e6502/e6502.c

#include "e6502.h"


// Instruction Addressing

typedef uint8_t* e6502_Address;

static inline e6502_Address e6502_mem_adr(e6502_Core* core,
                                          e6502_Mem_Addr adr) {
    return &core->memory[adr];
}

static inline e6502_Mem_Addr e6502_adr_mem(e6502_Core* core,
                                           e6502_Address adr) {
    return (adr - core->memory);
}



static inline uint8_t e6502_adr_r8(e6502_Core*,
                                   e6502_Address adr) {
    return *adr;
}

static inline uint8_t e6502_adr_r16(e6502_Core*,
                                    e6502_Address adr) {
    return *(uint16_t*)adr;
}

static inline void e6502_adr_w8(e6502_Core*,
                                e6502_Address adr,
                                uint8_t val) {
    *adr = val;
}

static inline void e6502_adr_w16(e6502_Core*,
                                 e6502_Address adr,
                                 uint16_t val) {
    *(uint16_t*)adr = val;
}


// Read values after opcodes

static inline uint8_t arg8(e6502_Core* core) {
    return e6502_r8(core, core->registers.PC++);
}

static inline uint16_t arg16(e6502_Core* core) {
    return (arg8(core) | ((uint16_t)arg8(core) << 8));
}


// Interpret opcode values

static inline e6502_Address adr_none(e6502_Core*) {
    return 0;
}

static inline e6502_Address adr_ind(e6502_Core* core) {
    return e6502_mem_adr(core, e6502_r16(core, arg16(core)));
}

static inline e6502_Address adr_ind_x(e6502_Core* core) {
    return e6502_mem_adr(core, e6502_r16(
            core,
            (uint8_t)(core->registers.X + arg8(core))
        ));
}

static inline e6502_Address adr_ind_y(e6502_Core* core) {
    uint16_t zp_adr = e6502_r16(core, arg8(core));
    uint8_t old_page = zp_adr >> 8;
    uint16_t new_adr = core->registers.Y + zp_adr;
    if (new_adr >> 8 != old_page) core->cycle++;
    return e6502_mem_adr(core, new_adr);
}

static inline e6502_Address adr_abs_i(e6502_Core* core,
                                      uint8_t i) {
    uint16_t abs_adr = arg16(core);
    uint8_t old_page = abs_adr >> 8;
    uint16_t new_adr = i + abs_adr;
    if (new_adr >> 8 != old_page) core->cycle++;
    return e6502_mem_adr(core, new_adr);
}

static inline e6502_Address adr_abs_x(e6502_Core* core) {
    return adr_abs_i(core, core->registers.X);
}

static inline e6502_Address adr_abs_y(e6502_Core* core) {
    return adr_abs_i(core, core->registers.Y);
}

static inline e6502_Address adr_zp(e6502_Core* core) {
    return e6502_mem_adr(core, arg8(core));
}

static inline e6502_Address adr_zp_x(e6502_Core* core) {
    return e6502_mem_adr(
            core, (uint8_t)(arg8(core) + core->registers.X)
        );
}

static inline e6502_Address adr_zp_y(e6502_Core* core) {
    return e6502_mem_adr(
            core, (uint8_t)(arg8(core) + core->registers.Y)
        );
}

static inline e6502_Address adr_abs(e6502_Core* core) {
    return e6502_mem_adr(core, arg16(core));
}

static inline e6502_Address adr_imm(e6502_Core* core) {
    return e6502_mem_adr(core, core->registers.PC++);
}

static inline e6502_Address adr_rel(e6502_Core* core) {
    return e6502_mem_adr(
            core, (int8_t)arg8(core) + core->registers.PC
        );
}

static inline e6502_Address adr_reg_a (e6502_Core* core) {
    return &core->registers.A;
}

static inline e6502_Address adr_reg_x (e6502_Core* core) {
    return &core->registers.X;
}

static inline e6502_Address adr_reg_y (e6502_Core* core) {
    return &core->registers.Y;
}

static inline e6502_Address adr_reg_s (e6502_Core* core) {
    return &core->registers.S;
}

static inline e6502_Address adr_reg_sp (e6502_Core* core) {
    return &core->registers.SP;
}

