Pass 1 of Two Pass Linking Loader

Introduction and Problem Description

i) Introduction:

1. Purpose of Two-Pass Linking Loader: Combines multiple object modules and resolves external references.

2. Function of Pass 1:

   • Establishes memory layout by processing external symbols and definitions.

   • Ensures modules are linked without overlapping memory addresses.

3. Key Output: Generates an external symbol table (ESTAB) mapping symbols to tentative addresses.

4. Objective: Prepares for actual address resolution and linking in Pass 2.

ii) Problem Description:

1. Primary Goal:

   • Scans input object modules to populate ESTAB.

   • Records control sections and external symbols with their names, addresses, and lengths.

2. Error Detection: Identifies and flags issues like duplicate symbol definitions.

3. Symbol References: Records unresolved external symbol references for resolution in Pass 2.

4. Outcome: Creates a foundation for accurate address resolution and linking in the subsequent pass.

Pass 1 of two pass linking loader: 

Code 

Pass 1 of two pass linking loader:

#include <stdio.h>

#include <string.h>

#include <stdlib.h>

#define MAX_SYMBOLS 100

#define MAX_NAME_LENGTH 20

#define MAX_REFERENCES 100

typedef struct

{

    char name[MAX_NAME_LENGTH];

    int address;

    int length;

} ESTABEntry;

typedef struct

{

    char name[MAX_NAME_LENGTH];

    char cname[MAX_NAME_LENGTH];

    int resolvedAddress;

} ReferenceEntry;

ESTABEntry ESTAB[MAX_SYMBOLS];

ReferenceEntry references[MAX_REFERENCES];

int estabCount = 0;

int referenceCount = 0;

int PROGADDR = 0;

int CSADDR = 0;

// Function to search ESTAB for a given name

int searchESTAB(const char *name)

{

    for (int i = 0; i < estabCount; i++)

    {

        if (strcmp(ESTAB[i].name, name) == 0)

        {

            return i;

        }

    }

    return -1;

}

// Function to add an entry to ESTAB

void addToESTAB(const char *name, int address, int length)

{

    strcpy(ESTAB[estabCount].name, name);

    ESTAB[estabCount].address = address;

    ESTAB[estabCount].length = length;

    estabCount++;

}

void addReference(const char *name, const char *cname)

{

    strcpy(references[referenceCount].name, name);

    strcpy(references[referenceCount].cname, cname);

    references[referenceCount].resolvedAddress = -1;

    referenceCount++;

}

int main()

{

    char line[100];

    FILE *inputFile = fopen("MDLL_input.txt", "r");

    FILE *outputFile = fopen("MDESTAB.txt", "w");

    if (!inputFile || !outputFile)

    {

        printf("Error opening file.\n");

        return 1;

    }

    PROGADDR = 0x7000;

    CSADDR = PROGADDR;

    printf("\n******* Control Sections and References *******\n");

    while (fgets(line, sizeof(line), inputFile))

    {

        if (line[0] == 'H')

        {

            char controlSectionName[MAX_NAME_LENGTH];

            int CSLTH;

            sscanf(line, "H%6s%*6x%6x", controlSectionName, &CSLTH);

            if (searchESTAB(controlSectionName) != -1)

            {

                printf("Error: Duplicate external symbol '%s'.\n", controlSectionName);

                continue;

            }

            // Add control section to ESTAB

            addToESTAB(controlSectionName, CSADDR, CSLTH);

            printf("\nControl Section: %s\n", controlSectionName);

            while (fgets(line, sizeof(line), inputFile) && line[0] != 'E')

            {

                if (line[0] == 'D') // Define record

                {

                    char symbolName[MAX_NAME_LENGTH];

                    int symbolAddress, offset = 1;

                    while (sscanf(&line[offset], "%6s%6x", symbolName, &symbolAddress) == 2)

                    {

                        offset += 12;

                        if (searchESTAB(symbolName) != -1)

                        {

                            printf("Error: Duplicate external symbol '%s'.\n", symbolName);

                        }

                        else

                        {

                            addToESTAB(symbolName, CSADDR + symbolAddress, 0);

                        }

                    }

                }

                else if (line[0] == 'R') // Reference record

                {

                    char symbolName[MAX_NAME_LENGTH];

                    int offset = 1;

                    printf("References:\n");

                    while (sscanf(&line[offset], "%6s", symbolName) == 1)

                    {

                        offset += 6;

                        addReference(symbolName, controlSectionName);

                        printf("  %s\n", symbolName);

                    }

                }

            }

            CSADDR += CSLTH;

        }

    }

    for (int i = 0; i < referenceCount; i++)

    {

        int idx = searchESTAB(references[i].name);

        if (idx != -1)

        {

            references[i].resolvedAddress = ESTAB[idx].address;

        }

    }

    printf("******* ESTAB Entries *******\n");

    printf("CSection SName\tAddress\tLength\n");

    for (int i = 0; i < estabCount; i++)

    {

        if (ESTAB[i].length > 0)

        {

            fprintf(outputFile, "%s\t\t%04X\t%04X\n", ESTAB[i].name, ESTAB[i].address, ESTAB[i].length);

            printf("%s\t\t%04X\t%04X\n", ESTAB[i].name, ESTAB[i].address, ESTAB[i].length);

        }

        else

        {

            fprintf(outputFile, "\t%s\t%04X\n", ESTAB[i].name, ESTAB[i].address);

            printf("\t%s\t%04X\n", ESTAB[i].name, ESTAB[i].address);

        }

    }

    printf("\n******* Resolved References Grouped by Control Section *******\n");

    for (int i = 0; i < estabCount; i++)

    {

        if (ESTAB[i].length > 0)

        {

            printf("\nControl Section: %s\n", ESTAB[i].name);

            printf("Symbol\t\tAddress\n");

            for (int j = 0; j < referenceCount; j++)

            {

                if (references[j].resolvedAddress != -1 && (strcmp(ESTAB[i].name, references[j].cname) == 0))

                {

                    printf("%s\t\t%04X\n", references[j].name, references[j].resolvedAddress);

                }

                /* else

                 {

                     printf("%s\t\tUNDEFINED\n", references[j].name);

                 }*/

            }

        }

    }

    fclose(inputFile);

    fclose(outputFile);

    return 0;

}