Pass1

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Function to perform the first pass of the assembler
void passOne(char label[10], char opcode[10], char operand[15], char code[10], char mnemonic[3]){
    int locctr, start, length;

    // File pointers for input, opcode table, symbol table, and intermediate files
    FILE *f1, *f2, *f3, *f4;

    // Open the necessary files
    f1 = fopen("input.txt", "r");           // Input assembly program
    f2 = fopen("optab.txt", "r");           // Opcode table
    f3 = fopen("symtab.txt", "w");          // Symbol table output
    f4 = fopen("intermediate.txt", "w");    // Intermediate file output

    // Check if files are opened successfully
    if (f1 == NULL || f2 == NULL || f3 == NULL || f4 == NULL) {
        printf("Error opening one of the files.\n");
        exit(1); // Exit if any file failed to open
    }

    // Read the first line of the input file
    fscanf(f1, "%s\t%s\t%s", label, opcode, operand);

    // Check if the first opcode is "START"
    if(strcmp(opcode, "START") == 0){
        start = strtol(operand, NULL, 16);  // Convert the starting address from string to integer
        locctr = start;         // Initialize LOCCTR with the starting address
        fprintf(f4, "\t%s\t%s\t%s\n", label, opcode, operand); // Write the first line to the intermediate file
        fscanf(f1, "%s\t%s\t%s", label, opcode, operand);  // Read the next line
    } else {
        locctr = 0;  // If no "START" directive, start LOCCTR at 0
    }

    // Process the input file line by line until "END" is encountered
    while(strcmp(opcode, "END") != 0){
        // Write the current line to the intermediate file with the LOCCTR
        fprintf(f4, "%X\t%s\t%s\t%s\n", locctr, label, opcode, operand);

        // If the label is not "-", add it to the symbol table
        if(strcmp(label, "-") != 0){
            fprintf(f3, "%s\t%X\n", label, locctr);
        }

        // Reset the file pointer to the start of the opcode table file
        rewind(f2);
        int found = 0;  // Flag to check if opcode is found

        // Search for the opcode in the opcode table
        while(fscanf(f2, "%s\t%s", code, mnemonic) != EOF){
            if(strcmp(opcode, code) == 0){
                locctr += 3;  // Increment LOCCTR by 3 for standard instructions
                found = 1;
                break;
            }
        }

        // If the opcode is not found in the table, handle directives
        if (!found) {
            // Check if opcode is a directive and update LOCCTR accordingly
            if(strcmp(opcode, "WORD") == 0){
                locctr += 3;  // WORD occupies 3 bytes
            } else if(strcmp(opcode, "RESW") == 0){
                locctr += (3 * atoi(operand));  // RESW reserves multiple words (3 bytes per word)
            } else if(strcmp(opcode, "RESB") == 0){
                locctr += atoi(operand);  // RESB reserves bytes
            } else if(strcmp(opcode, "BYTE") == 0){
                locctr += (strlen(operand) - 3);  // BYTE reserves space based on the length of the operand
            } else {
                // If the opcode is not recognized, print an error and break the loop
                printf("\nOPCODE ERROR: %s\n", opcode);
                break;
            }
        }

        // Read the next line from the input file
        fscanf(f1, "%s\t%s\t%s", label, opcode, operand);
    }

    // Write the last line to the intermediate file
    fprintf(f4, "%X\t%s\t%s\t%s\n", locctr, label, opcode, operand);

    // Close all the files
    fclose(f4);
    fclose(f3);
    fclose(f2);
    fclose(f1);

    // Calculate the length of the program
    length = locctr - start;
    // Print the length of the program
    printf("\nLength of program is %X\n", length);
}

// Main function
int main(){
    // Declare arrays to hold label, opcode, operand, code, and mnemonic
    char label[10], opcode[10], operand[15];
    char code[10], mnemonic[3];

    // Call the passOne function to process the input file
    passOne(label, opcode, operand, code, mnemonic);

    return 0;  // Return 0 to indicate successful execution
}


/*

    input.txt
------------------
-   START   2000
-   LDA FIVE
-   STA ALPHA
-   LDCH    CHARZ
-   STCH    C1
ALPHA   RESW    2
FIVE    WORD    5
CHARZ   BYTE    C'Z'
C1  RESB    1
-   END -
------------------



    optab.txt
------------------
LDA 03
STA 0f
LDCH    53
STCH    57
------------------



intermediate.txt
------------------
    -   START   2000
2000    -   LDA FIVE
2003    -   STA ALPHA
2006    -   LDCH    CHARZ
2009    -   STCH    C1
200C    ALPHA   RESW    2
2012    FIVE    WORD    5
2015    CHARZ   BYTE    C'Z'
2016    C1  RESB    1
2017    -   END -
------------------



    symtab.txt
------------------
ALPHA   200C
FIVE    2012
CHARZ   2015
C1  2016
------------------

*/