// Instructions

static inline void nop(e6502_Core*, e6502_Address) {
    // NOP
}

static inline void brk(e6502_Core *core, e6502_Address) {
    e6502_push16(core, core->registers.PC + 1);
    e6502_push8(core, core->registers.S |
                      e6502_Status_1 |
                      e6502_Status_B);
    core->registers.S |= e6502_Status_I;
    core->registers.PC = e6502_r16(core, e6502_IRQ_Vec);
}

static inline void rti(e6502_Core *core, e6502_Address) {
    core->registers.S = e6502_pop8(core) & ~(e6502_Status_B);
    core->registers.PC = e6502_pop16(core);
}

static inline void ora(e6502_Core *core, e6502_Address adr) {
    core->registers.A |= e6502_adr_r8(core, adr);
    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
    core->registers.S |= (core->registers.A & e6502_Status_N);
    if (!core->registers.A) core->registers.S |= e6502_Status_Z;
}

static inline void eor(e6502_Core *core, e6502_Address adr) {
    core->registers.A ^= e6502_adr_r8(core, adr);
    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
    core->registers.S |= (core->registers.A & e6502_Status_N);
    if (!core->registers.A) core->registers.S |= e6502_Status_Z;
}

static inline void asl(e6502_Core *core, e6502_Address adr) {
    uint8_t val = e6502_adr_r8(core, adr);
    core->registers.S &= ~(e6502_Status_C |
                           e6502_Status_Z |
                           e6502_Status_N);
    if (val & 0x80) core->registers.S |= e6502_Status_C;
    val <<= 1;
    core->registers.S |= (val & e6502_Status_N);
    if (!val) core->registers.S |= e6502_Status_Z;
    e6502_adr_w8(core, adr, val);
}

static inline void lsr(e6502_Core *core, e6502_Address adr) {
    uint8_t val = e6502_adr_r8(core, adr);
    core->registers.S &= ~(e6502_Status_C |
                           e6502_Status_Z |
                           e6502_Status_N);
    core->registers.S |= (val & e6502_Status_C);
    val >>= 1;
    core->registers.S |= (val & e6502_Status_N);
    if (!val) core->registers.S |= e6502_Status_Z;
    e6502_adr_w8(core, adr, val);
}

static inline void rol(e6502_Core *core, e6502_Address adr) {
    uint8_t val = e6502_adr_r8(core, adr);
    uint8_t new_c = (val & 0x80) ? e6502_Status_C : 0;
    val <<= 1;
    val |= (core->registers.S & e6502_Status_C);
    core->registers.S &= ~(e6502_Status_C |
                           e6502_Status_Z |
                           e6502_Status_N);
    core->registers.S |= new_c;
    if (!val) core->registers.S |= e6502_Status_Z;
    core->registers.S |= (val & e6502_Status_N);
    e6502_adr_w8(core, adr, val);
}

static inline void ror(e6502_Core *core, e6502_Address adr) {
    uint8_t val = e6502_adr_r8(core, adr);
    uint8_t new_c = (val & e6502_Status_C);
    val >>= 1;
    if (core->registers.S & e6502_Status_C) val |= e6502_Status_N;
    core->registers.S &= ~(e6502_Status_C |
                           e6502_Status_Z |
                           e6502_Status_N);
    core->registers.S |= new_c;
    if (!val) core->registers.S |= e6502_Status_Z;
    core->registers.S |= (val & e6502_Status_N);
    e6502_adr_w8(core, adr, val);
}

static inline void pha(e6502_Core *core, e6502_Address adr) {
    e6502_push8(core, core->registers.A);
}

static inline void php(e6502_Core *core, e6502_Address adr) {
    e6502_push8(core, core->registers.S |
                      e6502_Status_1 |
                      e6502_Status_B);
}

static inline void pla(e6502_Core *core, e6502_Address adr) {
    core->registers.A = e6502_pop8(core);
    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
    if (!core->registers.A) core->registers.S |= e6502_Status_Z;
    core->registers.S |= (core->registers.A & e6502_Status_N);
}

static inline void plp(e6502_Core *core, e6502_Address adr) {
    core->registers.S = (e6502_pop8(core) |
                         e6502_Status_1 |
                         e6502_Status_B);
}

