDX");

strcpy(op[5].val,"50");

printf("\n\t\tEnter the instruction");

printf("\nLabel\tOpcode\tOperand\n");

for(in=1;;in++)

{

scanf("\n%s\t%s\t%s",ip[in].label,ip[in].opc,ip[in].opr);

if(!(strcmp(ip[in].opc,"END")))

break;

}

if(!(strcmp(ip[1].opc,"START")))

lc=(atoi(ip[1].opr));

else

lc=0;

for(j=1;j<=in;j++)

{

temp=lc;

if(!(strcmp(ip[j].opc,"RESW")))

lc=(atoi(ip[j].opr))*3+lc;

else if(!(strcmp(ip[j].opc,"RESB")))

lc=(atoi(ip[j].opr))+lc;

else if(!(strcmp(ip[j].opc,"BYTE")))

lc=(strlen(ip[j].opr))+lc;

else if(!(strcmp(ip[j].opc,"START")));

else

lc=lc+3;

if(strcmp(ip[j].label,"-")!=0)

{

for(i=1;(i<=n)&&(strcmp(ip[j].label,s[i].sym));i++);

if(i==(n+1))

{

n++;

strcpy(s[n].sym,ip[j].label);

if((!(strcmp(ip[j].opc,"WORD"))||(!(strcmp(ip[j].opc,"BYTE")))))

strcpy(s[n].val,ip[j].opr);

else

strcpy(s[n].val,"0");

s[n].addr=temp;

}

}

flag=0;

for(i=1;i<=5;i++)

if(!(strcmp(op[i].opc,ip[j].opc)))

{

strcpy(ob[++o1].obj,op[i].val);

for(k=1;k<=n;k++)

if(!(strcmp(s[k].sym,ip[j].opr)))

{

itoa(s[k].addr,str,10);

strcat(ob[o1].obj,str);

}

flag=1;

}

if(flag==1);

else if((!(strcmp(ip[j].opc,"WORD")))||(!(strcmp(ip[j].opc,"BYTE"))))

strcpy(ob[++o1].obj,ip[j].opr);

else

if((!(strcmp(ip[j].opc,"RESW")))||(!(strcmp(ip[j].opc,"RESB")))||(!(strcmp(ip[j].opc,"START")))||(!(strcmp(ip[j].opc,"END"))))

strcpy(ob[++o1].obj,"-");

else

strcpy(ob[++o1].obj,"00");

ob[o1].addr=temp;

if((!(strcmp(ip[j].opc,"RESW")))||(!(strcmp(ip[j].opc,"RESB")))||(!(strcmp(ip[j].opc,"BYTE")))||(!(strcmp(ip[j].opc,"START")))||(!(strcmp(ip[j].opc,"END")))||(!(strcmp(ip[j].opc,"WORD"))));

else

{

for(i=1;(i<=n)&&(strcmp(ip[j].opr,s[i].sym));i++);

if(i==(n+1))

{

t1++;

strcpy(t[t1].sym,ip[j].opr);

t[t1].addr=temp;

}

}

}

if((!(strcmp(ip[1].opc,"START"))))

flag=2;

else

flag=1;

for(i=1;i<=t1;i++)

{

for(j=1;(j<=n)&&(strcmp(t[i].sym,s[j].sym));j++);

if(j!=(n+1))

{

for(k=flag;k<=o1;k++)

{

if(ob[k].addr==t[i].addr)

{

itoa(s[j].addr,str,10);

strcat(ob[k].obj,str);

}

}

}

}

printf("\n\nINPUT TABLE\n");

printf("\n\nADDRESS\tLABEL\tOPCODE\tOPERAND\n");

for(i=1;i<=in;i++)

printf("\n\n%d\t%s\t%s\t%s\t",ob[i].addr,ip[i].label,ip[i].opc,ip[i].opr);

printf("\n\nTABLE OF INCOMPLETE INSTRUCTIONS\n");

printf("\n\nADDRESS\tSYMBOL\n");

for(i=1;i<=t1;i++)

printf("\n%d\t%s\t",t[i].addr,t[i].sym);

printf("\n\nSYMBOL TABLE\n");

printf("\n\nSYMBOL\tADDRESS\tVALUE\n");

for(i=1;i<=n;i++)

printf("\n%s\t%d\t%s\t",s[i].sym,s[i].addr,s[i].val);

printf("\n\nOBJECT CODE\n");

printf("\n\nADDRESS\tOBJECTCODE\n");

for(i=1;i<=o1;i++)

printf("\n%d\t%s\t",ob[i].addr,ob[i].obj);

getch();

}

OUTPUT:

Enter the instruction

Label   Opcode  Operand

COPY    START   2000

-       ADD     ONE

-       SUB     TWO

ONE     WORD    1

TWO     WORD    2

-       END     -

INPUT TABLE

ADDRESS LABEL   OPCODE  OPERAND

2000    COPY    START   2000

2000    -       ADD     ONE

2003    -       SUB     TWO

2006    ONE     WORD    1

2009    TWO     WORD    2

2012    -       END     -

TABLE OF INCOMPLETE INSTRUCTIONS

ADDRESS SYMBOL

2000    ONE

2003    TWO

SYMBOL TABLE

SYMBOL  ADDRESS VALUE

COPY    2000    0

ONE     2006    1

TWO     2009    2

OBJECT CODE

ADDRESS OBJECTCODE

2000    -

2000    102006

2003    202009

2006    1

2009    2

2012    -

Result

Thus above program executed and output verified.