Linking loader pass1

Objective of Linking Loader Pass 1:

1. Resolve external symbol definitions and references across multiple object modules.

2. Allocate memory and calculate the starting addresses for each object module.

3. Identify addresses requiring modifications by processing relocation records.

4. Validate external symbols to ensure there are no undefined or duplicate definitions.

5. Prepare data structures to enable efficient object code loading and linking in Pass 2.

Data Structure Used in Linking Loader Pass 1:

The primary data structure used in Pass 1 is the External Symbol Table (ESTAB). This table stores the names and addresses of external symbols across the control sections being loaded. It is typically organized using a hash structure for efficient access.

begin

get PROOADDR from operating system

set CSADDR to PROOADDR {for first control section}

while not end of input do

begin

read next input record {Header record for control section}

set CSLTH to control section length

search ESTAB for control section name

if found then

set error flag {duplicate external symbol}

else

enter control section name into ESTAB with value CSADDR

while record type ~ 'E' do

begin

read next input record

if record type = 'D' then

for each symbol in the record do

begin

search ESTAB for symbol name

if found then

set error flag (duplicate external symbol)

else

enter symbol into ESTAB with value(CSADDR + indicated address)

end {for}

end {while ~ 'E'}

add CSLTH to CSADDR {starting address for next control section}

end {while not EOF}

end {Pass 1}

H PROGD 000000 000081

D LISTX 000050

R LISTY ENDX LISTZ ENDY

T 000024 0A 04201D 88100008 060014

T 000050 0F 110014 00000C 005051 000008 120000

M 000028 05 +LISTY

M 000050 06 +LISTZ

M 000058 06 +LISTY

M 000058 06 -LISTX

E 000024

H PROGE 000000 00009F

D LISTY 000070 ENDY 000090

R LISTX LISTZ ENDZ

T 000046 0B 04100000 882037 06100000

T 000090 0F 110000 00000C 005051 000008 120070

M 000047 05 +LISTX

M 00004E 05 -LISTX

M 000090 06 -LISTX

M 000090 06 +LISTZ

M 00009C 06 +PROGE

M 00009C 06 -LISTX

E 000000

H PROGF 000000 000067

D LISTZ 000040 ENDZ 000052

R LISTX LISTY ENDY

T 000028 0C 04100000 88100008 06100000

T 000052 0F 110040 00000C 005051 000008 120000

M 00002D 05 +LISTY

M 000031 05 -LISTX

M 000052 06 -LISTX

M 000052 06 +PROGF

M 00005E 06 +LISTY

M 00005E 06 -LISTX

E

END

#include<stdio.h>

#include<string.h>

struct estab

{

  char csname[10];

  char extsym[10];

  int address;

  int length;

}es[20];

void main()

{

char input[10],name[10],symbol[10],ch;  int count=0,progaddr,csaddr,add,len;

FILE *fp1,*fp2;

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

fp2=fopen("ESTAB.DAT","w");

printf("\n\nEnter the address where the program has to be loaded : ");

scanf("%x",&progaddr); // TAKING THE PROGRAM ADDRESS FROM THE USER,GENERALLY IT IS DONE BY THE OS

csaddr=progaddr;

fscanf(fp1,"%s",input);

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

{

if(strcmp(input,"H")==0)

{

fscanf(fp1,"%s",name);

strcpy(es[count].csname,name);

strcpy(es[count].extsym,"  ");

fscanf(fp1,"%x",&add);

es[count].address=add+csaddr;

fscanf(fp1,"%x",&len);

es[count].length=len;

fprintf(fp2,"%s ** %x %x\n",es[count].csname,es[count].address,es[count].length);

count++;

}

else if(strcmp(input,"D")==0)

{

fscanf(fp1,"%s",input);

while(strcmp(input,"R")!=0)

{

strcpy(es[count].csname,"  ");

strcpy(es[count].extsym,input);

fscanf(fp1,"%x",&add);

es[count].address=add+csaddr;

es[count].length=0;

fprintf(fp2,"** %s %x\n",es[count].extsym,es[count].address);

count++;

fscanf(fp1,"%s",input);

}

csaddr=csaddr+len;

}

else if(strcmp(input,"T")==0)

{

while(strcmp(input,"E")!=0)

fscanf(fp1,"%s",input);

}

fscanf(fp1,"%s",input);

}

  fclose(fp1);

  fclose(fp2);

  fp2=fopen("ESTAB.DAT","r");

  ch=fgetc(fp2);

  while(ch!=EOF)

  {

   printf("%c",ch);

   ch=fgetc(fp2);

  }

  fclose(fp2);

}