static inline void b(e6502_Core *core, e6502_Address adr) {
    uint8_t old_page = core->registers.PC >> 8;
    uint16_t new_addr = e6502_adr_mem(core, adr);
    if (old_page != (new_addr >> 8)) core->cycle++;
    core->registers.PC = new_addr;
    core->cycle++;
}

static inline void bpl(e6502_Core *core, e6502_Address adr) {
    if (!(core->registers.S & e6502_Status_N))  {
        b(core, adr);
    }
}

static inline void bmi(e6502_Core *core, e6502_Address adr) {
    if (core->registers.S & e6502_Status_N)  {
        b(core, adr);
    }
}

static inline void bcc(e6502_Core *core, e6502_Address adr) {
    if (!(core->registers.S & e6502_Status_C))  {
        b(core, adr);
    }
}

static inline void bcs(e6502_Core *core, e6502_Address adr) {
    if (core->registers.S & e6502_Status_C)  {
        b(core, adr);
    }
}

static inline void bvc(e6502_Core *core, e6502_Address adr) {
    if (!(core->registers.S & e6502_Status_V))  {
        b(core, adr);
    }
}

static inline void bvs(e6502_Core *core, e6502_Address adr) {
    if (core->registers.S & e6502_Status_V)  {
        b(core, adr);
    }
}

static inline void bne(e6502_Core *core, e6502_Address adr) {
    if (!(core->registers.S & e6502_Status_Z))  {
        b(core, adr);
    }
}

static inline void beq(e6502_Core *core, e6502_Address adr) {
    if (core->registers.S & e6502_Status_Z)  {
        b(core, adr);
    }
}

static inline void clc(e6502_Core *core, e6502_Address) {
    core->registers.S &= ~e6502_Status_C;
}

static inline void cli(e6502_Core *core, e6502_Address) {
    core->registers.S &= ~e6502_Status_I;
}

static inline void cld(e6502_Core *core, e6502_Address) {
    core->registers.S &= ~e6502_Status_D;
}

static inline void clv(e6502_Core *core, e6502_Address) {
    core->registers.S &= ~e6502_Status_V;
}

static inline void sec(e6502_Core *core, e6502_Address) {
    core->registers.S |= e6502_Status_C;
}

static inline void sei(e6502_Core *core, e6502_Address) {
    core->registers.S |= e6502_Status_I;
}

static inline void sed(e6502_Core *core, e6502_Address) {
    core->registers.S |= e6502_Status_D;
}

static inline void jsr(e6502_Core *core, e6502_Address adr) {
    e6502_push16(core, core->registers.PC - 1);
    core->registers.PC = e6502_adr_mem(core, adr);
}

static inline void rts(e6502_Core *core, e6502_Address) {
    core->registers.PC = e6502_pop16(core) + 1;
}

static inline void jmp(e6502_Core *core, e6502_Address adr) {
    core->registers.PC = e6502_adr_mem(core, adr);
}

static inline void bit(e6502_Core *core, e6502_Address adr) {
    uint8_t val = e6502_adr_r8(core, adr);
    core->registers.S &= ~(e6502_Status_Z |
                           e6502_Status_V |
                           e6502_Status_N);
    core->registers.S |= (val & (e6502_Status_V |
                                 e6502_Status_N));
    if (!(val & core->registers.A)) {
        core->registers.S |= e6502_Status_Z;
    }
}

static inline void cmp(e6502_Core *core,
                       e6502_Address adr,
                       uint8_t minuend) {
    uint8_t val = e6502_adr_r8(core, adr);
    uint8_t diff = minuend - val;
    core->registers.S &= ~(e6502_Status_C |
                           e6502_Status_Z |
                           e6502_Status_N);
    if (minuend >= val) core->registers.S |= e6502_Status_C;
    if (diff == 0) core->registers.S |= e6502_Status_Z;
    core->registers.S |= (diff & e6502_Status_N);
}

static inline void cpa(e6502_Core *core, e6502_Address adr) {
    cmp(core, adr, core->registers.A);
}

static inline void cpx(e6502_Core *core, e6502_Address adr) {
    cmp(core, adr, core->registers.X);
}

static inline void cpy(e6502_Core *core, e6502_Address adr) {
    cmp(core, adr, core->registers.Y);
}

