bradgrantham-apple2e/dis6502.cpp

#include <cstdio>
#include <cstring>
#include <string>
#include <tuple>

using namespace std;

tuple<int, string> disassemble_6502(int address, const unsigned char* buffer)
{
    static char cstr[512];
    int bytelength;
    char *p = cstr;

    int currentbyte;
    int previousbyte;
    int paramcount;
    int addrmode;
    const char *opcode;
    const char *pad;
    const char *pre;
    const char *post;

 // Padding for 1,2 & 3 byte instructions
    static const char *padding[3] = {"      ","   ",""};

 // 57 Instructions + Undefined ("???")
    static const char *instruction[58] = {
 //       0     1     2     3     4     5     6     7     8     9
        "ADC","AND","ASL","BCC","BCS","BEQ","BIT","BMI","BNE","BPL", // 0
        "BRK","BVC","BVS","CLC","CLD","CLI","CLV","CMP","CPX","CPY", // 1
        "DEC","DEX","DEY","EOR","INC","INX","INY","JMP","JSR","LDA", // 2
        "LDX","LDY","LSR","NOP","ORA","PHA","PHP","PLA","PLP","ROL", // 3
        "ROR","ROT","RTI","RTS","SBC","SEC","SED","SEI","STA","STX", // 4
        "STY","TAX","TAY","TSX","TXA","TXS","TYA","???"};            // 5

 // This is a lookup of the text formating required for mode output, plus one entry to distinguish relative mode
    static const char *modes[9][2]={{"",""},{"#",""},{"",",X"},{"",",Y"},{"(",",X)"},{"(","),Y"},{"(",")"},{"A",""},{"",""}};

 // Opcode Properties for 256 opcodes {length_in_bytes, mnemonic_lookup, mode_chars_lookup}
    static const int opcode_props[256][3] = {
 //         0        1        2        3        4        5        6        7        8        9        A        B        C        D        E        F
 //     ******** -------- ******** -------- ******** -------- ******** -------- ******** -------- ******** -------- ******** -------- ******** --------
        {1,10,0},{2,34,4},{1,57,0},{1,57,0},{1,57,0},{2,34,0},{2,2,0}, {1,57,0},{1,36,0},{2,34,1},{1,2,7}, {1,57,0},{1,57,0},{3,34,0},{3,2,0}, {1,57,0}, // 0
        {2,9,8}, {2,34,5},{1,57,0},{1,57,0},{1,57,0},{2,34,2},{2,2,2}, {1,57,0},{1,13,0},{3,34,3},{1,57,0},{1,57,0},{1,57,0},{3,34,2},{3,2,2}, {1,57,0}, // 1
        {3,28,0},{2,1,4}, {1,57,0},{1,57,0},{2,6,0}, {2,1,0}, {2,39,0},{1,57,0},{1,38,0},{2,1,1}, {1,39,7},{1,57,0},{3,6,0}, {3,1,0}, {3,39,0},{1,57,0}, // 2
        {2,7,8}, {2,1,5}, {1,57,0},{1,57,0},{1,57,0},{2,1,2}, {2,39,2},{1,57,0},{1,45,0},{3,1,3}, {1,57,0},{1,57,0},{1,57,0},{3,1,2}, {3,39,2},{1,57,0}, // 3
        {1,42,0},{2,23,4},{1,57,0},{1,57,0},{1,57,0},{2,23,0},{2,32,0},{1,57,0},{1,35,0},{2,23,1},{1,32,7},{1,57,0},{3,27,0},{3,23,0},{3,32,0},{1,57,0}, // 4
        {2,11,8},{2,23,5},{1,57,0},{1,57,0},{1,57,0},{2,23,2},{2,32,2},{1,57,0},{1,15,0},{3,23,3},{1,57,0},{1,57,0},{1,57,0},{3,23,2},{3,32,2},{1,57,0}, // 5
        {1,43,0},{2,0,4}, {1,57,0},{1,57,0},{1,57,0},{2,0,0}, {2,40,0},{1,57,0},{1,37,0},{2,0,1}, {1,41,7},{1,57,0},{3,27,6},{3,0,0}, {3,40,0},{1,57,0}, // 6
        {2,12,8},{2,0,5}, {1,57,0},{1,57,0},{1,57,0},{2,0,2}, {2,40,2},{1,57,0},{1,47,0},{3,0,3}, {1,57,0},{1,57,0},{1,57,0},{3,0,2}, {3,40,2},{1,57,0}, // 7
        {1,57,0},{2,48,4},{1,57,0},{1,57,0},{2,50,0},{2,48,0},{2,49,0},{1,57,0},{1,22,0},{1,57,0},{1,54,0},{1,57,0},{3,50,0},{3,48,0},{3,49,0},{1,57,0}, // 8
        {2,3,8}, {2,48,5},{1,57,0},{1,57,0},{2,50,2},{2,48,2},{2,49,3},{1,57,0},{1,56,0},{3,48,3},{1,55,0},{1,57,0},{1,57,0},{3,48,2},{1,57,0},{1,57,0}, // 9
        {2,31,1},{2,29,4},{2,30,1},{1,57,0},{2,31,0},{2,29,0},{2,30,0},{1,57,0},{1,52,0},{2,29,1},{1,51,0},{1,57,0},{3,31,0},{3,29,0},{3,30,0},{1,57,0}, // A
        {2,4,8}, {2,29,5},{1,57,0},{1,57,0},{2,31,2},{2,29,2},{2,30,3},{1,57,0},{1,16,0},{3,29,3},{1,53,0},{1,57,0},{3,31,2},{3,29,2},{3,30,3},{1,57,0}, // B
        {2,19,1},{2,17,4},{1,57,0},{1,57,0},{2,19,0},{2,17,0},{2,20,0},{1,57,0},{1,26,0},{2,17,1},{1,21,0},{1,57,0},{3,19,0},{3,17,0},{3,20,0},{1,57,0}, // C
        {2,8,8}, {2,17,5},{1,57,0},{1,57,0},{1,57,0},{2,17,2},{2,20,2},{1,57,0},{1,14,0},{3,17,3},{1,57,0},{1,57,0},{1,57,0},{3,17,2},{3,20,2},{1,57,0}, // D
        {2,18,1},{2,44,4},{1,57,0},{1,57,0},{2,18,0},{2,44,0},{2,24,0},{1,57,0},{1,25,0},{2,44,1},{1,33,0},{1,57,0},{3,18,0},{3,44,0},{3,24,0},{1,57,0}, // E
        {2,5,8}, {2,44,5},{1,57,0},{1,57,0},{1,57,0},{2,44,2},{2,24,2},{1,57,0},{1,46,0},{3,44,3},{1,57,0},{1,57,0},{1,57,0},{3,44,2},{3,24,2},{1,57,0}  // F
    };

