State-based 6502 emulator snippet

gistfile1.txt

//////////////////////////////////////////////////////////////////////////
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//
// Read/Absolute,X
//
//////////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////

static void T1_R_ABX(M6502 *);
static void T2_R_ABX(M6502 *);
static void T3_R_ABX(M6502 *);
static void T4_R_ABX(M6502 *);

static void T0_R_ABX(M6502 *s) {
    /* T0 phase 2 */
    /* (decode - already done) */

    /* T1 phase 1 */
    s->abus.w=s->pc.w++;
    s->read=1;
    s->tfn=&T1_R_ABX;
}

static void T1_R_ABX(M6502 *s) {
    /* T1 phase 2 */
    s->ad.b.l=s->dbus;

    /* T2 phase 1 */
    s->abus.w=s->pc.w++;
    s->read=1;
    s->tfn=&T2_R_ABX;
}

static void T2_R_ABX(M6502 *s) {
    /* T2 phase 2 */
    s->ad.b.h=s->dbus;

    /* T3 phase 1 */
    s->abus.w=s->ad.b.l+s->x;
    s->acarry=s->abus.b.h;
    s->abus.b.h=s->ad.b.h;
    s->read=1;
    if(s->acarry) {
        CheckForInterrupts(s);
    }
    s->tfn=&T3_R_ABX;
}

static void T3_R_ABX(M6502 *s) {
    /* T3 phase 2 */
    s->data=s->dbus;
    if(!s->acarry) {
        /* No carry - bail out early. */
        (*s->ifn)(s);
#ifdef _DEBUG
        s->ifn=NULL;
#endif

        /* T0 phase 1 */
        M6502_NextInstruction(s);
        return;
    }

    /* T4 phase 1 */
    s->abus.w=s->ad.w+s->x;
    s->read=1;
    s->tfn=&T4_R_ABX;
    CheckForInterrupts(s);
}

static void T4_R_ABX(M6502 *s) {
    /* T4 phase 2 */
    s->data=s->dbus;
    (*s->ifn)(s);
#ifdef _DEBUG
    s->ifn=NULL;
#endif

    /* T0 phase 1 */
    M6502_NextInstruction(s);
}