static inline void and(e6502_Core *core, e6502_Address adr) {
    core->registers.A &= e6502_adr_r8(core, adr);
    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
    core->registers.S |= (core->registers.A & e6502_Status_N);
    if (!core->registers.A) core->registers.S |= e6502_Status_Z;
}

static inline void adc(e6502_Core *core, e6502_Address adr) {
    uint8_t addend = e6502_adr_r8(core, adr);
    uint32_t sum = core->registers.A +
                   addend +
                   (core->registers.S & e6502_Status_C);
    core->registers.S &= ~(e6502_Status_C | e6502_Status_Z |
                           e6502_Status_V | e6502_Status_N);
    if (sum > 0xFF) core->registers.S |= e6502_Status_C;
    if (!(uint8_t)sum) core->registers.S |= e6502_Status_Z;
    if (    (addend & 0x80) == (core->registers.A & 0x80) &&
            (sum & 0x80) != (addend & 0x80)) {
        core->registers.S |= e6502_Status_V;
    }
    core->registers.S |= (sum & e6502_Status_N);
    core->registers.A = sum;
}

static inline void sbc(e6502_Core *core, e6502_Address adr) {
    uint8_t addend = e6502_adr_r8(core, adr);
    uint32_t sum = core->registers.A -
                   addend -
                   (1 - (core->registers.S & e6502_Status_C));
    core->registers.S &= ~(e6502_Status_C | e6502_Status_Z |
                           e6502_Status_V | e6502_Status_N);
    if (sum < 0x100) core->registers.S |= e6502_Status_C;
    if (!(uint8_t)sum) core->registers.S |= e6502_Status_Z;
    if (    (sum & 0x80) != (core->registers.A & 0x80) &&
            (addend & 0x80) != (core->registers.A & 0x80)) {
        core->registers.S |= e6502_Status_V;
    }
    core->registers.S |= (sum & e6502_Status_N);
    core->registers.A = sum;
}

static inline void inc(e6502_Core *core, e6502_Address adr) {
    uint8_t val = e6502_adr_r8(core, adr) + 1;
    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
    core->registers.S |= (val & e6502_Status_N);
    if (!val) core->registers.S |= e6502_Status_Z;
    e6502_adr_w8(core, adr, val);
}

static inline void dec(e6502_Core *core, e6502_Address adr) {
    uint8_t val = e6502_adr_r8(core, adr) - 1;
    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
    core->registers.S |= (val & e6502_Status_N);
    if (!val) core->registers.S |= e6502_Status_Z;
    e6502_adr_w8(core, adr, val);
}

static inline void sta(e6502_Core *core, e6502_Address adr) {
    e6502_adr_w8(core, adr, core->registers.A);
}

static inline void stx(e6502_Core *core, e6502_Address adr) {
    e6502_adr_w8(core, adr, core->registers.X);
}

static inline void sty(e6502_Core *core, e6502_Address adr) {
    e6502_adr_w8(core, adr, core->registers.Y);
}

static inline void t(e6502_Core *core,
                     e6502_Address adr,
                     uint8_t reg) {
    core->registers.S &= ~(e6502_Status_Z | e6502_Status_N);
    core->registers.S |= (reg & e6502_Status_N);
    if (!reg) core->registers.S |= e6502_Status_Z;
    e6502_adr_w8(core, adr, reg);
}

static inline void ta(e6502_Core *core, e6502_Address adr) {
    t(core, adr, core->registers.A);
}

static inline void txs(e6502_Core *core, e6502_Address adr) {
    core->registers.SP = core->registers.X;
}

static inline void txa(e6502_Core *core, e6502_Address adr) {
    t(core, &core->registers.A, core->registers.X);
}

static inline void ty(e6502_Core *core, e6502_Address adr) {
    t(core, adr, core->registers.Y);
}

static inline void ts(e6502_Core *core, e6502_Address adr) {
    t(core, adr, core->registers.SP);
}

static inline void lda(e6502_Core *core, e6502_Address adr) {
    t(core, &core->registers.A, e6502_adr_r8(core, adr));
}

static inline void ldx(e6502_Core *core, e6502_Address adr) {
    t(core, &core->registers.X, e6502_adr_r8(core, adr));
}

static inline void ldy(e6502_Core *core, e6502_Address adr) {
    t(core, &core->registers.Y, e6502_adr_r8(core, adr));
}


typedef struct {
    void(*operator)(e6502_Core*, e6502_Address);
    e6502_Address(*address)(e6502_Core*);
    uint16_t cycles;
} e6502_Instruction;

static e6502_Instruction e6502_instructions[256] = {
    // 0x00
    [0b00000000] = {brk, adr_none,  7},

