aiie/cpu.cpp

#ifdef TEENSYDUINO
#include <Arduino.h>
#endif
#include "cpu.h"
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include "mmu.h"

#include "serialize.h"

#include "globals.h"

#ifdef TEENSYDUINO
#include "teensy-println.h"
#include "iocompat.h"
#endif

// define DEBUGSTEPS to show disassembly of each instruction as it's processed
//#define DEBUGSTEPS
#ifdef DEBUGSTEPS
#include "disassembler.h"
extern Disassembler dis;
#endif

// To see calls to unimplemented opcodes, define this:
//#define VERBOSE_CPU_ERRORS

// To exit on illegals:
//#define EXIT_ON_ILLEGAL

// Macros to set negative and zero flags based on param, X, Y, whatever
#define SETNZ  { FLAG(F_N, param & 0x80); FLAG(F_Z, !param); }
#define SETNZX { FLAG(F_N, x & 0x80);     FLAG(F_Z, !x); }
#define SETNZY { FLAG(F_N, y & 0x80);     FLAG(F_Z, !y); }
#define SETNZA { FLAG(F_N, a & 0x80);     FLAG(F_Z, !a); }

#define FLAG(bit, condition) { if (condition) {flags |= bit;} else {flags &= ~bit;} }

#define writemem(addr, val) mmu->write(addr, val)
#define readmem(addr) mmu->read(addr)