    paramcount = 0;
    int stored = 0;

    for (int i = 0; i < 4; ++i) {                                 //Start proccessing loop.
        previousbyte = currentbyte;
        currentbyte = ((unsigned char * ) buffer)[i];
        if (paramcount == 0) {
            sprintf(p, "%04X   %n", address, &stored);                             //Display current address at beginning of line
            p += stored;
            paramcount = opcode_props[currentbyte][0];              //Get instruction length
            bytelength = 1 + paramcount;
            opcode = instruction[opcode_props[currentbyte][1]];     //Get opcode name
            addrmode = opcode_props[currentbyte][2];                //Get info required to display addressing mode
            pre = modes[addrmode][0];                               //Look up pre-operand formatting text
            post = modes[addrmode][1];                              //Look up post-operand formatting text
            pad = padding[(paramcount - 1)];                        //Calculate correct padding for output alignment
            address = address + paramcount;                         //Increment address
        }
        if (paramcount != 0)                                        //Keep track of possition within instruction
            paramcount = paramcount - 1;
        sprintf(p, "%02X %n", currentbyte, &stored);                               //Display the current byte in HEX
        p += stored;
        if (paramcount == 0) {
            sprintf(p, " %s %s %s%n", pad, opcode, pre, &stored);                  //Pad text, display instruction name and pre-operand chars
            p += stored;
            if (strcmp(pad, "   ") == 0) {                                     //Check if single operand instruction
                if (addrmode != 8) {                                //If not using relative addressing ...
                    sprintf(p, "$%02X%n", currentbyte, &stored);                   //...display operand
                    p += stored;
                } else {                                            //Addressing mode is relative...
                    sprintf(p, "$%02X%n", (address + ((currentbyte < 128) ? currentbyte : currentbyte - 256)), &stored); //...display relative address.
                    p += stored;
                }
            }
            if (strcmp(pad, "") == 0) {                                          //Check if two operand instruction and if so...
                sprintf(p, "$%02X%02X%n", currentbyte, previousbyte, &stored);     //...display operand
                p += stored;
            }
            sprintf(p, "%s%n", post, &stored);                                   //Display post-operand chars
            p += stored;
            break;
        }
    }

    return make_tuple(bytelength, string(cstr));
}