6502.c

/* MoarNES source - CPU.c

   This open-source software is (c)2010-2013 Mike Chambers, and is released
   under the terms of the GNU GPL v2 license.

   This is my implementation of a MOS Technology 6502 CPU emulator. The
   NES technically has a Ricoh 2A03, not a true 6502, however it is nearly
   identical to it.

   The differences are small:
     - The 2A03 does not support binary-coded decimal (BCD) arithmetic.
	 - It has the audio processor built into it.

   The audio emulation is, however, handled in APU.c.
*/

#include "config-6502.h"
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <stdlib.h>

#define _6502_PRIVATE
#include "6502.h"


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

//helper variables
uint64_t instructions = 0;	//keep track of total instructions executed
uint64_t clockticks6502 = 0, clockgoal6502 = 0;
uint16_t oldpc, ea, reladdr, value, result;
uint8_t opcode, oldstatus;
uint8_t mempage;		// address holding the memory page to use (low 4 bits)

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

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

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

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

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


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

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

static void acc(void)
{				//accumulator
}

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

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

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

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

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

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

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

#if 0
static uint16_t getvalue16(void)
{
	return ((uint16_t) read6502(ea) | ((uint16_t) read6502(ea + 1) << 8));
}
#endif

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(void)
{
	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(void)
{
	penaltyop = 1;
	value = getvalue();
	result = (uint16_t) a & value;

	zerocalc(result);
	signcalc(result);

	saveaccum(result);
}

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

	carrycalc(result);
	zerocalc(result);
	signcalc(result);

	putvalue(result);
}

static void bcc(void)
{
	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(void)
{
	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(void)
{
	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(void)
{
	value = getvalue();
	result = (uint16_t) a & value;

	zerocalc(result);
	status = (status & 0x3F) | (uint8_t) (value & 0xC0);
}

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

static void bvc(void)
{
	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(void)
{
	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(void)
{
	clearcarry();
}

static void cld(void)
{
	cleardecimal();
}

static void cli(void)
{
	clearinterrupt();
}

static void clv(void)
{
	clearoverflow();
}

static void cmp(void)
{
	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(void)
{
	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(void)
{
	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(void)
{
	value = getvalue();
	result = value - 1;

	zerocalc(result);
	signcalc(result);

	putvalue(result);
}

static void dex(void)
{
	x--;

	zerocalc(x);
	signcalc(x);
}

static void dey(void)
{
	y--;

	zerocalc(y);
	signcalc(y);
}

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

	zerocalc(result);
	signcalc(result);

	saveaccum(result);
}

static void inc(void)
{
	value = getvalue();
	result = value + 1;

	zerocalc(result);
	signcalc(result);

	putvalue(result);
}

static void inx(void)
{
	x++;

	zerocalc(x);
	signcalc(x);
}

static void iny(void)
{
	y++;

	zerocalc(y);
	signcalc(y);
}

static void jmp(void)
{
	pc = ea;
}

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

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

	zerocalc(a);
	signcalc(a);
}

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

	zerocalc(x);
	signcalc(x);
}

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

	zerocalc(y);
	signcalc(y);
}

static void lsr(void)
{
	value = getvalue();
	result = value >> 1;

	if (value & 1)
		setcarry();
	else
		clearcarry();
	zerocalc(result);
	signcalc(result);

	putvalue(result);
}

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

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

	zerocalc(result);
	signcalc(result);

	saveaccum(result);
}

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

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

static void pla(void)
{
	a = pull8();

	zerocalc(a);
	signcalc(a);
}

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

static void rol(void)
{
	value = getvalue();
	result = (value << 1) | (status & FLAG_CARRY);

	carrycalc(result);
	zerocalc(result);
	signcalc(result);

	putvalue(result);
}

static void ror(void)
{
	value = getvalue();
	result = (value >> 1) | ((status & FLAG_CARRY) << 7);

	if (value & 1)
		setcarry();
	else
		clearcarry();
	zerocalc(result);
	signcalc(result);

	putvalue(result);
}

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

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

static void sbc(void)
{
	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(void)
{
	setcarry();
}

static void sed(void)
{
	setdecimal();
}

static void sei(void)
{
	setinterrupt();
}

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

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

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

static void tax(void)
{
	x = a;

	zerocalc(x);
	signcalc(x);
}

static void tay(void)
{
	y = a;

