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wrote a 6502 CPU emu core in C if anybody wants to use it

miker00lz

New member
i wrote this for my NES emulator, and it's been pretty thoroughly tested with a ton of NES games. i've also ran some very thorough CPU test ROMs, it all passes and has 100% accurate instruction timings. that's important if you're doing a console emulator. let me know if you need some help getting it to work. enjoy! :gunman:

it's very straightforward and simple to use (and actually readable unlike marat fayzullin's!) .... usage is explained in the comments at the top. you basically just supply it read6502 and write6502 functions, and tell it to execute. you can also use it's hookexternal(void *funcptr) function to tell it you have a function you want called after every instruction is executed.

there are approximately a metric assload of old computers and game systems that were based around the 6502 or a derivative. the NES, commodore 64, the apple 2 line, just to name a few. most of these (especially the apple 2) are an excellent first choice for somebody wanting to get into emulation programming.

screenshot of it hard at work in my NES emu, which i plan to release fairly soon incidentally. i know there are a billion, but why not eh?

megaman2-bubbleman.png


EDIT: i forgot to mention, it does also emulate all of the undocumented opcodes in addition to the documented ones.


Code:
/* Fake6502 CPU emulator core v1.1 *******************
 * (c)2011 Mike Chambers ([email protected])       *
 *****************************************************
 * v1.1 - Small bugfix in BIT opcode, but it was the *
 *        difference between a few games in my NES   *
 *        emulator working and being broken!         *
 *        I went through the rest carefully again    *
 *        after fixing it just to make sure I didn't *
 *        have any other typos! (Dec. 17, 2011)      *
 *                                                   *
 * v1.0 - First release (Nov. 24, 2011)              *
 *****************************************************
 * LICENSE: This source code is released into the    *
 * public domain, but if you use it please do give   *
 * credit. I put a lot of effort into writing this!  *
 *                                                   *
 *****************************************************
 * Fake6502 is a MOS Technology 6502 CPU emulation   *
 * engine in C. It was written as part of a Nintendo *
 * Entertainment System emulator I've been writing.  *
 *                                                   *
 * It has been pretty well-tested in the NES emu,    *
 * and the clock-cycle timing in particular has been *
 * VERY thoroughly checked out. It matches with the  *
 * real 6502 processor 100%.                         *
 *                                                   *
 * A couple important things to know about are two   *
 * defines in the code. One is "UNDOCUMENTED" which, *
 * when defined, allows Fake6502 to compile with     *
 * full support for the more predictable             *
 * undocumented instructions of the 6502. If it is   *
 * undefined, undocumented opcodes just act as NOPs. *
 *                                                   *
 * The other define is "NES_CPU", which causes the   *
 * code to compile without support for binary-coded  *
 * decimal (BCD) support for the ADC and SBC         *
 * opcodes. The Ricoh 2A03 CPU in the NES does not   *
 * support BCD, but is otherwise identical to the    *
 * standard MOS 6502. (Note that this define is      *
 * enabled in this file if you haven't changed it    *
 * yourself. If you're not emulating a NES, you      *
 * should comment it out.)                           *
 *                                                   *
 * If you do discover an error in timing accuracy,   *
 * or operation in general please e-mail me at the   *
 * address above so that I can fix it. Thank you!    *
 *                                                   *
 *****************************************************
 * Usage:                                            *
 *                                                   *
 * Fake6502 requires you to provide two external     *
 * functions:                                        *
 *                                                   *
 * uint8_t read6502(uint16_t address)                *
 * void write6502(uint16_t address, uint8_t value)   *
 *                                                   *
 * You may optionally pass Fake6502 the pointer to a *
 * function which you want to be called after every  *
 * emulated instruction. This function should be a   *
 * void with no parameters expected to be passed to  *
 * it.                                               *
 *                                                   *
 * This can be very useful. For example, in a NES    *
 * emulator, you check the number of clock ticks     *
 * that have passed so you can know when to handle   *
 * APU events.                                       *
 *                                                   *
 * To pass Fake6502 this pointer, use the            *
 * hookexternal(void *funcptr) function provided.    *
 *                                                   *
 * To disable the hook later, pass NULL to it.       *
 *****************************************************
 * Useful functions in this emulator:                *
 *                                                   *
 * void reset6502()                                  *
 *   - Call this once before you begin execution.    *
 *                                                   *
 * void exec6502(uint32_t tickcount)                 *
 *   - Execute 6502 code up to the next specified    *
 *     count of clock ticks.                         *
 *                                                   *
 * void step6502()                                   *
 *   - Execute a single instrution.                  *
 *                                                   *
 * void irq6502()                                    *
 *   - Trigger a hardware IRQ in the 6502 core.      *
 *                                                   *
 * void nmi6502()                                    *
 *   - Trigger an NMI in the 6502 core.              *
 *                                                   *
 * void hookexternal(void *funcptr)                  *
 *   - Pass a pointer to a void function taking no   *
 *     parameters. This will cause Fake6502 to call  *
 *     that function once after each emulated        *
 *     instruction.                                  *
 *                                                   *
 *****************************************************
 * Useful variables in this emulator:                *
 *                                                   *
 * uint32_t clockticks6502                           *
 *   - A running total of the emulated cycle count.  *
 *                                                   *
 * uint32_t instructions                             *
 *   - A running total of the total emulated         *
 *     instruction count. This is not related to     *
 *     clock cycle timing.                           *
 *                                                   *
 *****************************************************/

