PASS2_OF_2_PASS_ASSEMBLER

IMPLEMENTATION OF PASS-2 OF 2-PASS ASSEMBLER

AIM

To write a C-program to implement pass-2  assembler.

ALGORITHM

1.Read first line from the intermediate file.

2.Check to see if the opcode from the first line read is “START”.If so then write label,opcode and operand field values of corresponding statement directly to final output files.

3.Start the following processing for other lines in intermediate file if it is not a comment line until an “END” statement is reached.

4.Start writing labels LOCCTR opcode and operand fields of corresponding statements to the output file along with the object code.The object code is found by assembling each statement opcode machine equivalent with the label address.

5.If there is no symbol or label in the operand field , then the operand address is assigned as zero and it is assembled with object code of instruction.

6.If OPCODE is BYTE,WORD,RESB etc are convert constants to object code close operand file and exit.

PROGRAM:

#include<stdio.h>

#include<conio.h>

#include<string.h>

void main()

 {

  char a[10],ad[10],label[10],opcode[10],operand[10],symbol[10],ch;

  int st,diff,i,address,add,len,actual_len,finaddr,prevaddr,j=0;

  char mnemonic[15][15]={"LDA","STA","LDCH","STCH"};

  char code[15][15]={"33","44","53","57"};

  FILE *fp1,*fp2,*fp3,*fp4;

  clrscr();

  fp1=fopen("ASSMLIST.DAT","w");

  fp2=fopen("SYMTAB2.DAT","r");

  fp3=fopen("INTERMED.DAT","r");

  fp4=fopen("OBJCODE.DAT","w");

  fscanf(fp3,"%s%s%s",label,opcode,operand);

  while(strcmp(opcode,"END")!=0)

   {

     prevaddr=address;

     fscanf(fp3,"%d%s%s%s",&address,label,opcode,operand);

   }

  finaddr=address;

  fclose(fp3);

  fp3=fopen("INTERMED.DAT","r");

  fscanf(fp3,"%s%s%s",label,opcode,operand);

  if(strcmp(opcode,"START")==0)

   {

  fprintf(fp1,"\t%s\t%s\t%s\n",label,opcode,operand);

  fprintf(fp4,"H^%s^00%s^00%d\n",label,operand,finaddr);

  fscanf(fp3,"%d%s%s%s",&address,label,opcode,operand);

  st=address;

  diff=prevaddr-st;

  fprintf(fp4,"T^00%d^%d",address,diff);

 }

while(strcmp(opcode,"END")!=0)

 {

   if(strcmp(opcode,"BYTE")==0)

    {

                fprintf(fp1,"%d\t%s\t%s\t%s\t",address,label,opcode,operand);

                len=strlen(operand);

                actual_len=len-3;

                fprintf(fp4,"^");

                for(i=2;i<(actual_len+2);i++)

                 {

                                itoa(operand[i],ad,16);

                                fprintf(fp1,"%s",ad);

                                fprintf(fp4,"%s",ad);

                 }

                fprintf(fp1,"\n");

    }

   else if(strcmp(opcode,"WORD")==0)

    {

                len=strlen(operand);

                itoa(atoi(operand),a,10);

                fprintf(fp1,"%d\t%s\t%s\t%s\t00000%s\n",address,label,opcode,operand,a);

                fprintf(fp4,"^00000%s",a);

    }

   else if((strcmp(opcode,"RESB")==0)||(strcmp(opcode,"RESW")==0))

   fprintf(fp1,"%d\t%s\t%s\t%s\n",address,label,opcode,operand);

   else

    {

                while(strcmp(opcode,mnemonic[j])!=0)

                j++;

                if(strcmp(operand,"COPY")==0)

                fprintf(fp1,"%d\t%s\t%s\t%s\t%s0000\n",address,label,opcode,operand,code[j]);

                else

                 {

                                rewind(fp2);

                                fscanf(fp2,"%s%d",symbol,&add);

                                while(strcmp(operand,symbol)!=0)

                                fscanf(fp2,"%s%d",symbol,&add);

                                fprintf(fp1,"%d\t%s\t%s\t%s\t%s%d\n",address,label,opcode,operand,code[j],add);

                                fprintf(fp4,"^%s%d",code[j],add);

                 }

    }

   fscanf(fp3,"%d%s%s%s",&address,label,opcode,operand);

 }

fprintf(fp1,"%d\t%s\t%s\t%s\n",address,label,opcode,operand);

fprintf(fp4,"\nE^00%d",st);

printf("\nIntermediate file is converted into object code");

fcloseall();

printf("\n\nThe contents of intermediate file:\n\n\t");

fp3=fopen("INTERMED.DAT","r");

ch=fgetc(fp3);

while(ch!=EOF)

 {

   printf("%c",ch);

   ch=fgetc(fp3);

 }

printf("\n\nThe contents of symbol table:\n\n");

fp2=fopen("SYMTAB2.DAT","r");

ch=fgetc(fp2);

while(ch!=EOF)

 {

   printf("%c",ch);

   ch=fgetc(fp2);

 }

printf("\n\nThe contents of output file:\n\n");

fp1=fopen("ASSMLIST.DAT","r");

ch=fgetc(fp1);

while(ch!=EOF)

 {

   printf("%c",ch);

   ch=fgetc(fp1);

 }

printf("\n\nThe contents of Object code file:\n\n");

fp4=fopen("OBJCODE.DAT","r");

ch=fgetc(fp4);

while(ch!=EOF)

 {

   printf("%c",ch);

   ch=fgetc(fp4);

 }

fcloseall();

getch();

}

INPUT FILES:

SYMTAB.DAT

ALPHA 2012

FIVE 2015

CHARZ 2018

C1 2019

INTERMED.DAT

COPY     START   2000

2000       **           LDA        FIVE

2003       **           STA        ALPHA

2006       **           LDCH     CHARZ

2009       **           STCH      C1

2012       ALPHA  RESW    1

2015       FIVE       WORD   5

2018       CHARZ  BYTE      C'EOF'

2019       C1           RESB      1

2020       **           END       **

OUTPUT:

RESULT

  Thus pass -2 assembler has been successfully implemented using C –programme.