// serialize suspend/restore token
#define CPUMAGIC 0x65

optype_t opcodes[256] = {
  { O_BRK,     A_IMP,     7 }, // 0x00
  { O_ORA    , A_INX    , 6 }, // 0x01 [2]  i.e. "ORA ($44,X)"
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x02
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x03
  { O_TSB    , A_ZER    , 5 }, // 0x04 [2]
  { O_ORA    , A_ZER    , 3 }, // 0x05 [2]  i.e. "ORA $44"
  { O_ASL    , A_ZER    , 5 }, // 0x06 [2]
  { O_RMB    , A_ZER    , 5 }, // 0x07
  { O_PHP    , A_IMP    , 3 }, // 0x08
  { O_ORA    , A_IMM    , 2 }, // 0x09      i.e. "ORA #$44"
  { O_ASL_ACC, A_ACC    , 2 }, // 0x0A
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x0B
  { O_TSB    , A_ABS    , 6 }, // 0x0C
  { O_ORA    , A_ABS    , 4 }, // 0x0D      i.e. "ORA $4400"
  { O_ASL    , A_ABS    , 6 }, // 0x0E
  { O_BBR    , A_ZPREL  , 5 }, // 0x0F
  { O_BPL    , A_REL    , 2 }, // 0x10 [8]
  { O_ORA    , A_INY    , 5 }, // 0x11 [2,3] i.e. "ORA ($44),Y"
  { O_ORA    , A_ZIND   , 5 }, // 0x12 [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x13
  { O_TRB    , A_ZER    , 5 }, // 0x14 [2]
  { O_ORA    , A_ZEX    , 4 }, // 0x15 [2]   i.e. "ORA $44,X"
  { O_ASL    , A_ZEX    , 6 }, // 0x16 [2]
  { O_RMB    , A_ZER    , 5 }, // 0x17
  { O_CLC    , A_IMP    , 2 }, // 0x18
  { O_ORA    , A_ABY    , 4 }, // 0x19 [3]   i.e. "ORA $4400,Y"
  { O_INC_ACC, A_ACC    , 2 }, // 0x1A
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x1B
  { O_TRB    , A_ABS    , 6 }, // 0x1C [3]
  { O_ORA    , A_ABX    , 4 }, // 0x1D [3]   i.e. "ORA $4400,X"
  { O_ASL    , A_ABX    , 6 }, // 0x1E [6]
  { O_BBR    , A_ZPREL  , 5 }, // 0x1F
  { O_JSR    , A_ABS    , 6 }, // 0x20
  { O_AND    , A_INX    , 6 }, // 0x21 [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x22
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x23
  { O_BIT    , A_ZER    , 3 }, // 0x24 [2]
  { O_AND    , A_ZER    , 3 }, // 0x25 [2]
  { O_ROL    , A_ZER    , 5 }, // 0x26 [2]
  { O_RMB    , A_ZER    , 5 }, // 0x27
  { O_PLP    , A_IMP    , 4 }, // 0x28
  { O_AND    , A_IMM    , 2 }, // 0x29 
  { O_ROL_ACC, A_ACC    , 2 }, // 0x2A
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x2B
  { O_BIT    , A_ABS    , 4 }, // 0x2C 
  { O_AND    , A_ABS    , 4 }, // 0x2D 
  { O_ROL    , A_ABS    , 6 }, // 0x2E
  { O_BBR    , A_ZPREL  , 5 }, // 0x2F
  { O_BMI    , A_REL    , 2 }, // 0x30 [8]
  { O_AND    , A_INY    , 5 }, // 0x31 [2,3]
  { O_AND    , A_ZIND   , 5 }, // 0x32 [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x33
  { O_BIT    , A_ZEX    , 4 }, // 0x34 [2]
  { O_AND    , A_ZEX    , 4 }, // 0x35 [2]
  { O_ROL    , A_ZEX    , 6 }, // 0x36 [2]
  { O_RMB    , A_ZER    , 5 }, // 0x37
  { O_SEC    , A_IMP    , 2 }, // 0x38
  { O_AND    , A_ABY    , 4 }, // 0x39 [3]
  { O_DEC_ACC, A_ACC    , 2 }, // 0x3A [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x3B
  { O_BIT    , A_ABX    , 4 }, // 0x3C [3]
  { O_AND    , A_ABX    , 4 }, // 0x3D [3]
  { O_ROL    , A_ABX    , 6 }, // 0x3E [6]
  { O_BBR    , A_ZPREL  , 5 }, // 0x3F
  { O_RTI    , A_IMP    , 6 }, // 0x40
  { O_EOR    , A_INX    , 6 }, // 0x41 [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x42
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x43
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x44
  { O_EOR    , A_ZER    , 3 }, // 0x45 [2]
  { O_LSR    , A_ZER    , 5 }, // 0x46 [2]
  { O_RMB    , A_ZER    , 5 }, // 0x47
  { O_PHA    , A_IMP    , 3 }, // 0x48 
  { O_EOR    , A_IMM    , 2 }, // 0x49 
  { O_LSR_ACC, A_ACC    , 2 }, // 0x4A
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x4B
  { O_JMP    , A_ABS    , 3 }, // 0x4C
  { O_EOR    , A_ABS    , 4 }, // 0x4D 
  { O_LSR    , A_ABS    , 6 }, // 0x4E
  { O_BBR    , A_ZPREL  , 5 }, // 0x4F
  { O_BVC    , A_REL    , 2 }, // 0x50 [8]
  { O_EOR    , A_INY    , 5 }, // 0x51 [2,3]
  { O_EOR    , A_ZIND   , 5 }, // 0x52 
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x53
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x54