#include <stdio.h>
#include <stdint.h>

//6502 defines
#define UNDOCUMENTED //when this is defined, undocumented opcodes are handled.
                     //otherwise, they're simply treated as NOPs.

#define NES_CPU      //when this is defined, the binary-coded decimal (BCD)
                     //status flag is not honored by ADC and SBC. the 2A03
                     //CPU in the Nintendo Entertainment System does not
                     //support BCD operation.

#define FLAG_CARRY     0x01
#define FLAG_ZERO      0x02
#define FLAG_INTERRUPT 0x04
#define FLAG_DECIMAL   0x08
#define FLAG_BREAK     0x10
#define FLAG_CONSTANT  0x20
#define FLAG_OVERFLOW  0x40
#define FLAG_SIGN      0x80

#define BASE_STACK     0x100

#define saveaccum(n) a = (uint8_t)((n) & 0x00FF)


//flag modifier macros
#define setcarry() status |= FLAG_CARRY
#define clearcarry() status &= (~FLAG_CARRY)
#define setzero() status |= FLAG_ZERO
#define clearzero() status &= (~FLAG_ZERO)
#define setinterrupt() status |= FLAG_INTERRUPT
#define clearinterrupt() status &= (~FLAG_INTERRUPT)
#define setdecimal() status |= FLAG_DECIMAL
#define cleardecimal() status &= (~FLAG_DECIMAL)
#define setbreak() status |= FLAG_BREAK
#define clearbreak() status &= (~FLAG_BREAK)
#define setoverflow() status |= FLAG_OVERFLOW
#define clearoverflow() status &= (~FLAG_OVERFLOW)
#define setsign() status |= FLAG_SIGN
#define clearsign() status &= (~FLAG_SIGN)


//flag calculation macros
#define zerocalc(n) {\
    if ((n) & 0x00FF) clearzero();\
        else setzero();\
}

#define signcalc(n) {\
    if ((n) & 0x0080) setsign();\
        else clearsign();\
}

#define carrycalc(n) {\
    if ((n) & 0xFF00) setcarry();\
        else clearcarry();\
}

#define overflowcalc(n, m, o) { /* n = result, m = accumulator, o = memory */ \
    if (((n) ^ (uint16_t)(m)) & ((n) ^ (o)) & 0x0080) setoverflow();\
        else clearoverflow();\
}


//6502 CPU registers
uint16_t pc;
uint8_t sp, a, x, y, status;


//helper variables
uint32_t instructions = 0; //keep track of total instructions executed
uint32_t clockticks6502 = 0, clockgoal6502 = 0;
uint16_t oldpc, ea, reladdr, value, result;
uint8_t opcode, oldstatus;

//externally supplied functions
extern uint8_t read6502(uint16_t address);
extern void write6502(uint16_t address, uint8_t value);

//a few general functions used by various other functions
void push16(uint16_t pushval) {
    write6502(BASE_STACK + sp, (pushval >> 8) & 0xFF);
    write6502(BASE_STACK + ((sp - 1) & 0xFF), pushval & 0xFF);
    sp -= 2;
}

void push8(uint8_t pushval) {
    write6502(BASE_STACK + sp--, pushval);
}

uint16_t pull16() {
    uint16_t temp16;
    temp16 = read6502(BASE_STACK + ((sp + 1) & 0xFF)) | ((uint16_t)read6502(BASE_STACK + ((sp + 2) & 0xFF)) << 8);
    sp += 2;
    return(temp16);
}

uint8_t pull8() {
    return (read6502(BASE_STACK + ++sp));
}

void reset6502() {
    pc = (uint16_t)read6502(0xFFFC) | ((uint16_t)read6502(0xFFFD) << 8);
    a = 0;
    x = 0;
    y = 0;
    sp = 0xFD;
    status |= FLAG_CONSTANT;
}


static void (*addrtable[256])();
static void (*optable[256])();
uint8_t penaltyop, penaltyaddr;

