PASS ONE OF TWO PASS ASSEMBLER

AIM:

To write a C program to implement the operation of pass1 of two pass assembler.

ALGORITHM:

Step 1: Start the program.

Step 2: Create a structure to declare the input variables and another structure for output variables.

Step 3: Create a structure for symbol table declaration.

Step 4: In the main function, get the label, operand and opcode.

Step 5: Compare the opcode with START and set the location counter.

Step 6: Compare the output address with the location counter, if true copy the inputs to the output.

Step 7: Using while compare opcode with END and if its not true compare the labels.

Step 8: Set the flag value and equals the symbol table address with location counter value.

Step 9: Compare the opcode with WORD if true add 3 to location counter.

Step 10: If the opcode is BYTE then find the length of operand and equate to location counter.

Step 11: If it is RESW, then add 3 into operand value to location counter.

Step 12: If it is RESB, then equate the value of operand value to location counter.

Step 13: Print the table.

Step 14: Stop the program.

SOURCE CODE:

/* PASS ONE OF TWO PASS ASSEMBLER */

#include<stdio.h>

#include<conio.h>

struct input

{

char label[10],opcode[10],operand[10];

}in[20];

struct output

{

char label[10],opcode[10],operand[10];

int address;

}op[20];

struct symbol

{

char label[10],value[10];

int address;

}sym[20];

int s=1,o=1,i=1,x,flag=0;

int locctr,a,temp,l,t;

char loc[10];

void main()

{

clrscr();

printf("\nEnter the label opcode and operand: ");

scanf("%s%s%s",in[i].label,in[i].opcode,in[i].operand);

if(strcmp(in[i].opcode,"START")==0)

{

locctr=atoi(in[i].operand,10);

}

else

{

locctr=0;

}

op[o].address=locctr;

strcpy(op[o].label,in[i].label);

strcpy(op[o].opcode,in[i].opcode);

strcpy(op[o].operand,in[i].operand);

o++;

i++;

temp=locctr;

printf("\nEnter the label opcode and operand: ");

scanf("%s%s%s",in[i].label,in[i].opcode,in[i].operand);

while(strcmp(in[i].opcode,"END")!=0)

{

if(strcmp(in[i].label,"-")!=0)

{

for(x=1;x<=s;x++)

{

if(strcmp(in[i].label,sym[s].label)==0)

{

flag=1;

break;

}

}

if(flag==0)

{

strcpy(sym[s].label,in[i].label);

sym[s].address=locctr;

}

else

{

printf("\nAlready exists");

}

}

if(strcmp(in[i].opcode,"WORD")==0)

{

strcpy(sym[s].value,in[i].operand);

locctr=locctr+3;

s++;

}

else if(strcmp(in[i].opcode,"BYTE")==0)

{

strcpy(sym[s].value,in[i].operand);

l=strlen(in[i].operand);

locctr=locctr+l;

s++;

}

else if(strcmp(in[i].opcode,"RESW")==0)

{

locctr=locctr+(3*atoi(in[i].operand));

s++;

}

else if(strcmp(in[i].opcode,"RESB")==0)

{

locctr=atoi(in[i].operand);

s++;

}

else

{

locctr=locctr+3;

}

op[o].address=temp;

strcpy(op[o].label,in[i].label);

strcpy(op[o].opcode,in[i].opcode);

strcpy(op[o].operand,in[i].operand);

o++;

i++;

temp=locctr;

printf("\nEnter the label opcode and operand: ");

scanf("%s%s%s",in[i].label,in[i].opcode,in[i].operand);

}

printf("\n\n\t\t\tINPUT PROGRAM\n");

printf("\nLABEL\tOPCODE\tOPERAND");

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

{

printf("\n\n%s",in[l].label);

printf("\t%s",in[l].opcode);

printf("\t%s",in[l].operand);

}

printf("\n\n\t\t\tOUTPUT PROGRAM");

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

for(l=1;l<o;l++)

{

printf("\n\n%d",op[l].address);

printf("\t %s",op[l].label);

printf("\t %s",op[l].opcode);

printf("\t %s",op[l].operand);

}

printf("\n\n\t\tSYMBOL TABLE");

printf("\n\nADDRESS\tLABEL\tVALUE");

for(l=1;l<=s;l++)

{

printf("\n\n%d",sym[l].address);

printf("\t%s",sym[l].label);

printf("\t%s",sym[l].value);

}

getch();

}

OUTPUT:

Enter the label opcode and operand: COPY START 1000

Enter the label opcode and operand: -   ADD     ONE

Enter the label opcode and operand: -   SUB     TWO

Enter the label opcode and operand: ONE WORD    1

Enter the label opcode and operand: TWO WORD    2

Enter the label opcode and operand: -   END     1000

                        INPUT PROGRAM

LABEL   OPCODE  OPERAND

COPY    START   1000

-       ADD     ONE

-       SUB     TWO

ONE     WORD    1

TWO     WORD    2

-       END     1000

                        OUTPUT PROGRAM

ADDRESS LABEL   OPCODE  OPERAND

1000     COPY    START   1000

1000     -       ADD     ONE

1003     -       SUB     TWO

1006     ONE     WORD    1

1009     TWO     WORD    2

                SYMBOL TABLE

ADDRESS LABEL   VALUE

1006    ONE     1

1009    TWO     2

Result

Thus above program executed and output verified.