QBoy/disassembler.c

#include <stdio.h>
#include <stdlib.h>
#include <cjson/cJSON.h>
#include <string.h> // for easier work with cJSON

// How to compile? Like this!
// gcc -o disassembler disassembler.c -lcjson

// How to run? Like this!
// ./disassembler snake.gb

// Large chunks of the code are from my Intel 8080 emulator

typedef struct buffer {
    unsigned char* buffer;
    int length;
} buffer;

buffer* read_file(char** argv) {
    FILE* file = fopen(argv[1], "rb");
    if (file == NULL) {
        printf("Issue opening the file.\n");
        return NULL;
    }
    fseek(file, 0L, SEEK_END);
    int fsize = ftell(file);
    fseek(file, 0L, SEEK_SET);

    buffer *b = malloc(sizeof(buffer));
    b->buffer = malloc(fsize);
    b->length = fsize;

    fread(b->buffer, fsize, 1, file);
    fclose(file);

    return b;
}

cJSON* read_cjson() {
    FILE *fs = fopen("Opcodes.json", "rb");
    if (fs == NULL) {
        printf("ERROR: Unable to open the Opcodes.json file.\n");
        return NULL;
    }
    // read the file contents into a string 
    fseek(fs, 0L, SEEK_END);
    int fsize = ftell(fs);
    fseek(fs, 0L, SEEK_SET);

    buffer *b = malloc(sizeof(buffer));
    b->buffer = malloc(fsize);
    b->length = fsize;

    fread(b->buffer, fsize, 1, fs);
    fclose(fs);
  
    // parse the JSON data 
    cJSON *json = cJSON_Parse(b->buffer);
    if (json == NULL) {
        const char *error_ptr = cJSON_GetErrorPtr();
        if (error_ptr != NULL) {
            printf("Error: %s\n", error_ptr);
        }
        cJSON_Delete(json);
        return NULL;
    }
    free(b->buffer); // almost forgot that dataleak
    free(b);
    return json;
}

void print_buffer(buffer* b) {
    // just a check
    // equivalent to $ xxd snake.gb
    for (int i = 0; i < 40; i++) {
        printf("%02x \n", b->buffer[i]);
    }
}

int disassemble8080(buffer* b, int pc, cJSON* json) {
    unsigned char *code = &(b->buffer[pc]);
    int opbytes = 1;
    printf("%04x ", pc);
    char mnemonic[10];

    char opcode_str[5];
    sprintf(opcode_str, "0x%02X", *code);
    //printf("%s\n", opcode_str);


    if (strcmp(opcode_str, "0xCB") == 0) {
        pc++;
        unsigned char *code = &(b->buffer[pc]);

        sprintf(opcode_str, "0x%02X", *code);
        //printf("%s\n", opcode_str);

        cJSON *prefixed = cJSON_GetObjectItem(json, "cbprefixed");
        if (prefixed == NULL) {
            printf("Error: 'prefixed' not found in JSON.\n");
            return 0;
        }
        cJSON *command = cJSON_GetObjectItem(prefixed, opcode_str);
        if (command == NULL) {
            printf("Error: Command with opcode %s not found.\n", opcode_str);
            return 0;
        }
        cJSON *mnemonic_item = cJSON_GetObjectItem(command, "mnemonic");
        cJSON *bytes_item = cJSON_GetObjectItem(command, "bytes");


        if (mnemonic_item != NULL && cJSON_IsString(mnemonic_item)) {
            strcpy(mnemonic, mnemonic_item->valuestring);
        }

        if (bytes_item != NULL && cJSON_IsNumber(bytes_item)) {
            opbytes = bytes_item->valueint;
        }

        printf("%s  \t", mnemonic);
        
        // Extract and print operands
        int operand_count = 0;
        cJSON *operands = cJSON_GetObjectItem(command, "operands");
        
        if (operands != NULL && cJSON_IsArray(operands)) {
            
            operand_count = cJSON_GetArraySize(operands); 
            //printf("Number of operands: %d\n", operand_count); 
            for (int i = 0; i < operand_count; i++) { 
                if (i == 1) {
                    printf(", ");
                }
                cJSON *operand = cJSON_GetArrayItem(operands, i); 
                cJSON *name = cJSON_GetObjectItem(operand, "name"); 
                cJSON *immediate = cJSON_GetObjectItem(operand, "immediate");
                cJSON *bytes = cJSON_GetObjectItem(operand, "bytes");
                if (immediate != NULL && cJSON_IsBool(immediate)) { 
                    printf("%s", cJSON_IsTrue(immediate) ? "" : "("); 
                }
                if (bytes != NULL && cJSON_IsNumber(bytes)) {
                    if (bytes->valueint == 1) {
                        unsigned char *code = &(b->buffer[pc+1]);
                        sprintf(opcode_str, "0x%02X", *code);
                        printf("%s", opcode_str);
                    }
                    if (bytes->valueint == 2) {
                        unsigned char *code = &(b->buffer[pc+2]);
                        sprintf(opcode_str, "0x%02X", *code);
                        printf("%s", opcode_str);
                        sprintf(opcode_str, "%02X", *(code-1));
                        printf("%s", opcode_str);
                    }