//addressing mode functions, calculates effective addresses
static void imp() { //implied
}

static void acc() { //accumulator
}

static void imm() { //immediate
    ea = pc++;
}

static void zp() { //zero-page
    ea = (uint16_t)read6502((uint16_t)pc++);
}

static void zpx() { //zero-page,X
    ea = ((uint16_t)read6502((uint16_t)pc++) + (uint16_t)x) & 0xFF; //zero-page wraparound
}

static void zpy() { //zero-page,Y
    ea = ((uint16_t)read6502((uint16_t)pc++) + (uint16_t)y) & 0xFF; //zero-page wraparound
}

static void rel() { //relative for branch ops (8-bit immediate value, sign-extended)
    reladdr = (uint16_t)read6502(pc++);
    if (reladdr & 0x80) reladdr |= 0xFF00;
}

static void abso() { //absolute
    ea = (uint16_t)read6502(pc) | ((uint16_t)read6502(pc+1) << 8);
    pc += 2;
}

static void absx() { //absolute,X
    uint16_t startpage;
    ea = ((uint16_t)read6502(pc) | ((uint16_t)read6502(pc+1) << 8));
    startpage = ea & 0xFF00;
    ea += (uint16_t)x;

    if (startpage != (ea & 0xFF00)) { //one cycle penlty for page-crossing on some opcodes
        penaltyaddr = 1;
    }

    pc += 2;
}

static void absy() { //absolute,Y
    uint16_t startpage;
    ea = ((uint16_t)read6502(pc) | ((uint16_t)read6502(pc+1) << 8));
    startpage = ea & 0xFF00;
    ea += (uint16_t)y;

    if (startpage != (ea & 0xFF00)) { //one cycle penlty for page-crossing on some opcodes
        penaltyaddr = 1;
    }

    pc += 2;
}

static void ind() { //indirect
    uint16_t eahelp, eahelp2;
    eahelp = (uint16_t)read6502(pc) | (uint16_t)((uint16_t)read6502(pc+1) << 8);
    eahelp2 = (eahelp & 0xFF00) | ((eahelp + 1) & 0x00FF); //replicate 6502 page-boundary wraparound bug
    ea = (uint16_t)read6502(eahelp) | ((uint16_t)read6502(eahelp2) << 8);
    pc += 2;
}

static void indx() { // (indirect,X)
    uint16_t eahelp;
    eahelp = (uint16_t)(((uint16_t)read6502(pc++) + (uint16_t)x) & 0xFF); //zero-page wraparound for table pointer
    ea = (uint16_t)read6502(eahelp & 0x00FF) | ((uint16_t)read6502((eahelp+1) & 0x00FF) << 8);
}

static void indy() { // (indirect),Y
    uint16_t eahelp, eahelp2, startpage;
    eahelp = (uint16_t)read6502(pc++);
    eahelp2 = (eahelp & 0xFF00) | ((eahelp + 1) & 0x00FF); //zero-page wraparound
    ea = (uint16_t)read6502(eahelp) | ((uint16_t)read6502(eahelp2) << 8);
    startpage = ea & 0xFF00;
    ea += (uint16_t)y;

    if (startpage != (ea & 0xFF00)) { //one cycle penlty for page-crossing on some opcodes
        penaltyaddr = 1;
    }
}

static uint16_t getvalue() {
    if (addrtable[opcode] == acc) return((uint16_t)a);
        else return((uint16_t)read6502(ea));
}

static uint16_t getvalue16() {
    return((uint16_t)read6502(ea) | ((uint16_t)read6502(ea+1) << 8));
}

static void putvalue(uint16_t saveval) {
    if (addrtable[opcode] == acc) a = (uint8_t)(saveval & 0x00FF);
        else write6502(ea, (saveval & 0x00FF));
}


//instruction handler functions
static void adc() {
    penaltyop = 1;
    value = getvalue();
    result = (uint16_t)a + value + (uint16_t)(status & FLAG_CARRY);
   
    carrycalc(result);
    zerocalc(result);
    overflowcalc(result, a, value);
    signcalc(result);
    
    #ifndef NES_CPU
    if (status & FLAG_DECIMAL) {
        clearcarry();
        
        if ((a & 0x0F) > 0x09) {
            a += 0x06;
        }
        if ((a & 0xF0) > 0x90) {
            a += 0x60;
            setcarry();
        }
        
        clockticks6502++;
    }
    #endif
   
    saveaccum(result);
}

static void and() {
    penaltyop = 1;
    value = getvalue();
    result = (uint16_t)a & value;
   
    zerocalc(result);
    signcalc(result);
   
    saveaccum(result);
}

static void asl() {
    value = getvalue();
    result = value << 1;

    carrycalc(result);
    zerocalc(result);
    signcalc(result);
   
    putvalue(result);
}

static void bcc() {
    if ((status & FLAG_CARRY) == 0) {
        oldpc = pc;
        pc += reladdr;
        if ((oldpc & 0xFF00) != (pc & 0xFF00)) clockticks6502 += 2; //check if jump crossed a page boundary
            else clockticks6502++;
    }
}