  { O_EOR    , A_ZEX    , 4 }, // 0x55 [2]
  { O_LSR    , A_ZEX    , 6 }, // 0x56 [2]
  { O_RMB    , A_ZER    , 5 }, // 0x57
  { O_CLI    , A_IMP    , 2 }, // 0x58
  { O_EOR    , A_ABY    , 4 }, // 0x59 [3]
  { O_PHY    , A_IMP    , 3 }, // 0x5A
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x5B
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x5C
  { O_EOR    , A_ABX    , 4 }, // 0x5D [3]
  { O_LSR    , A_ABX    , 6 }, // 0x5E [6]
  { O_BBR    , A_ZPREL  , 5 }, // 0x5F
  { O_RTS    , A_IMP    , 6 }, // 0x60
  { O_ADC    , A_INX    , 6 }, // 0x61 [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x62
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x63
  { O_STZ    , A_ZER    , 3 }, // 0x64 [2]
  { O_ADC    , A_ZER    , 3 }, // 0x65 [2]
  { O_ROR    , A_ZER    , 5 }, // 0x66 [2]
  { O_RMB    , A_ZER    , 5 }, // 0x67
  { O_PLA    , A_IMP    , 4 }, // 0x68 
  { O_ADC    , A_IMM    , 2 }, // 0x69
  { O_ROR_ACC, A_ACC    , 2 }, // 0x6A [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x6B
  { O_JMP    , A_ABI    , 6 }, // 0x6C
  { O_ADC    , A_ABS    , 4 }, // 0x6D
  { O_ROR    , A_ABS    , 6 }, // 0x6E
  { O_BBR    , A_ZPREL  , 5 }, // 0x6F
  { O_BVS    , A_REL    , 2 }, // 0x70 [8]
  { O_ADC    , A_INY    , 5 }, // 0x71 [2,3]
  { O_ADC    , A_ZIND   , 5 }, // 0x72 [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x73
  { O_STZ    , A_ZEX    , 4 }, // 0x74 [2]
  { O_ADC    , A_ZEX    , 4 }, // 0x75 [2]
  { O_ROR    , A_ZEX    , 6 }, // 0x76 [2]
  { O_RMB    , A_ZER    , 5 }, // 0x77
  { O_SEI    , A_IMP    , 2 }, // 0x78
  { O_ADC    , A_ABY    , 4 }, // 0x79 [3]
  { O_PLY    , A_IMP    , 4 }, // 0x7A
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x7B
  { O_JMP    , A_ABXI   , 6 }, // 0x7C JMP (ABS, X)
  { O_ADC    , A_ABX    , 4 }, // 0x7D [3] Absolute,X    ADC $4400,X
  { O_ROR    , A_ABX    , 6 }, // 0x7E [6]
  { O_BBR    , A_ZPREL  , 5 }, // 0x7F
  { O_BRA    , A_REL    , 3 }, // 0x80 [8]
  { O_STA    , A_INX    , 6 }, // 0x81 [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x82
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x83
  { O_STY    , A_ZER    , 3 }, // 0x84 [2]
  { O_STA    , A_ZER    , 3 }, // 0x85 [2]
  { O_STX    , A_ZER    , 3 }, // 0x86 [2]
  { O_SMB    , A_ZER    , 5 }, // 0x87
  { O_DEY    , A_IMP    , 2 }, // 0x88
  { O_BIT    , A_IMM    , 2 }, // 0x89 
  { O_TXA    , A_IMP    , 2 }, // 0x8A
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x8B
  { O_STY    , A_ABS    , 4 }, // 0x8C
  { O_STA    , A_ABS    , 4 }, // 0x8D 
  { O_STX    , A_ABS    , 4 }, // 0x8E
  { O_BBS    , A_ZPREL  , 5 }, // 0x8F
  { O_BCC    , A_REL    , 2 }, // 0x90 [8]
  { O_STA    , A_INY    , 6 }, // 0x91 [2]
  { O_STA    , A_ZIND   , 5 }, // 0x92 [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x93
  { O_STY    , A_ZEX    , 4 }, // 0x94 [2]
  { O_STA    , A_ZEX    , 4 }, // 0x95 [2]
  { O_STX    , A_ZEY    , 4 }, // 0x96 [2]
  { O_SMB    , A_ZER    , 5 }, // 0x97
  { O_TYA    , A_IMP    , 2 }, // 0x98
  { O_STA    , A_ABY    , 5 }, // 0x99 
  { O_TXS    , A_IMP    , 2 }, // 0x9A [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0x9B
  { O_STZ    , A_ABS    , 4 }, // 0x9C
  { O_STA    , A_ABX    , 5 }, // 0x9D 
  { O_STZ    , A_ABX    , 5 }, // 0x9E 
  { O_BBS    , A_ZPREL  , 5 }, // 0x9F
  { O_LDY    , A_IMM    , 2 }, // 0xA0
  { O_LDA    , A_INX    , 6 }, // 0xA1 [2]
  { O_LDX    , A_IMM    , 2 }, // 0xA2
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xA3
  { O_LDY    , A_ZER    , 3 }, // 0xA4 [2]
  { O_LDA    , A_ZER    , 3 }, // 0xA5 [2]
  { O_LDX    , A_ZER    , 3 }, // 0xA6 [2]
  { O_SMB    , A_ZER    , 5 }, // 0xA7
  { O_TAY    , A_IMP    , 2 }, // 0xA8
  { O_LDA    , A_IMM    , 2 }, // 0xA9 
  { O_TAX    , A_IMP    , 2 }, // 0xAA
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xAB
  { O_LDY    , A_ABS    , 4 }, // 0xAC
  { O_LDA    , A_ABS    , 4 }, // 0xAD 
  { O_LDX    , A_ABS    , 4 }, // 0xAE
  { O_BBS    , A_ZPREL  , 5 }, // 0xAF
