dirty_trick.c

// Option 1: two dispatches, separated address modes and operations

#define ABSOLUTE_X \
         0x1C: case 0x1D: case 0x1E: case 0x1F: \
    case 0x3C: case 0x3D: case 0x3E: case 0x3F: \
    case 0x5C: case 0x5D: case 0x5E: case 0x5F: \
    case 0x7C: case 0x7D: case 0x7E: case 0x7F: \
    case 0x9C: case 0x9D: \
    case 0xBC: case 0xBD: \
    case 0xDC: case 0xDD: case 0xDE: case 0xDF: \
    case 0xFC: case 0xFD: case 0xFE: case 0xFF

// ...

#define ADC \
         0x61: case 0x65: case 0x69: case 0x6D: \
    case 0x71: case 0x75: case 0x79: case 0x7D

// ...

switch (instruction) {
    // Absolute 16-bit address plus X
    case ABSOLUTE_X:
        address = READ16(pc + 1) + x;
        pc += 3;
        break;

    // ...
}

// ...

switch (instruction) {
    // Add with carry
    case ADC: {
        int sum = (int)a + (int)b + (int)c;
        SET_CV(a, b, sum);
        a = (uint8_t)sum;
        SET_ZN(a);
        break;
    }

    // ...
}

// Option 2: one dispatch, fused address modes and operations

// Absolute 16-bit address plus X
#define ABSOLUTE_X \
    { \
        address = READ16(pc + 1) + x; \
        pc += 3; \
    }

// ...

// Add with carry
#define ADC \
    { \
        int sum = (int)a + (int)b + (int)c; \
        SET_CV(a, b, sum); \
        a = (uint8_t)sum; \
        SET_ZN(a); \
    }

// ...

#define CASE(byte, operation, addressing_mode) \
    case byte: { \
        addressing_mode \
        b = READ8(address); \
        operation \
        break; \
    }

// ...

switch (instruction) {
    CASE(0x7D, ADC, ABSOLUTE_X)
    // ...
}

// Option 3: Like option 2 but with forced-inline functions rather than macros. Relies heavily on optimizer doing the right thing.

FORCE_INLINE
void ABSOLUTE_X(cpu_t *cpu) {
    cpu->address = READ16(cpu->pc + 1) + cpu->x;
    cpu->pc += 3;
}

// ...

FORCE_INLINE
void ADC(cpu_t *cpu) {
    int sum = (int)cpu->a + (int)cpu->b + (int)cpu->c;
    SET_CV(cpu->a, cpu->b, sum);
    cpu->a = (uint8_t)sum;
    SET_ZN(cpu->a);
}

// ...

#define CASE(byte, operation, addressing_mode) \
    case byte: { \
        addressing_mode(cpu); \
        cpu->b = READ8(cpu->address); \
        operation(cpu); \
        break; \
    }

// ...

switch (instruction) {
    CASE(0x7D, ADC, ABSOLUTE_X)
    // ...
}