static void bcs() {
    if ((status & FLAG_CARRY) == FLAG_CARRY) {
        oldpc = pc;
        pc += reladdr;
        if ((oldpc & 0xFF00) != (pc & 0xFF00)) clockticks6502 += 2; //check if jump crossed a page boundary
            else clockticks6502++;
    }
}

static void beq() {
    if ((status & FLAG_ZERO) == FLAG_ZERO) {
        oldpc = pc;
        pc += reladdr;
        if ((oldpc & 0xFF00) != (pc & 0xFF00)) clockticks6502 += 2; //check if jump crossed a page boundary
            else clockticks6502++;
    }
}

static void bit() {
    value = getvalue();
    result = (uint16_t)a & value;
   
    zerocalc(result);
    status = (status & 0xBF) | (uint8_t)(value & 0xC0);
}

static void bmi() {
    if ((status & FLAG_SIGN) == FLAG_SIGN) {
        oldpc = pc;
        pc += reladdr;
        if ((oldpc & 0xFF00) != (pc & 0xFF00)) clockticks6502 += 2; //check if jump crossed a page boundary
            else clockticks6502++;
    }
}

static void bne() {
    if ((status & FLAG_ZERO) == 0) {
        oldpc = pc;
        pc += reladdr;
        if ((oldpc & 0xFF00) != (pc & 0xFF00)) clockticks6502 += 2; //check if jump crossed a page boundary
            else clockticks6502++;
    }
}

static void bpl() {
    if ((status & FLAG_SIGN) == 0) {
        oldpc = pc;
        pc += reladdr;
        if ((oldpc & 0xFF00) != (pc & 0xFF00)) clockticks6502 += 2; //check if jump crossed a page boundary
            else clockticks6502++;
    }
}

static void brk() {
    pc++;
    push16(pc); //push next instruction address onto stack
    setbreak(); //set break flag
    push8(status); //push CPU status to stack
    setinterrupt(); //set interrupt flag
    pc = (uint16_t)read6502(0xFFFE) | ((uint16_t)read6502(0xFFFF) << 8);
}

static void bvc() {
    if ((status & FLAG_OVERFLOW) == 0) {
        oldpc = pc;
        pc += reladdr;
        if ((oldpc & 0xFF00) != (pc & 0xFF00)) clockticks6502 += 2; //check if jump crossed a page boundary
            else clockticks6502++;
    }
}

static void bvs() {
    if ((status & FLAG_OVERFLOW) == FLAG_OVERFLOW) {
        oldpc = pc;
        pc += reladdr;
        if ((oldpc & 0xFF00) != (pc & 0xFF00)) clockticks6502 += 2; //check if jump crossed a page boundary
            else clockticks6502++;
    }
}

static void clc() {
    clearcarry();
}

static void cld() {
    cleardecimal();
}

static void cli() {
    clearinterrupt();
}

static void clv() {
    clearoverflow();
}

static void cmp() {
    penaltyop = 1;
    value = getvalue();
    result = (uint16_t)a - value;
   
    if (a >= (uint8_t)(value & 0x00FF)) setcarry();
        else clearcarry();
    if (a == (uint8_t)(value & 0x00FF)) setzero();
        else clearzero();
    signcalc(result);
}

static void cpx() {
    value = getvalue();
    result = (uint16_t)x - value;
   
    if (x >= (uint8_t)(value & 0x00FF)) setcarry();
        else clearcarry();
    if (x == (uint8_t)(value & 0x00FF)) setzero();
        else clearzero();
    signcalc(result);
}

static void cpy() {
    value = getvalue();
    result = (uint16_t)y - value;
   
    if (y >= (uint8_t)(value & 0x00FF)) setcarry();
        else clearcarry();
    if (y == (uint8_t)(value & 0x00FF)) setzero();
        else clearzero();
    signcalc(result);
}

static void dec() {
    value = getvalue();
    result = value - 1;
   
    zerocalc(result);
    signcalc(result);
   
    putvalue(result);
}

static void dex() {
    x--;
   
    zerocalc(x);
    signcalc(x);
}

static void dey() {
    y--;
   
    zerocalc(y);
    signcalc(y);
}

static void eor() {
    penaltyop = 1;
    value = getvalue();
    result = (uint16_t)a ^ value;
   
    zerocalc(result);
    signcalc(result);
   
    saveaccum(result);
}

static void inc() {
    value = getvalue();
    result = value + 1;
   
    zerocalc(result);
    signcalc(result);
   
    putvalue(result);
}

static void inx() {
    x++;
   
    zerocalc(x);
    signcalc(x);
}

static void iny() {
    y++;
   
    zerocalc(y);
    signcalc(y);
}

static void jmp() {
    pc = ea;
}

static void jsr() {
    push16(pc - 1);
    pc = ea;
}

static void lda() {
    penaltyop = 1;
    value = getvalue();
    a = (uint8_t)(value & 0x00FF);
   