  { O_BCS    , A_REL    , 2 }, // 0xB0 [8]
  { O_LDA    , A_INY    , 5 }, // 0xB1 [2,3]
  { O_LDA    , A_ZIND   , 5 }, // 0xB2 [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xB3
  { O_LDY    , A_ZEX    , 4 }, // 0xB4 [2]
  { O_LDA    , A_ZEX    , 4 }, // 0xB5 [2]
  { O_LDX    , A_ZEY    , 4 }, // 0xB6 [2]
  { O_SMB    , A_ZER    , 5 }, // 0xB7
  { O_CLV    , A_IMP    , 2 }, // 0xB8
  { O_LDA    , A_ABY    , 4 }, // 0xB9 [3]
  { O_TSX    , A_IMP    , 2 }, // 0xBA
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xBB
  { O_LDY    , A_ABX    , 4 }, // 0xBC [3]
  { O_LDA    , A_ABX    , 4 }, // 0xBD [3]
  { O_LDX    , A_ABY    , 4 }, // 0xBE [3]
  { O_BBS    , A_ZPREL  , 5 }, // 0xBF
  { O_CPY    , A_IMM    , 2 }, // 0xC0
  { O_CMP    , A_INX    , 6 }, // 0xC1 [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xC2
  //  { O_DCP,     A_INX,     2 }, // 0xC3 -- fixme -- cycle count of this illegal instruction?
  { O_ILLEGAL, A_ILLEGAL,     2 }, // 0xC3 -- as a nop for tests
  { O_CPY    , A_ZER    , 3 }, // 0xC4 [2]
  { O_CMP    , A_ZER    , 3 }, // 0xC5 [2]
  { O_DEC    , A_ZER    , 5 }, // 0xC6 [2]
  { O_SMB    , A_ZER    , 5 }, // 0xC7
  { O_INY    , A_IMP    , 2 }, // 0xC8
  { O_CMP    , A_IMM    , 2 }, // 0xC9 
  { O_DEX    , A_IMP    , 2 }, // 0xCA
  { O_WAI    , A_IMP    , 2 }, // 0xCB
  { O_CPY    , A_ABS    , 4 }, // 0xCC
  { O_CMP    , A_ABS    , 4 }, // 0xCD
  { O_DEC    , A_ABS    , 6 }, // 0xCE
  { O_BBS    , A_ZPREL  , 5 }, // 0xCF
  { O_BNE    , A_REL    , 2 }, // 0xD0 [8]
  { O_CMP    , A_INY    , 5 }, // 0xD1 [2,3]
  { O_CMP    , A_ZIND   , 5 }, // 0xD2 [2]
  { O_ILLEGAL, A_ILLEGAL,     2 }, // 0xD3 -- as a nop for tests
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xD4
  { O_CMP    , A_ZEX    , 4 }, // 0xD5 [2]
  { O_DEC    , A_ZEX    , 6 }, // 0xD6 [2]
  { O_SMB    , A_ZER    , 5 }, // 0xD7
  { O_CLD    , A_IMP    , 2 }, // 0xD8
  { O_CMP    , A_ABY    , 4 }, // 0xD9 [3]
  { O_PHX    , A_IMP    , 3 }, // 0xDA
  { O_DCP    , A_ABY    , 2 }, // 0xDB -- fixme -- cycle count of this illegal instruction?
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xDC
  { O_CMP    , A_ABX    , 4 }, // 0xDD [3]
  { O_DEC    , A_ABX    , 6 }, // 0xDE [6]
  { O_BBS    , A_ZPREL  , 5 }, // 0xDF
  { O_CPX    , A_IMM    , 2 }, // 0xE0
  { O_SBC    , A_INX    , 6 }, // 0xE1 [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xE2
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xE3
  { O_CPX    , A_ZER    , 3 }, // 0xE4 [2]
  { O_SBC    , A_ZER    , 3 }, // 0xE5 [2]
  { O_INC    , A_ZER    , 5 }, // 0xE6 [2]
  { O_SMB    , A_ZER    , 5 }, // 0xE7
  { O_INX    , A_IMP    , 2 }, // 0xE8
  { O_SBC    , A_IMM    , 2 }, // 0xE9
  { O_NOP    , A_IMP    , 2 }, // 0xEA
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xEB
  { O_CPX    , A_ABS    , 4 }, // 0xEC
  { O_SBC    , A_ABS    , 4 }, // 0xED
  { O_INC    , A_ABS    , 6 }, // 0xEE
  { O_BBS    , A_ZPREL  , 5 }, // 0xEF
  { O_BEQ    , A_REL    , 2 }, // 0xF0 [8]
  { O_SBC    , A_INY    , 5 }, // 0xF1 [2,3]
  { O_SBC    , A_ZIND   , 5 }, // 0xF2 [2]
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xF3
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xF4
  { O_SBC    , A_ZEX    , 4 }, // 0xF5 [2]
  { O_INC    , A_ZEX    , 6 }, // 0xF6 [2]
  { O_SMB    , A_ZER    , 5 }, // 0xF7
  { O_SED    , A_IMP    , 2 }, // 0xF8
  { O_SBC    , A_ABY    , 4 }, // 0xF9 [3]
  { O_PLX    , A_IMP    , 4 }, // 0xFA
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xFB
  { O_ILLEGAL, A_ILLEGAL, 2 }, // 0xFC
  { O_SBC    , A_ABX    , 4 }, // 0xFD [3]
  { O_INC    , A_ABX    , 6 }, // 0xFE [6]
  { O_BBS    , A_ZPREL  , 5 }, // 0xFF
};
/* cycle count footnotes:
2: Add 1 cycle if low byte of Direct Page Register is non-zero 
3 Add 1 cycle if adding index crosses a page boundary 
6 Add 1 cycle if 65C02 and page boundary crossed 
8 Add 1 cycle if branch taken crosses page boundary on 6502, 65C02, or 65816's 6502 emulation mode (e=1)
*/

