Pass 1 of Two Pass Assembler

Introduction and Problem Description:

a. Introduction:

• Pass 1 is the initial stage of a two-pass assembler, converting assembly language into machine code.

• Focuses on analyzing the source program to extract essential information for generating the final object code.

• Builds a symbol table mapping symbolic labels to memory locations.

• Calculates memory addresses for instructions and data using the Location Counter (LOCCTR).

• Processes assembler directives like START, END, BYTE, WORD, RESW, and RESB to allocate memory and define program boundaries.

• Detects errors, such as duplicate labels or invalid instructions, enabling corrections before Pass 2.

• Produces an intermediate file containing calculated addresses and program details.

b. Problem Description:

• Symbol Table Construction:

  • Identifies and maps all labels to their respective memory addresses.

  • Detects and reports duplicate or undefined labels.

• Memory Address Calculation:

  • Manages the LOCCTR to assign sequential addresses.

  • Accounts for varying instruction sizes and memory allocation directives like RESW and RESB.

• Intermediate File Generation:

  • Ensures the file is accurate and well-structured with line numbers, mnemonics, operands, and addresses.

  • Prevents propagation of errors to Pass 2.

• Directive Handling:

  • Processes START and END to define program boundaries.

  • Handles BYTE, WORD, RESW, and RESB for memory allocation.

• Error Detection and Reporting:

  • Identifies syntax errors, invalid instructions, and misplaced labels.

  • Provides clear error messages for effective debugging.

Pass 1 of two pass assembler algorithm: 

Codes: 

SIC Pass 1 of two pass assembler: 

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

typedef struct INPUT

{

    char label[20], opcode[20], operand[20];

} INSTR;

typedef struct OpTab

{

    char opCode[10], value[3], format[2];

} OPTAB;

int main()

{

    INSTR a[100];

    OPTAB b[102];

    char sd[100];

    char pName[30];

    FILE *fp = fopen("SICinput.txt", "r");

    FILE *ip = fopen("SICINT.txt", "w");

    FILE *kp = fopen("SICSymTab.txt", "w");

    FILE *optab = fopen("OpTab.txt", "r");

    if (fp == NULL || ip == NULL || kp == NULL || optab == NULL)

    {

        printf("Unable to open one or more files\n");

        return 1;

    }

    int Size = 0;

    while (fscanf(optab, "%s %s %s", b[Size].opCode, b[Size].value, b[Size].format) != EOF)

    {

        Size++;

    }

    fclose(optab);

    int i = 0;

    int startValue = 0, locctr = 0;

    while (fgets(sd, sizeof(sd), fp) && i < 100)

    {

        char lab[20] = "", oper[20] = "", open[20] = "";

        int tokens = sscanf(sd, "%s %s %s", lab, oper, open);

        strcpy(a[i].label, "");

        strcpy(a[i].opcode, "");

        strcpy(a[i].operand, "");

        if (tokens == 1)

            strcpy(a[i].opcode, lab);

        else if (tokens == 2)

        {

            strcpy(a[i].opcode, lab);

            strcpy(a[i].operand, oper);

        }

        else if (tokens == 3)

        {

            strcpy(a[i].label, lab);

            strcpy(a[i].opcode, oper);

            strcpy(a[i].operand, open);

            if (i == 1)

                strcpy(pName, a[i].label);

        }

        if ((strcmp(a[i].opcode, "START") == 0) && (i == 0))

        {

            strcpy(pName, a[i].label);

            locctr = startValue = strtoul(a[0].operand, NULL, 16);

            fprintf(ip, "%04X\t%s", startValue, sd);

            if (tokens == 3)

                fprintf(kp, "%s\t%X\n", a[i].label, startValue);

        }

        else

        {

            if (tokens == 3)

            {

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

                {

                    if (strcmp(a[i].label, a[j].label) == 0 && i != j)

                    {

                        printf("Duplicate label found: %s\n", a[j].label);

                        return 1;

                    }

                }

                fprintf(kp, "%s\t%X\n", a[i].label, locctr);

            }

            if (strcmp(a[i].opcode, "END") == 0)

                continue;

            else

            {

                fprintf(ip, "%04X\t%s", locctr, sd);

                printf("PC address is: %X\n", locctr);

            }

            int found = 0, k = 0;

            while (k < Size || strcmp(a[k].opcode, "END") == 0)

            {

                if (strcmp(b[k].opCode, a[i].opcode) == 0)

                {

                    locctr += 3;

                    found = 1;

                    break;

                }

                k++;

            }

            if (found == 0)

            {

                if (strcmp(a[i].opcode, "WORD") == 0)

                {

                    locctr += 3;

                }

                else if (strcmp(a[i].opcode, "RESW") == 0)

                {

                    locctr += 3 * atoi(a[i].operand);

                }

                else if (strcmp(a[i].opcode, "RESB") == 0)

                {

                    locctr += atoi(a[i].operand);

                }

                else if (strcmp(a[i].opcode, "BYTE") == 0)

                {

                    if (a[i].operand[0] == 'X')

                    {

                        locctr += (strlen(a[i].operand) - 3) / 2;

                    }

                    else if (a[i].operand[0] == 'C')

                    {

                        locctr += strlen(a[i].operand) - 3;

                    }

                }

                else if (strcmp(a[i].opcode, "END") == 0)

                    continue;

                else

                {

                    printf("Invalid opcode found: %s\n", a[i].opcode);

                }

            }

        }

        i++;

    }

    // printf("%s\n",pName);

    fprintf(ip, "%04X\t\t\tEND\t%s", locctr, pName);

    int prLength = locctr - startValue;

    printf("The program length is: %X\n", prLength);

    fclose(fp);

    fclose(ip);

    fclose(kp);

    return 0;

}

Sample Inputs and Outputs of SIC Pass 1 of two pass assembler: 

Sample Input and Output 1:

Sample Input and Output 3:

Sample Inputs and Outputs of SIC/XE Pass 1 of two pass assembler: 

Sample Input and Output 1

Sample Input and Output 2

Sample Input and Output 3