    zerocalc(a);
    signcalc(a);
}

static void ldx() {
    penaltyop = 1;
    value = getvalue();
    x = (uint8_t)(value & 0x00FF);
   
    zerocalc(x);
    signcalc(x);
}

static void ldy() {
    penaltyop = 1;
    value = getvalue();
    y = (uint8_t)(value & 0x00FF);
   
    zerocalc(y);
    signcalc(y);
}

static void lsr() {
    value = getvalue();
    result = value >> 1;
   
    if (value & 1) setcarry();
        else clearcarry();
    zerocalc(result);
    signcalc(result);
   
    putvalue(result);
}

static void nop() {
    switch (opcode) {
        case 0x1C:
        case 0x3C:
        case 0x5C:
        case 0x7C:
        case 0xDC:
        case 0xFC:
            penaltyop = 1;
            break;
    }
}

static void ora() {
    penaltyop = 1;
    value = getvalue();
    result = (uint16_t)a | value;
   
    zerocalc(result);
    signcalc(result);
   
    saveaccum(result);
}

static void pha() {
    push8(a);
}

static void php() {
    push8(status);
}

static void pla() {
    a = pull8();
   
    zerocalc(a);
    signcalc(a);
}

static void plp() {
    status = pull8() | FLAG_CONSTANT;
}

static void rol() {
    value = getvalue();
    result = (value << 1) | (status & FLAG_CARRY);
   
    carrycalc(result);
    zerocalc(result);
    signcalc(result);
   
    putvalue(result);
}

static void ror() {
    value = getvalue();
    result = (value >> 1) | ((status & FLAG_CARRY) << 7);
   
    if (value & 1) setcarry();
        else clearcarry();
    zerocalc(result);
    signcalc(result);
   
    putvalue(result);
}

static void rti() {
    status = pull8();
    value = pull16();
    pc = value;
}

static void rts() {
    value = pull16();
    pc = value + 1;
}

static void sbc() {
    penaltyop = 1;
    value = getvalue() ^ 0x00FF;
    result = (uint16_t)a + value + (uint16_t)(status & FLAG_CARRY);
   
    carrycalc(result);
    zerocalc(result);
    overflowcalc(result, a, value);
    signcalc(result);

    #ifndef NES_CPU
    if (status & FLAG_DECIMAL) {
        clearcarry();
        
        a -= 0x66;
        if ((a & 0x0F) > 0x09) {
            a += 0x06;
        }
        if ((a & 0xF0) > 0x90) {
            a += 0x60;
            setcarry();
        }
        
        clockticks6502++;
    }
    #endif
   
    saveaccum(result);
}

static void sec() {
    setcarry();
}

static void sed() {
    setdecimal();
}

static void sei() {
    setinterrupt();
}

static void sta() {
    putvalue(a);
}

static void stx() {
    putvalue(x);
}

static void sty() {
    putvalue(y);
}

static void tax() {
    x = a;
   
    zerocalc(x);
    signcalc(x);
}

static void tay() {
    y = a;
   
    zerocalc(y);
    signcalc(y);
}

static void tsx() {
    x = sp;
   
    zerocalc(x);
    signcalc(x);
}

static void txa() {
    a = x;
   
    zerocalc(a);
    signcalc(a);
}

static void txs() {
    sp = x;
}

static void tya() {
    a = y;
   
    zerocalc(a);
    signcalc(a);
}

//undocumented instructions
#ifdef UNDOCUMENTED
    static void lax() {
        lda();
        ldx();
    }

    static void sax() {
        sta();
        stx();
        putvalue(a & x);
        if (penaltyop && penaltyaddr) clockticks6502--;
    }

    static void dcp() {
        dec();
        cmp();
        if (penaltyop && penaltyaddr) clockticks6502--;
    }

    static void isb() {
        inc();
        sbc();
        if (penaltyop && penaltyaddr) clockticks6502--;
    }

    static void slo() {
        asl();
        ora();
        if (penaltyop && penaltyaddr) clockticks6502--;
    }

    static void rla() {
        rol();
        and();
        if (penaltyop && penaltyaddr) clockticks6502--;
    }

    static void sre() {
        lsr();
        eor();
        if (penaltyop && penaltyaddr) clockticks6502--;
    }

    static void rra() {
        ror();
        adc();
        if (penaltyop && penaltyaddr) clockticks6502--;
    }
#else
    #define lax nop
    #define sax nop
    #define dcp nop
    #define isb nop
    #define slo nop
    #define rla nop
    #define sre nop
    #define rra nop
#endif