// Initialization sequence comes from http://www.pagetable.com/?p=410
// (the Visual 6502 project).

Cpu::Cpu()
{
  mmu = NULL;
  Reset();
}

Cpu::~Cpu()
{
  mmu = NULL;
}

bool Cpu::Serialize(int8_t fd)
{
  serializeMagic(CPUMAGIC);
  serialize16(pc);
  serialize8(sp);
  serialize8(a);
  serialize8(x);
  serialize8(y);
  serialize8(flags);
  serialize32(cycles);
  serialize8(irqPending ? 1 : 0);

  if (!mmu->Serialize(fd)) {
    printf("MMU serialization failed\n");
    goto err;
  }
  serializeMagic(CPUMAGIC);

  return true;
  
 err:
  return false;
}

bool Cpu::Deserialize(int8_t fd)
{
  deserializeMagic(CPUMAGIC);
  deserialize16(pc);
  deserialize8(sp);
  deserialize8(a);
  deserialize8(x);
  deserialize8(y);
  deserialize8(flags);
  deserialize32(cycles);
  deserialize8(irqPending);
  
  if (!mmu->Deserialize(fd)) {
    printf("MMU deserialization failed\n");
    return false;
  }

  deserializeMagic(CPUMAGIC);

  printf("CPU deserialization complete\n");
  return true;

 err:
  return false;
}

void Cpu::Reset()
{
  a = 0;
  x = 0;
  y = 0;
  flags = F_Z | F_UNK; // FIXME: is that F_UNK flag right here?
  irqPending = false;

  if (mmu) {
    pc = readmem(0xFFFC) | (readmem(0xFFFD) << 8);
  } else {
    pc = 0x0000;
  }

  sp = 0xFD;

  cycles = 6; // according to the datasheet, the reset routine takes 6 clock cycles

  realtimeProcessing = false;
}

void Cpu::nmi()
{
  flags &= ~F_B; // clear break flag

  pushS16(pc);
  pushS8(flags);
  flags |= 0x20; // FIXME: what flag is this?

