Implementation of the Pass-II of the assembler

Problem Statement: Implementation of the Pass-II of the assembler produces a target program which is in the machine language of the target computer.

Theory: Important data structures used by Pass-II are

SYMTAB
LITTAB &
POOLTAB

LC : Location counter

littab_ptr : Points to an entry in LITTAB

pooltab_ptr : Points to an entry in POOLTAB

machine_code_buffer : Area for constructing code for one statement

code_area : Area for assembling the target program

code_area_address : Contains address of code_area

Input program: An Assembly Language Program

START 501

NEXT: MOVER AREG,A

PRINT ONE

MOVER AREG,='5'

ADD CREG,='1'

LTORG

ADD DREG,='9'

ONE DC '1'

A DS 3

F EQU 200

END

Output of PASS-I for the above assembly language program will generate following tables:

SYMTAB:

NEXT 501 1

A 519 3

ONE 518 1

F 200 1

LITTAB:

='5' 513

='1' 514

='9' 522

POOLTAB:

IC (Intermediate Code):

(AD,01) (C,501)

(IS,04) 1 (S,02)

(IS,09) (S,03)

(IS,04) 1 (L,01)

(IS,01) 3 (L,02)

(AD,05)

(IS,01) 4 (L,03)

(DL,01) (C,1)

(DL,02) (C,3)

(AD,04) (C,200)

(AD,02)

Now this output of PASS-I is given as input to the Pass-II. PASS-II will perform all the required operations to produce target program which is in machine understandable format.

The output of PASS-II is shown here.

OUTPUT of PASS-II:

1. What is the role of PASS-II in assembler?

2. What is the input & output of PASS-II.

3. Explain the processing steps of PASS-II in short.

4. Explain the output of PASS-II in detail.

MAIN PROGRAM:

#include<iostream.h>
#include<conio.h>
#include<string.h>
#include<fstream.h>
#include<stdlib.h>
# define MAX 80
# define MAX1 40
int get_symbol(int);
int get_lit(int);
void main()
{
int pool_tab[MAX1],ptr=0,sr_no,addr,i,next_srno;
fstream f,fp;
char str[MAX1],str1[MAX],str2[MAX],str3[MAX],mem_buf[MAX1],a[20];
clrscr();
f.open("sp\\pt.txt",ios::beg|ios::in);
while(f.getline(str,MAX))
{
     pool_tab[ptr]=atoi(str);
     ptr++;
     if(f.eof())
     break;
}
   ptr=0;
   f.close();
f.open("sp\\ic.txt",ios::beg|ios::in);
while(f.getline(str,MAX))
{
      strcpy(str1,strtok(str," "));
      strcpy(str2,strtok(NULL," "));
      strcpy(str3,strtok(NULL," "));
      strcpy(str1,strtok(str1,"("));
      strcpy(str1,strtok(str1,","));
      if(strcmp(str1,"AD")==0)
      {
       strcpy(str1,strtok(NULL,","));
       strcpy(str1,strtok(str1,")"));
       if(strcmp(str1,"05")==0)
       {
       fp.open("sp\\lt.txt",ios::beg|ios::in);
    sr_no=pool_tab[ptr];
    ptr++;
    next_srno=pool_tab[ptr];
    for(i=0;i<sr_no;i++)
    {
       fp.getline(str,MAX);
    }
    for(i=sr_no;i<next_srno;i++)
    {
        fp.getline(str,MAX);
        strcpy(str1,strtok(str," "));
        strcpy(str1,strtok(str1,"'"));
        strcpy(str1,strtok(NULL,"'"));
        strcpy(mem_buf,"+00 0 ");
        strcat(mem_buf,str1);
        cout<<mem_buf<<endl;
       }
    fp.close();
    }
    if(strcmp(str1,"02")==0)
       {
    sr_no=pool_tab[ptr];
    fp.open("sp\\lt.txt",ios::beg|ios::in);
    for(i=0;i<sr_no;i++)
    {
       fp.getline(str,MAX);
    }
    while(fp.getline(str,MAX))
    {
        strcpy(str1,strtok(str," "));
        strcpy(str1,strtok(str1,"'"));
        strcpy(str1,strtok(NULL,"'"));
        strcpy(mem_buf,"+00 0 ");
        strcat(mem_buf,str1);
        cout<<mem_buf<<endl;
       if(fp.eof())
       break;
       }
    }
}
if(strcmp(str1,"IS")==0)
      {
    strcpy(str1,strtok(NULL,","));
       strcpy(str1,strtok(str1,")"));
       strcpy(mem_buf,"+");
       strcat(mem_buf,str1);
       strcat(mem_buf," ");
       if(!strstr(str2,"("))
       {
    strcat(mem_buf,str2);
    strcat(mem_buf," ");
       }
       else
       {
    strcpy(str3,str2);
    strcat(mem_buf,"0");
    strcat(mem_buf," ");
       }
       strcpy(str1,strtok(str3,"("));
       strcpy(str1,strtok(str1,","));
       if(strcmp(str1,"S")==0)
       {
    strcpy(str1,strtok(NULL,","));
       strcpy(str1,strtok(str1,")"));
       sr_no=atoi(str1);
       addr=get_symbol(sr_no);
       itoa(addr,a,10);
       strcat(mem_buf,a);
       }
       if(strcmp(str1,"L")==0)
       {
    strcpy(str1,strtok(NULL,","));
       strcpy(str1,strtok(str1,")"));
       sr_no=atoi(str1);
       addr=get_lit(sr_no);
       itoa(addr,a,10);
       strcat(mem_buf,a);
       }
       cout<<mem_buf<<endl;
}
    if(strcmp(str1,"DL")==0)
    {
    strcpy(str1,strtok(NULL,","));
    strcpy(str1,strtok(str1,")"));
    if(strcmp(str1,"01")==0)      //For DC only
    {
    strcpy(str1,strtok(str2,"("));
    strcpy(str1,strtok(str1,","));
    if(strcmp(str1,"C")==0)
    {
       strcpy(str1,strtok(NULL,","));
       strcpy(str1,strtok(str1,")"));
    strcpy(mem_buf,"+00 0 ");
    strcat(mem_buf,str1);
    cout<<mem_buf<<endl;
       }
     }

}
}
f.close();
fp.close();
getch();
}
int get_symbol( int sr_no)
{
fstream f;
char str[MAX1],str1[MAX1];
f.open("sp\\st.txt",ios::beg|ios::in);
for(int i=0;i<sr_no;i++)
{
   f.getline(str,MAX);
}
strcpy(str1,strtok(str," "));
strcpy(str1,strtok(NULL," "));
int addr=atoi(str1);
f.close();
return addr;
}
int get_lit(int sr_no)
{
fstream f;
char str[MAX1],str1[MAX1];
f.open("sp\\lt.txt",ios::beg|ios::in);
for(int i=0;i<sr_no;i++)
{
   f.getline(str,MAX);
}
strcpy(str1,strtok(str," "));
strcpy(str1,strtok(NULL," "));
int addr=atoi(str1);
f.close();
return addr;
}

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Implementation of the Pass-II of the assembler

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