static void (*addrtable[256])() = {
/*        |  0  |  1  |  2  |  3  |  4  |  5  |  6  |  7  |  8  |  9  |  A  |  B  |  C  |  D  |  E  |  F  |     */
/* 0 */     imp, indx,  imp, indx,   zp,   zp,   zp,   zp,  imp,  imm,  acc,  imm, abso, abso, abso, abso, /* 0 */
/* 1 */     rel, indy,  imp, indy,  zpx,  zpx,  zpx,  zpx,  imp, absy,  imp, absy, absx, absx, absx, absx, /* 1 */
/* 2 */    abso, indx,  imp, indx,   zp,   zp,   zp,   zp,  imp,  imm,  acc,  imm, abso, abso, abso, abso, /* 2 */
/* 3 */     rel, indy,  imp, indy,  zpx,  zpx,  zpx,  zpx,  imp, absy,  imp, absy, absx, absx, absx, absx, /* 3 */
/* 4 */     imp, indx,  imp, indx,   zp,   zp,   zp,   zp,  imp,  imm,  acc,  imm, abso, abso, abso, abso, /* 4 */
/* 5 */     rel, indy,  imp, indy,  zpx,  zpx,  zpx,  zpx,  imp, absy,  imp, absy, absx, absx, absx, absx, /* 5 */
/* 6 */     imp, indx,  imp, indx,   zp,   zp,   zp,   zp,  imp,  imm,  acc,  imm,  ind, abso, abso, abso, /* 6 */
/* 7 */     rel, indy,  imp, indy,  zpx,  zpx,  zpx,  zpx,  imp, absy,  imp, absy, absx, absx, absx, absx, /* 7 */
/* 8 */     imm, indx,  imm, indx,   zp,   zp,   zp,   zp,  imp,  imm,  imp,  imm, abso, abso, abso, abso, /* 8 */
/* 9 */     rel, indy,  imp, indy,  zpx,  zpx,  zpy,  zpy,  imp, absy,  imp, absy, absx, absx, absy, absy, /* 9 */
/* A */     imm, indx,  imm, indx,   zp,   zp,   zp,   zp,  imp,  imm,  imp,  imm, abso, abso, abso, abso, /* A */
/* B */     rel, indy,  imp, indy,  zpx,  zpx,  zpy,  zpy,  imp, absy,  imp, absy, absx, absx, absy, absy, /* B */
/* C */     imm, indx,  imm, indx,   zp,   zp,   zp,   zp,  imp,  imm,  imp,  imm, abso, abso, abso, abso, /* C */
/* D */     rel, indy,  imp, indy,  zpx,  zpx,  zpx,  zpx,  imp, absy,  imp, absy, absx, absx, absx, absx, /* D */
/* E */     imm, indx,  imm, indx,   zp,   zp,   zp,   zp,  imp,  imm,  imp,  imm, abso, abso, abso, abso, /* E */
/* F */     rel, indy,  imp, indy,  zpx,  zpx,  zpx,  zpx,  imp, absy,  imp, absy, absx, absx, absx, absx  /* F */
};

static void (*optable[256])() = {
/*        |  0  |  1  |  2  |  3  |  4  |  5  |  6  |  7  |  8  |  9  |  A  |  B  |  C  |  D  |  E  |  F  |      */
/* 0 */      brk,  ora,  nop,  slo,  nop,  ora,  asl,  slo,  php,  ora,  asl,  nop,  nop,  ora,  asl,  slo, /* 0 */
/* 1 */      bpl,  ora,  nop,  slo,  nop,  ora,  asl,  slo,  clc,  ora,  nop,  slo,  nop,  ora,  asl,  slo, /* 1 */
/* 2 */      jsr,  and,  nop,  rla,  bit,  and,  rol,  rla,  plp,  and,  rol,  nop,  bit,  and,  rol,  rla, /* 2 */
/* 3 */      bmi,  and,  nop,  rla,  nop,  and,  rol,  rla,  sec,  and,  nop,  rla,  nop,  and,  rol,  rla, /* 3 */
/* 4 */      rti,  eor,  nop,  sre,  nop,  eor,  lsr,  sre,  pha,  eor,  lsr,  nop,  jmp,  eor,  lsr,  sre, /* 4 */
/* 5 */      bvc,  eor,  nop,  sre,  nop,  eor,  lsr,  sre,  cli,  eor,  nop,  sre,  nop,  eor,  lsr,  sre, /* 5 */
/* 6 */      rts,  adc,  nop,  rra,  nop,  adc,  ror,  rra,  pla,  adc,  ror,  nop,  jmp,  adc,  ror,  rra, /* 6 */
/* 7 */      bvs,  adc,  nop,  rra,  nop,  adc,  ror,  rra,  sei,  adc,  nop,  rra,  nop,  adc,  ror,  rra, /* 7 */
/* 8 */      nop,  sta,  nop,  sax,  sty,  sta,  stx,  sax,  dey,  nop,  txa,  nop,  sty,  sta,  stx,  sax, /* 8 */
/* 9 */      bcc,  sta,  nop,  nop,  sty,  sta,  stx,  sax,  tya,  sta,  txs,  nop,  nop,  sta,  nop,  nop, /* 9 */
/* A */      ldy,  lda,  ldx,  lax,  ldy,  lda,  ldx,  lax,  tay,  lda,  tax,  nop,  ldy,  lda,  ldx,  lax, /* A */
/* B */      bcs,  lda,  nop,  lax,  ldy,  lda,  ldx,  lax,  clv,  lda,  tsx,  lax,  ldy,  lda,  ldx,  lax, /* B */
/* C */      cpy,  cmp,  nop,  dcp,  cpy,  cmp,  dec,  dcp,  iny,  cmp,  dex,  nop,  cpy,  cmp,  dec,  dcp, /* C */
/* D */      bne,  cmp,  nop,  dcp,  nop,  cmp,  dec,  dcp,  cld,  cmp,  nop,  dcp,  nop,  cmp,  dec,  dcp, /* D */
/* E */      cpx,  sbc,  nop,  isb,  cpx,  sbc,  inc,  isb,  inx,  sbc,  nop,  sbc,  cpx,  sbc,  inc,  isb, /* E */
/* F */      beq,  sbc,  nop,  isb,  nop,  sbc,  inc,  isb,  sed,  sbc,  nop,  isb,  nop,  sbc,  inc,  isb  /* F */
};