	zerocalc(y);
	signcalc(y);
}

static void tsx(void)
{
	x = sp;

	zerocalc(x);
	signcalc(x);
}

static void txa(void)
{
	a = x;

	zerocalc(a);
	signcalc(a);
}

static void txs(void)
{
	sp = x;
}

static void tya(void)
{
	a = y;

	zerocalc(a);
	signcalc(a);
}

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

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

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

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

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

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

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

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

static const uint32_t ticktable[256] = {
	7, 6, 2, 8, 3, 3, 5, 5, 3, 2, 2, 2, 4, 4, 6, 6,
	2, 5, 2, 8, 4, 4, 6, 6, 2, 4, 2, 7, 4, 4, 7, 7,
	6, 6, 2, 8, 3, 3, 5, 5, 4, 2, 2, 2, 4, 4, 6, 6,
	2, 5, 2, 8, 4, 4, 6, 6, 2, 4, 2, 7, 4, 4, 7, 7,
	6, 6, 2, 8, 3, 3, 5, 5, 3, 2, 2, 2, 3, 4, 6, 6,
	2, 5, 2, 8, 4, 4, 6, 6, 2, 4, 2, 7, 4, 4, 7, 7,
	6, 6, 2, 8, 3, 3, 5, 5, 4, 2, 2, 2, 5, 4, 6, 6,
	2, 5, 2, 8, 4, 4, 6, 6, 2, 4, 2, 7, 4, 4, 7, 7,
	2, 6, 2, 6, 3, 3, 3, 3, 2, 2, 2, 2, 4, 4, 4, 4,
	2, 6, 2, 6, 4, 4, 4, 4, 2, 5, 2, 5, 5, 5, 5, 5,
	2, 6, 2, 6, 3, 3, 3, 3, 2, 2, 2, 2, 4, 4, 4, 4,
	2, 5, 2, 5, 4, 4, 4, 4, 2, 4, 2, 4, 4, 4, 4, 4,
	2, 6, 2, 8, 3, 3, 5, 5, 2, 2, 2, 2, 4, 4, 6, 6,
	2, 5, 2, 8, 4, 4, 6, 6, 2, 4, 2, 7, 4, 4, 7, 7,
	2, 6, 2, 8, 3, 3, 5, 5, 2, 2, 2, 2, 4, 4, 6, 6,
	2, 5, 2, 8, 4, 4, 6, 6, 2, 4, 2, 7, 4, 4, 7, 7
};


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

void irq6502(void)
{
	if ((status & FLAG_INTERRUPT) == FLAG_INTERRUPT)
		return;		//abort if interrupts are inhibited
	push16(pc);
	push8(status);
	status |= FLAG_INTERRUPT;
	pc = (uint16_t) read6502(0xFFFE) | ((uint16_t) read6502(0xFFFF) << 8);
}

static void (*loopexternal) (void);

int log_6502 = 0;

uint64_t exec6502(uint64_t tickcount)
{
	uint64_t startticks;
	clockgoal6502 += tickcount;

	startticks = clockticks6502;
	while (clockticks6502 < clockgoal6502) {
		opcode = read6502(pc++);
		/* Track for 6509 emulation */
		mempage = 0;
		if (opcode == 0xB1 || opcode == 0x91)
			mempage = 1;
		status |= FLAG_CONSTANT;
		if (log_6502) {
			uint8_t c[3];
			char *dis;
			c[0] = opcode;
			c[1] = read6502_debug(pc);
			c[2] = read6502_debug(pc + 1);
			dis = dis6502(pc - 1, c);
			fprintf(stderr, "%02X %02X %02X %02X %02X | %04X %s\n",
				a, x, y, sp, status, pc - 1, dis);
		}
		penaltyop = 0;
		penaltyaddr = 0;

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

		instructions++;

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

	return (clockticks6502 - startticks);
}

void step6502(void)
{
	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 (loopexternal)
		(*loopexternal) ();
}

void hookexternal(void (*funcptr) (void))
{
	loopexternal = funcptr;
}

uint16_t getPC(void)
{
	return (pc);
}

uint64_t getclockticks(void)
{
	return (clockticks6502);
}

void waitstates(uint32_t n)
{
	clockticks6502 += n;
}

void init6502(void)
{
	disassembler_init();
}