    [0b00000001] = {ora, adr_ind_x, 6},
    [0b00000101] = {ora, adr_zp,    3},
    [0b00001001] = {ora, adr_imm,   2},
    [0b00001101] = {ora, adr_abs,   4},

    [0b00000110] = {asl, adr_zp,    5},
    [0b00001010] = {asl, adr_reg_a, 2},
    [0b00001110] = {asl, adr_abs,   6},

    [0b00001000] = {php, adr_none,  3},

    // 0x10
    [0b00010000] = {bpl, adr_rel,   2},

    [0b00010001] = {ora, adr_ind_y, 5},
    [0b00010101] = {ora, adr_zp_x,  4},
    [0b00011001] = {ora, adr_abs_y, 4},
    [0b00011101] = {ora, adr_abs_x, 4},

    [0b00011000] = {clc, adr_none,  2},

    [0b00010110] = {asl, adr_zp_x,  6},
    [0b00011110] = {asl, adr_abs_x, 7},

    // 0x20
    [0b00100000] = {jsr, adr_abs,   6},

    [0b00100100] = {bit, adr_zp,    3},
    [0b00101100] = {bit, adr_abs,   4},

    [0b00100001] = {and, adr_ind_x, 6},
    [0b00100101] = {and, adr_zp,    3},
    [0b00101001] = {and, adr_imm,   2},
    [0b00101101] = {and, adr_abs,   4},

    [0b00100110] = {rol, adr_zp,    5},
    [0b00101010] = {rol, adr_reg_a, 2},
    [0b00101110] = {rol, adr_abs,   6},

    [0b00101000] = {plp, adr_none,  4},

    // 0x30
    [0b00110000] = {bmi, adr_rel,   2},

    [0b00110001] = {and, adr_ind_y, 5},
    [0b00110101] = {and, adr_zp_x,  4},
    [0b00111001] = {and, adr_abs_y, 3},
    [0b00111101] = {and, adr_abs_x, 3},

    [0b00110110] = {rol, adr_zp_x,  6},
    [0b00111110] = {rol, adr_abs_x, 7},

    [0b00111000] = {sec, adr_none,  2},

    // 0x40
    [0b01000000] = {rti, adr_none,  6},

    [0b01000001] = {eor, adr_ind_x, 6},
    [0b01000101] = {eor, adr_zp,    3},
    [0b01001001] = {eor, adr_imm,   2},
    [0b01001101] = {eor, adr_abs,   4},

    [0b01000110] = {lsr, adr_zp,    5},
    [0b01001010] = {lsr, adr_reg_a, 2},
    [0b01001110] = {lsr, adr_abs,   6},

    [0b01001000] = {pha, adr_none,  3},

    [0b01001100] = {jmp, adr_abs,   3},

    // 0x50
    [0b01010000] = {bvc, adr_rel,   2},

    [0b01010001] = {eor, adr_ind_y, 5},
    [0b01010101] = {eor, adr_zp_x,  4},
    [0b01011001] = {eor, adr_abs_y, 4},
    [0b01011101] = {eor, adr_abs_x, 4},

    [0b01010110] = {lsr, adr_zp_x,  6},
    [0b01011110] = {lsr, adr_abs_x, 7},

    [0b01011000] = {cli, adr_none,  2},

    // 0x60
    [0b01100000] = {rts, adr_none,  6},

    [0b01100001] = {adc, adr_ind_x, 6},
    [0b01100101] = {adc, adr_zp,    3},
    [0b01101001] = {adc, adr_imm,   2},
    [0b01101101] = {adc, adr_abs,   4},

    [0b01100110] = {ror, adr_zp,    5},
    [0b01101010] = {ror, adr_reg_a, 2},
    [0b01101110] = {ror, adr_abs,   6},

    [0b01101000] = {pla, adr_none,  4},

    [0b01101100] = {jmp, adr_ind,   5},

    // 0x70
    [0b01110000] = {bvs, adr_rel,   2},

    [0b01110001] = {adc, adr_ind_y, 5},
    [0b01110101] = {adc, adr_zp_x,  4},
    [0b01111001] = {adc, adr_abs_y, 4},
    [0b01111101] = {adc, adr_abs_x, 4},