static const uint32_t ticktable[256] = {
/*        |  0  |  1  |  2  |  3  |  4  |  5  |  6  |  7  |  8  |  9  |  A  |  B  |  C  |  D  |  E  |  F  |     */
/* 0 */      7,    6,    2,    8,    3,    3,    5,    5,    3,    2,    2,    2,    4,    4,    6,    6,  /* 0 */
/* 1 */      2,    5,    2,    8,    4,    4,    6,    6,    2,    4,    2,    7,    4,    4,    7,    7,  /* 1 */
/* 2 */      6,    6,    2,    8,    3,    3,    5,    5,    4,    2,    2,    2,    4,    4,    6,    6,  /* 2 */
/* 3 */      2,    5,    2,    8,    4,    4,    6,    6,    2,    4,    2,    7,    4,    4,    7,    7,  /* 3 */
/* 4 */      6,    6,    2,    8,    3,    3,    5,    5,    3,    2,    2,    2,    3,    4,    6,    6,  /* 4 */
/* 5 */      2,    5,    2,    8,    4,    4,    6,    6,    2,    4,    2,    7,    4,    4,    7,    7,  /* 5 */
/* 6 */      6,    6,    2,    8,    3,    3,    5,    5,    4,    2,    2,    2,    5,    4,    6,    6,  /* 6 */
/* 7 */      2,    5,    2,    8,    4,    4,    6,    6,    2,    4,    2,    7,    4,    4,    7,    7,  /* 7 */
/* 8 */      2,    6,    2,    6,    3,    3,    3,    3,    2,    2,    2,    2,    4,    4,    4,    4,  /* 8 */
/* 9 */      2,    6,    2,    6,    4,    4,    4,    4,    2,    5,    2,    5,    5,    5,    5,    5,  /* 9 */
/* A */      2,    6,    2,    6,    3,    3,    3,    3,    2,    2,    2,    2,    4,    4,    4,    4,  /* A */
/* B */      2,    5,    2,    5,    4,    4,    4,    4,    2,    4,    2,    4,    4,    4,    4,    4,  /* B */
/* C */      2,    6,    2,    8,    3,    3,    5,    5,    2,    2,    2,    2,    4,    4,    6,    6,  /* C */
/* D */      2,    5,    2,    8,    4,    4,    6,    6,    2,    4,    2,    7,    4,    4,    7,    7,  /* D */
/* E */      2,    6,    2,    8,    3,    3,    5,    5,    2,    2,    2,    2,    4,    4,    6,    6,  /* E */
/* F */      2,    5,    2,    8,    4,    4,    6,    6,    2,    4,    2,    7,    4,    4,    7,    7   /* F */
};


void nmi6502() {
    push16(pc);
    push8(status);
    status |= FLAG_INTERRUPT;
    pc = (uint16_t)read6502(0xFFFA) | ((uint16_t)read6502(0xFFFB) << 8);
}

void irq6502() {
    push16(pc);
    push8(status);
    status |= FLAG_INTERRUPT;
    pc = (uint16_t)read6502(0xFFFE) | ((uint16_t)read6502(0xFFFF) << 8);
}

uint8_t callexternal = 0;
void (*loopexternal)();

void exec6502(uint32_t tickcount) {
    clockgoal6502 += tickcount;
   
    while (clockticks6502 < clockgoal6502) {
        opcode = read6502(pc++);
        status |= FLAG_CONSTANT;

        penaltyop = 0;
        penaltyaddr = 0;

