Assembler
Pass-2

Pass 2 of the SIC/XE assembler focuses on generating the final machine code. During this phase, the assembler uses the symbol table created in Pass 1 to resolve all addresses and replace the placeholders with actual memory locations. It translates the intermediate code into machine language instructions, ensuring that all labels and references are correctly linked. Pass 2 also handles the generation of the object code, which can be executed by the SIC/XE machine. This pass produces the final executable program.  

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}

Intermediate:

- COPY START 1000

1000 - +LDA ALPHA

1004 - ADD #1

1007 - DIV #2

100A - STA BETA

100D - SUB #4

1010 - STA GAMMA

1013 - +LDA GAMMA

1017 - ADD #2

101A - +LDA BETA

101E ALPHA BYTE C'COF'

1021 BETA RESW 2

1027 GAMMA RESW 2

102D - END 1000

Optab:

DIV      06

SUB 05 

CMP 03

LDA 00

STA 23

ADD 01

JNC 08

Symbol Tab:

ALPHA 101E

BETA 1021

GAMMA 1027

#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);

}