Assembler Pass-2

Objective of Assembler Pass 2:-

1. Generate machine code and produce the final object program.

2. Convert symbolic machine op-codes to their corresponding bit patterns.

3. Store machine op-codes in the MOT (Machine Opcode Table), which includes the symbolic code, length, and bit configuration.

4. Process pseudo-operations and store them in the POT (Pseudo-Operation Table).

5. Generate object code: Replace labels with their corresponding addresses and translate instructions into their binary formats.

6. Write the final object program and generate a listing of the source code.

During the first pass, the assembler scans the assembly code, checks for errors, assigns memory locations to instructions and symbols, and creates an intermediate file for use in Pass 2.

Data Structures Used in Assembler Pass 2:-

1. Symbol Table: A key data structure that holds symbols (labels) and their corresponding memory addresses. In Pass 2, the assembler uses this table to resolve symbolic addresses, replacing references to labels with actual memory addresses, which is essential for generating correct object code.

2. Literal Table: This table stores literals defined in the source program along with their assigned memory addresses and values. During Pass 2, the assembler uses this table to map literals to their memory locations, ensuring that literal values are accurately included in the object code.

3. Location Counter (LC): A dynamic structure that tracks the memory address of each instruction and data item. In Pass 2, the Location Counter assigns memory addresses to each program component, ensuring proper placement and alignment of the object code segments.

4. Opcode Table (OPTAB): Contains machine opcodes and instruction formats for all mnemonics in the assembly language. The assembler uses this table in Pass 2 to retrieve the correct opcode and instruction format, which are necessary for converting mnemonics into executable machine code.

begin

read first input line (from intermediate file)

if OPCODE ='START' then

begin

write listing line

read next input line

end {if START}

write Header record to object program

initialize first Text record

while OPCODE != 'END' do

begin

if this is not a comment line then

begin

search OPTAB for OPCODE

if found then

begin

if there is a symbol in OPERAND field then

begin

search SYMTAB for OPERAND

if found then

store symbol value as operand address

else

begin

store 0 as operand address

set error flag (undefined symbol)

end

end {if symbol}

else

store 0 as operand address

assemble the object code instruction

end {if opcode found}

else if OPCODE ='BYTE' or 'WORD' then

convert constant to object code

if object code will not fit into the current Text record then

begin

write Text record to object program

initialize new Text record

end

add object code to Text record

end {if not comment}

write listing line

read next input line

end(while not END)

write last Text record to object program

write End record to object program

write last listing line

end{Pass 2}

#iinclude<stdio.h>

#include<stdlib.h>

#include<string.h>

#include<stdbool.h>

char *decimalToHex(int decimal) {

    static char hex_string[20];

    sprintf(hex_string, "%X", decimal);

    return hex_string;

}

void main() {

    printf("\nPASS 2.\n");

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

    char optab_opcode[10], optab_machine_code[10];

    char symtab_label[10], symtab_address[10];

    int count = 0, start, end, length = 0;

    bool opcode_found = false;

    bool label_found = false;

    FILE *input, *output, *symtab, *optab;

    input = fopen("intermediate.txt", "r");

    output = fopen("object.txt", "w");

    symtab = fopen("symtab.txt", "r");

    optab = fopen("optab.txt", "r");

    if (input == NULL || output == NULL || symtab == NULL || optab == NULL){

        printf("Error! File not found!");

        exit(0);

    } else {

        printf("All files opened successfully!\n");

    }

    fscanf(input, "%s %s %s %s", loc, label, opcode, operand);

    start = strtol(operand, NULL, 16);

    while (!feof(input)) {

        if(strcmp(opcode,"BYTE")==0){

            count+=strlen(operand)-3;

        }

        if (!(strcmp(opcode, "START") == 0 || strcmp(opcode, "END") == 0 || strcmp(opcode, "RESW") == 0 || strcmp(opcode, "RESB") == 0 || strcmp(opcode,"BYTE")==0)) {

            count+=3;

        }

        fscanf(input, "%s %s %s %s", loc, label, opcode, operand);

    }

    end = strtol(loc, NULL, 16);

    printf("END: %d/ START: %d", end, start);

    length = end - start;

    rewind(input);

    fscanf(input, "%s %s %s %s", loc, label, opcode, operand);

    fprintf(output, "H^%s^00%s^0000%d\n", label, operand, length);

    fprintf(output, "T^00%s^%d^", operand,count);

    while (!feof(input)) {

        if (strcmp(opcode, "START") == 0 || strcmp(opcode, "END") == 0 || strcmp(opcode, "RESW") == 0 || strcmp(opcode, "RESB") == 0) {

            fscanf(input, "%s %s %s %s", loc, label, opcode, operand);

            continue;

        } else {

            rewind(optab);

            opcode_found = false;

            fscanf(optab, "%s %s", optab_opcode, optab_machine_code);

            while (!feof(optab)) {

                if (strcmp(opcode, optab_opcode) == 0) {

                    opcode_found = true;

                    break;

                }

                fscanf(optab, "%s %s", optab_opcode, optab_machine_code);

            }

            if (opcode_found) {

                rewind(symtab);

                label_found = false;

                fscanf(symtab, "%s %s", symtab_label, symtab_address);

                while (!feof(symtab)) {

                    if (strcmp(symtab_label, operand) == 0) {

                        label_found = true;

                        break;

                    }

                    fscanf(symtab, "%s %s", symtab_label, symtab_address);

                }

                if (label_found) {

                    fprintf(output, "%s%s^", optab_machine_code, symtab_address);

                } else {

                    printf("Error! Label not found in symtab!\n");

                    exit(0);

                }

            } else if (strcmp(opcode, "WORD") == 0) {

                fprintf(output, "00000%s ", operand);

            }

            else if (strcmp(opcode, "BYTE") == 0) {

                for (int i = 2; i < strlen(operand) - 1; i++) {

                    fprintf(output, "%s", decimalToHex(operand[i]));

                }

                fprintf(output, "^");

            } else {

                printf("Error!\n %s", opcode);

                exit(0);

            }

        }

        fscanf(input, "%s %s %s %s", loc, label, opcode, operand);

    }

    fprintf(output, "\nE^00%s\n", decimalToHex(start));

    fclose(input);

    fclose(output);

    fclose(symtab);

    fclose(optab);

    output = fopen("object.txt", "r");

    char str = fgetc(output);

    printf("\n\nObject program file/ Output file :  \n\n");

    while (str != EOF) {

        printf("%c", str);

        str = fgetc(output);

    }

        fclose(output);

}