        (*addrtable[opcode])();
        (*optable[opcode])();
        clockticks6502 += ticktable[opcode];
        if (penaltyop && penaltyaddr) clockticks6502++;

        instructions++;

        if (callexternal) (*loopexternal)();
    }

}

void step6502() {
    opcode = read6502(pc++);
    status |= FLAG_CONSTANT;

    penaltyop = 0;
    penaltyaddr = 0;

    (*addrtable[opcode])();
    (*optable[opcode])();
    clockticks6502 += ticktable[opcode];
    if (penaltyop && penaltyaddr) clockticks6502++;
    clockgoal6502 = clockticks6502;

    instructions++;

    if (callexternal) (*loopexternal)();
}

void hookexternal(void *funcptr) {
    if (funcptr != (void *)NULL) {
        loopexternal = funcptr;
        callexternal = 1;
    } else callexternal = 0;
}
 
Last edited:

ShizZy

Emulator Developer
Thanks for sharing this, miker00lz. It's very clean and easy to read, and looks to be well written. I'm sure it will be helpful for some people who are just starting out.
 
OP
M

miker00lz

New member
yeah, i hope somebody can make use of it. by using big function jump tables for addressing modes and ops, it's probably not the fastest 6502 emu by any means but i thought it's the best way to make the code easy to understand and work with. that way, people who want to get into emulation don't just use the code but if they read it they'll quickly see exactly how it works without getting a migraine. :ph34r:

besides, it's still fast enough easily. i got on one of my oldest systems, an 800 MHz Pentium 3 (Coppermine) running XP -- i benchmarked it running most code at about the speed of a 35 to 40 MHz 6502. it was close to emulating a 200 MHz 6502 on my Phenom II X4. the NES has a 1.79 MHz CPU. iirc, the C64 and Apple 2 systems were even slower. i may be wrong but it's in the same ballpark anyway.
 
Last edited:

Cyberman

Moderator
Moderator
It's difficult to compare speed in this case. I suspect if you used DREC (Dynamic Recompilation) with a self modifying code detector you could get a larger throughput boost.

It's good for learning that is a certainty.

I believe VICE and mame have 6502 emulators that are in the mature state. You might want to see what they did for inspiration or other ways of doing the same thing.

Fortunately the 6502 is fairly simple, UNFORTUNATELY the x86 and 64 bit extensions etc. require some careful consideration about DREC.

Some DREC designs I've seen convert the code into a series of function pointers in a queue.

Although DREC is significantly faster it is also significantly more complicated.

Having written a code translator before it can be quite complicated dealing with the rule set for converting from one IS (instruction set) to another. My application was a conversion of 8048 assembler syntax to 8051. It should have been simple, they are similar IS architectures, but it was all the screwy things the original code relied on that made it difficult. Things that were on the 8048 were no longer in the 8051 (instructions) and clock features and other things. 'Clever' code sometimes causes more problems than it is worth. So consider that.

DREC is best applied without using REAL assembly for the target system by the way.

So enough about this and that... :D

Thanks for contributing something someone can use to learn from.

Cyb - learning is good.
 
OP
M

miker00lz

New member
It's difficult to compare speed in this case. I suspect if you used DREC (Dynamic Recompilation) with a self modifying code detector you could get a larger throughput boost.

It's good for learning that is a certainty.

I believe VICE and mame have 6502 emulators that are in the mature state. You might want to see what they did for inspiration or other ways of doing the same thing.

Fortunately the 6502 is fairly simple, UNFORTUNATELY the x86 and 64 bit extensions etc. require some careful consideration about DREC.

Some DREC designs I've seen convert the code into a series of function pointers in a queue.

Although DREC is significantly faster it is also significantly more complicated.

Having written a code translator before it can be quite complicated dealing with the rule set for converting from one IS (instruction set) to another. My application was a conversion of 8048 assembler syntax to 8051. It should have been simple, they are similar IS architectures, but it was all the screwy things the original code relied on that made it difficult. Things that were on the 8048 were no longer in the 8051 (instructions) and clock features and other things. 'Clever' code sometimes causes more problems than it is worth. So consider that.

DREC is best applied without using REAL assembly for the target system by the way.

So enough about this and that... :D

Thanks for contributing something someone can use to learn from.

Cyb - learning is good.

yeah, i must admit i've never tried to do any sort of dynarec emulator although i more or less understand how they work. luckily, no machine using a 6502 that you might want to emulate requires a super-fast emu.

also, i found a bug i had in that emulator code, and i changed my original post to fix it. it was in the BIT opcode. it looked like this:

Code:
static void bit() {
    value = getvalue();
    result = (uint16_t)a & value;
   
    zerocalc(result);
    status = (status & 0xBF) | (uint8_t)(value & 0xC0);
}

where it should have been this:

Code:
static void bit() {
    value = getvalue();
    result = (uint16_t)a & value;
   
    zerocalc(result);
    status = (status & 0x3F) | (uint8_t)(value & 0xC0);
}

it's just a small typo, but it was the difference between a few games running and not running in my NES emu!
 

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