Pass 2 of Two Pass Linking Loader

Introduction and Problem Description

a. Introduction:

1. Purpose of the Program: 

• Implements the Pass 2 of a Direct Linking Loader, which takes object code from multiple object modules and produces a memory image for execution.

2. Function of Pass 2:

• Uses the External Symbol Table (ESTAB) generated in Pass 1 to resolve external references.

• Processes text records, modifies addresses based on relocation information, and outputs the final object program in a structured format.

3. Main Tasks Performed:

• Reads and processes the ESTAB file to load external symbols and their addresses.

• Reads the INPUT file containing object code records, resolves addresses, and applies modifications.

• Generates the OUTPUT file with the final memory image in hexadecimal format.

4. Outcome: 

• Produces a fully linked memory image ready for execution.

b. Problem Description:

1. Reading and Storing External Symbols:

• The program first opens the ESTAB file and loads all external symbols, their addresses, and lengths into a table.

• The EXECADDR (execution address) is initialized to the starting address of the first control section.

2. Processing Object Code Records:

• Reads the INPUT file line by line to process Header (H), Text (T), and Modification (M) records.

• The Header Record (H) defines the control section name and its starting address.

• The Text Record (T) contains the actual object code and its length.

• The Modification Record (M) specifies addresses that require adjustments based on external symbol references.

3. Handling Address Relocation and Modifications:

• The program adjusts the addresses in the Modification Records by either adding or subtracting the address of the referenced external symbol from ESTAB.

• It supports both positive (+) and negative (-) modifications.

• The modified addresses are converted back into hexadecimal format and updated in the object code array.

4. Generating Output:

• • The program formats the final output into a structured memory image file (OUTPUT.TXT).

  • The memory image contains memory addresses and corresponding object code, formatted with tabular spacing.

  • The output is printed to the console for verification.

5. Error Handling and Validation:

• Ensures that all symbols referenced in modification records exist in the ESTAB.

• Handles potential mismatches between symbol references and definitions.

6. Objective:

• • The program resolves all external references and generates a fully linked and ready-to-execute memory image in Pass 2.

  • Ensures that all text records are correctly relocated and all modification records are processed accurately.

Pass 2 of two pass linking loader: 

Code :

Pass 2 of two pass linking loader:

#include<stdio.h>

#include<string.h>

#include<stdlib.h>

struct exttable

{

  char cextsym[20], extsym[20];

  int address, length;

} estab[20];

struct objectcode

{

 unsigned char code[15];

 int add;

} obcode[500];

void main()

{

    char temp[10];

    FILE *fp1, *fp2, *fp3;

    int i, j, x, y, pstart, exeloc, start, textloc, loc, textlen, length, location, st, s;

    int n = 0, num = 0, inc = 0, count = 0, record = 0, mloc[30], mlen[30];

    signed long int newadd;

    char operation, lbl[10], input[10], label[50][10], opr[30], ch, *add1, address[10];

    fp1 = fopen("INPUT.TXT", "r");

    fp2 = fopen("ESTAB.TXT", "r");

    fp3 = fopen("OUTPUT.TXT", "w");

    while (!feof(fp2))

    {

        fscanf(fp2, "%s %s %x %x", estab[num].cextsym, estab[num].extsym, &estab[num].address, &estab[num].length);

        num++;

    }

    exeloc = estab[0].address;

    loc = exeloc;

    start = loc;

    st = start;

    while (!feof(fp1))

    {

        fscanf(fp1, "%s", input);

        if (strcmp(input, "H") == 0)

        {

            fscanf(fp1, "%s", input);

            for (i = 0; i < num; i++)

                if (strcmp(input, estab[i].cextsym) == 0)

                {

                    pstart = estab[i].address;

                    break;

                }

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

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

        }

        do

        {

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

            {

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

                textloc = textloc + pstart;

                for (i = 0; i < (textloc - loc); i++)

                {

                    strcpy(obcode[inc].code, "..");

                    obcode[inc++].add = start++;

                }

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

                loc = textloc + textlen;

            }

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

            {

                fscanf(fp1, "%x", &mloc[record]);

                mloc[record] = mloc[record] + pstart;

                fscanf(fp1, "%x", &mlen[record]);

                fscanf(fp1, "%s", label[record++]);

            }

            else

            {

                length = strlen(input);

                x = 0;

                for (i = 0; i < length; i++)

                {

                    obcode[inc].code[x++] = input[i];

                    if (x > 1)

                    {

                        obcode[inc++].add = start++;

                        x = 0;

                    }

                }

            }

            fscanf(fp1, "%s", input);

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

        if (strcmp(input, "E") == 0)

            fscanf(fp1, "%s", input);

    }

    for (n = 0; n < record; n++)

    {

        operation = label[n][0];

        length = strlen(label[n]);

        for (i = 1; i < length; i++)

        {

            lbl[i - 1] = label[n][i];

        }

        lbl[length - 1] = '\0';

        length = 0;

        strcpy(address, "\0");

        location = mloc[n] - exeloc;

        loc = location;

        count = 0;

        while (length < mlen[n])

        {

            strcat(address, obcode[location++].code);

            count++;

            length += 2;

        }

        for (i = 0; i < num; i++)

        {

            if (strcmp(lbl, estab[i].cextsym) == 0)

                break;

            if (strcmp(lbl, estab[i].extsym) == 0)

                break;

        }

        switch (operation)

        {

            case '+':

                newadd = strtol(address, &add1, 16) + (long int)estab[i].address;

                break;

            case '-':

                newadd = strtol(address, &add1, 16) - (long int)estab[i].address;

                break;

        }

        ltoa(newadd, address, 16);

        x = 0;

        y = 0;

        while (count > 0)

        {

            obcode[loc].code[x++] = address[y++];

            if (x > 1)

            {

                x = 0;

                loc++;

                count--;

            }

        }

    }

    count = 0;

    n = 0;

    s = st - 16;

    fprintf(fp3, "%x\t", s);

    for (i = 1; i <= 16; i++)

    {

        fprintf(fp3, "xx");

        if (i == 4 || i == 8 || i == 12)

        {

            fprintf(fp3, "\t");

        }

    }

    fprintf(fp3, "\n\n%x\t", obcode[0].add);

    for (i = 0; i < inc; i++)

    {

        fprintf(fp3, "%s", obcode[i].code);

        n++;

        if (n > 3)

        {

            fprintf(fp3, "\t");

            n = 0;

            count++;

        }

        if (count > 3)

        {

            fprintf(fp3, "\n\n%x\t", obcode[i + 1].add);

            count = 0;

        }

    }

    fclose(fp1);

    fclose(fp2);

    fclose(fp3);

    printf("\n\t***** PASS TWO OF A DIRECT-LINKING LOADER *****\n");

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

    printf("\n---------------------------------------------------------------");

    printf("\nAddress\t\t\t\tContents");

    printf("\n---------------------------------------------------------------\n");

    fp3 = fopen("OUTPUT.TXT", "r");

    ch = fgetc(fp3);

    while (ch != EOF)

    {

        printf("%c", ch);

        ch = fgetc(fp3);

    }

    fclose(fp3);

}