                }
                if (name != NULL && cJSON_IsString(name) && (bytes == NULL)) { 
                    //printf("Operand %d: %s", i + 1, name->valuestring); 
                    printf("%s", name->valuestring);    
                } 
                if (bytes != NULL && cJSON_IsNumber(bytes)) {
                    //printf(", Bytes: %d", bytes->valueint);
                } 
                if (immediate != NULL && cJSON_IsBool(immediate)) { 
                    printf("%s", cJSON_IsTrue(immediate) ? "" : ")"); 
                    //printf(", Immediate: %s", cJSON_IsTrue(immediate) ? "true" : "false"); 
                }
            }    
        }
    

        printf("\n");
        printf(" ^ We have a prefixed command!\n");
        return opbytes+1;
        
    } 

    cJSON *unprefixed = cJSON_GetObjectItem(json, "unprefixed");
    if (unprefixed == NULL) {
        printf("Error: 'unprefixed' not found in JSON.\n");
        return 0;
    }
    cJSON *command = cJSON_GetObjectItem(unprefixed, opcode_str);
    if (command == NULL) {
        printf("Error: Command with opcode %s not found.\n", opcode_str);
        return 0;
    }
    cJSON *mnemonic_item = cJSON_GetObjectItem(command, "mnemonic");
    cJSON *bytes_item = cJSON_GetObjectItem(command, "bytes");


    if (mnemonic_item != NULL && cJSON_IsString(mnemonic_item)) {
        strcpy(mnemonic, mnemonic_item->valuestring);
    }

    if (bytes_item != NULL && cJSON_IsNumber(bytes_item)) {
        opbytes = bytes_item->valueint;
    }

    printf("%s  \t", mnemonic);
    
    // Extract and print operands
    int operand_count = 0;
    cJSON *operands = cJSON_GetObjectItem(command, "operands");
     
    if (operands != NULL && cJSON_IsArray(operands)) {
         
        operand_count = cJSON_GetArraySize(operands); 
        //printf("Number of operands: %d\n", operand_count); 
        for (int i = 0; i < operand_count; i++) { 
            if (i == 1) {
                printf(", ");
            }
            cJSON *operand = cJSON_GetArrayItem(operands, i); 
            cJSON *name = cJSON_GetObjectItem(operand, "name"); 
            cJSON *immediate = cJSON_GetObjectItem(operand, "immediate");
            cJSON *bytes = cJSON_GetObjectItem(operand, "bytes");
            if (immediate != NULL && cJSON_IsBool(immediate)) { 
                printf("%s", cJSON_IsTrue(immediate) ? "" : "("); 
            }
            if (bytes != NULL && cJSON_IsNumber(bytes)) {
                if (bytes->valueint == 1) {
                    unsigned char *code = &(b->buffer[pc+1]);
                    sprintf(opcode_str, "0x%02X", *code);
                    printf("%s", opcode_str);
                }
                if (bytes->valueint == 2) {
                    unsigned char *code = &(b->buffer[pc+2]);
                    sprintf(opcode_str, "0x%02X", *code);
                    printf("%s", opcode_str);
                    sprintf(opcode_str, "%02X", *(code-1));
                    printf("%s", opcode_str);
                }

            }
            if (name != NULL && cJSON_IsString(name) && (bytes == NULL)) { 
                //printf("Operand %d: %s", i + 1, name->valuestring); 
                printf("%s", name->valuestring);    
            } 
            if (bytes != NULL && cJSON_IsNumber(bytes)) {
                //printf(", Bytes: %d", bytes->valueint);
            } 
            if (immediate != NULL && cJSON_IsBool(immediate)) { 
                printf("%s", cJSON_IsTrue(immediate) ? "" : ")"); 
                //printf(", Immediate: %s", cJSON_IsTrue(immediate) ? "true" : "false"); 
            }
        }    
    }
    

    printf("\n");
    return opbytes;
}

void print_keys(cJSON *json) {
    // Used only for debug purposes
    // Lists all the keys
    cJSON *unprefixed = cJSON_GetObjectItem(json, "unprefixed");
    if (unprefixed == NULL) {
        printf("Error: 'unprefixed' not found in JSON.\n");
        return;
    }

    cJSON *item = NULL;
    cJSON_ArrayForEach(item, unprefixed) {
        printf("Key: %s\n", item->string);
    }
}


int main (int argc, char** argv)  {
    int pc = 0x150; //Program Counter, note GB roms typically start at 0x100

    buffer* buff = read_file(argv);
    if (buff == NULL) {
        printf("Fatal error.\n");
        return 1;
    }
    //print_buffer(buff);
    cJSON *json = read_cjson();

    if (json == NULL) {
        printf("main terminated\n");
        return 1;
    }

    //print_keys(json);


    while (pc < 0x400) {//buff->length) {
        pc += disassemble8080(buff, pc, json);
    }
    free(buff);
    cJSON_Delete(json); // basically free(json)
    return 0;
}