    [0b01110110] = {ror, adr_zp_x,  6},
    [0b01111110] = {ror, adr_abs_x, 7},

    [0b01111000] = {sei, adr_none,  2},

    // 0x80
    [0b10000001] = {sta, adr_ind_x, 6},
    [0b10000101] = {sta, adr_zp,    3},
    [0b10001101] = {sta, adr_abs,   4},

    [0b10000110] = {stx, adr_zp,    3},
    [0b10001110] = {stx, adr_abs,   4},

    [0b10000100] = {sty, adr_zp,    3},
    [0b10001100] = {sty, adr_abs,   4},

    [0b10001000] = {dec, adr_reg_y, 2},

    [0b10001010] = {txa, adr_none, 2},

    // 0x90
    [0b10010000] = {bcc, adr_rel,   2},

    [0b10010001] = {sta, adr_ind_y, 6},
    [0b10010101] = {sta, adr_zp_x,  4},
    [0b10011001] = {sta, adr_abs_y, 5},
    [0b10011101] = {sta, adr_abs_x, 5},

    [0b10010110] = {stx, adr_zp_y,  4},

    [0b10010100] = {sty, adr_zp_x,  4},

    [0b10011000] = {ty,  adr_reg_a, 2},
    [0b10011010] = {txs, adr_none,  2},

    // 0xA0
    [0b10100001] = {lda, adr_ind_x, 6},
    [0b10100101] = {lda, adr_zp,    3},
    [0b10101001] = {lda, adr_imm,   2},
    [0b10101101] = {lda, adr_abs,   4},

    [0b10100010] = {ldx, adr_imm,   2},
    [0b10100110] = {ldx, adr_zp,    3},
    [0b10101110] = {ldx, adr_abs,   4},

    [0b10100000] = {ldy, adr_imm,   2},
    [0b10100100] = {ldy, adr_zp,    3},
    [0b10101100] = {ldy, adr_abs,   4},

    [0b10101010] = {ta,  adr_reg_x, 2},
    [0b10101000] = {ta,  adr_reg_y, 2},

    // 0xB0
    [0b10110000] = {bcs, adr_rel,   2},

    [0b10110001] = {lda, adr_ind_y, 5},
    [0b10110101] = {lda, adr_zp_x,  4},
    [0b10111001] = {lda, adr_abs_y, 4},
    [0b10111101] = {lda, adr_abs_x, 4},

    [0b10110110] = {ldx, adr_zp_y,  4},
    [0b10111110] = {ldx, adr_abs_y, 4},

    [0b10110100] = {ldy, adr_zp_x,  4},
    [0b10111100] = {ldy, adr_abs_x, 4},

    [0b10111000] = {clv, adr_none,  2},

    [0b10111010] = {ts,  adr_reg_x, 2},

    // 0xC0
    [0b11000000] = {cpy, adr_imm,   2},
    [0b11000100] = {cpy, adr_zp,    3},
    [0b11001100] = {cpy, adr_abs,   4},

    [0b11000001] = {cpa, adr_ind_x, 6},
    [0b11000101] = {cpa, adr_zp,    3},
    [0b11001001] = {cpa, adr_imm,   2},
    [0b11001101] = {cpa, adr_abs,   4},

    [0b11001000] = {inc, adr_reg_y, 2},

    [0b11000110] = {dec, adr_zp,    5},
    [0b11001110] = {dec, adr_abs,   6},
    [0b11010110] = {dec, adr_zp_x,  6},
    [0b11011110] = {dec, adr_abs_x, 7},

    [0b11001010] = {dec, adr_reg_x, 2},

    // 0xD0
    [0b11010000] = {bne, adr_rel,   2},

    [0b11011000] = {cld, adr_none,  2},

    [0b11010001] = {cpa, adr_ind_y, 5},
    [0b11010101] = {cpa, adr_zp_x,  4},
    [0b11011001] = {cpa, adr_abs_y, 4},
    [0b11011101] = {cpa, adr_abs_x, 4},

    // 0xE0
    [0b11100000] = {cpx, adr_imm,   2},
    [0b11100100] = {cpx, adr_zp,    3},
    [0b11101100] = {cpx, adr_abs,   4},

    [0b11100001] = {sbc, adr_ind_x, 6},
    [0b11100101] = {sbc, adr_zp,    3},
    [0b11101001] = {sbc, adr_imm,   2},
    [0b11101101] = {sbc, adr_abs,   4},

    [0b11100110] = {inc, adr_zp,    5},
    [0b11101110] = {inc, adr_abs,   6},

    [0b11101000] = {inc, adr_reg_x, 2},

    [0b11101010] = {nop, adr_none,  2},

    // 0xF0
    [0b11110000] = {beq, adr_rel,   2},

    [0b11110001] = {sbc, adr_ind_y, 5},
    [0b11110101] = {sbc, adr_zp_x,  4},
    [0b11111001] = {sbc, adr_abs_y, 4},
    [0b11111101] = {sbc, adr_abs_x, 4},

    [0b11110110] = {inc, adr_zp_x,  6},
    [0b11111110] = {inc, adr_abs_x, 7},

    [0b11111000] = {sed, adr_none,  2},

};

int e6502_instr_size(uint8_t opcode) {
    e6502_Instruction* instr = &e6502_instructions[opcode];
    if (    instr->address == adr_none ||
            instr->address == adr_reg_a ||
            instr->address == adr_reg_x ||
            instr->address == adr_reg_y ||
            instr->address == adr_reg_s ||
            instr->address == adr_reg_sp) {
        return 1;
    } else if ( instr->address == adr_abs ||
                instr->address == adr_abs_x ||
                instr->address == adr_abs_y) {
        return 3;
    } else {
        return 2;
    }
}

/*
int e6502_dump_instr(e6502_Core* core, e6502_Mem_Addr addr,
                     FILE* file) {
    uint8_t opcode = e6502_r8(core, addr);
    int size = e6502_instr_size(opcode);
    fprintf(file, "$%02x", opcode);
    if (size == 2) {
        fprintf(file, " $%02x", e6502_r8(core, addr + 1));
    } else if (size == 3) {
        fprintf(file, " $%04x", e6502_r16(core, addr + 1));
    }
    fputc('\n', file);
}

int e6502_dump_regs(e6502_Core* core, FILE* file) {
    fprintf(file, "S:%02x A:%02x X:%02x Y:%02x P:%02x\n",
            core->registers.SP, core->registers.A,
            core->registers.X, core->registers.Y,
            core->registers.S);
}
*/

int e6502_run(e6502_Core* core,
              int remaining,
              int* run,
              int instructions) {
    int status = 0;
    int start = (instructions ? core->instr : core->cycle);
    int end = start + remaining;
    int last_pc = -1;
    while ((instructions ? core->instr : core->cycle) < end) {
        if (core->registers.PC == last_pc) {
            // Trapped.
            status = -2;
            break;
        }
        last_pc = core->registers.PC;
        uint8_t opcode = e6502_r8(core, core->registers.PC);
        e6502_Instruction* instr = &e6502_instructions[opcode];
        if (!instr->operator) {
/*
            fprintf(stdout, "$%04x: %02x\n", last_pc, opcode);
*/
            status = -1;
            break;
        } else {
            core->registers.PC++;
            e6502_Address adr = instr->address(core);
/*
            uint16_t size = core->registers.PC - last_pc;
*/
            instr->operator(core, adr);
            core->cycle += instr->cycles;
            core->instr++;
/*
            fprintf(stdout, "$%04x: $%02x", last_pc, opcode);
            if (size == 2) {
                fprintf(stdout, " $%02x", e6502_r8(core, last_pc + 1));
            } else if (size == 3) {
                fprintf(stdout, " $%02x%02x", e6502_r8(core, last_pc + 2), e6502_r8(core, last_pc + 1));
            }
            fputc('\n', stdout);
            e6502_dump_regs(core, stdout);
*/
        }
    }
    if (run) {
        *run = (instructions ? core->instr : core->cycle) - start;
    }
    return status;
}

int e6502_reset(e6502_Core* core) {
    core->registers.PC = e6502_r16(core, e6502_Reset_Vec);
    core->registers.SP -= 3;
    core->registers.S |= (e6502_Status_B | e6502_Status_1);
}