  pc = readmem(0xFFFA) | (readmem(0xFFFB) << 8);
  cycles += 2;
}

void Cpu::rst()
{
  // On reset, the CPU initializes the instruction register (IR) to 0.
  // That's BRK. So we spend 3 cycles breaking (0, 1, 2)?

  // Then we would pushS16(pc), but the Reset logic has the R/W line set
  // to R, so the value is discarded. Twice. That's cycles 3 and 4.
  cycles += 2; sp -= 2;

  // Then we would pushS8(flags) the status register, but again, R/W
  // is still R.  That's cycle 5.
  cycles++; sp--;

  // Note that in the real CPU, the SP register doesn't get updated
  // until now - pushS16/pushS8 wouldn't have updated it. Not sure
  // that's ever important to me.

  // In cycle 6, we read 0xFFFC; in cycle 7, we read 0xFFFD. Setting the 
  // PC takes no time.
  pc = readmem(0xFFFC) | (readmem(0xFFFD) << 8);
  cycles+=2;
  // And now we're going to go fetch and operate on the first instruction.
}

void Cpu::brk()
{
  pc++;
  pushS16(pc);
  pushS8(flags | F_B); // FIXME: does this have the missing status bit set?
  // FIXME: is setting the BRK bit a 65C02-specific thing? I think it is

  FLAG(F_D, 0); // 65c02...
  FLAG(F_I, 1);

  pc = readmem(0xFFFE) | (uint16_t)(readmem(0xFFFF) << 8);
  cycles += 2;
}

void Cpu::assertIrq()
{
  irqPending = true;
}

void Cpu::deassertIrq()
{
  irqPending = false;
}

void Cpu::irq()
{
  // If interrupts are disabled, then do nothing
  if (flags & F_I)
    return;

  pushS16(pc);
  flags &= ~F_B; // clear BRK flag
  pushS8(flags);
  flags |= F_I; // set interrupt flag

  pc = readmem(0xFFFE) | (readmem(0xFFFF) << 8);
  cycles += 2;
}

uint8_t Cpu::Run(uint8_t numSteps)
{
  uint8_t runtime = 0;
  realtimeProcessing = false;
  while (runtime < numSteps && !realtimeProcessing) {
    runtime += step();
  }
  return runtime;
}

uint8_t Cpu::step()
{
  if (irqPending) {
    irq();
  }

#ifdef DEBUGSTEPS
  static uint8_t cmdbuf[10];
  static char buf[50];

  uint16_t loc=g_cpu->pc;
  for (int idx=0; idx<sizeof(cmdbuf); idx++) {
    cmdbuf[idx] = g_vm->getMMU()->read(loc+idx);
  }
  dis.instructionToMnemonic(loc, cmdbuf, buf, sizeof(buf));
  while (strlen(buf) < 25) {
    strcat(buf, " ");
  }
  printf("%s ;", buf);

  uint8_t p = g_cpu->flags;
  printf("BS/BT: %02x/%02x A: %02x  X: %02x  Y: %02x  SP: %02x  Flags: %c%cx%c%c%c%c%c\n",
	 g_vm->getMMU()->read(0x3D),
	 g_vm->getMMU()->read(0x41),
	 g_cpu->a, g_cpu->x, g_cpu->y, g_cpu->sp,
	 p & (1<<7) ? 'N':' ',
	 p & (1<<6) ? 'V':' ',
	 p & (1<<4) ? 'B':' ',
	 p & (1<<3) ? 'D':' ',
	 p & (1<<2) ? 'I':' ',
	 p & (1<<1) ? 'Z':' ',
	 p & (1<<0) ? 'C':' '
	 );

#endif
  
  uint8_t m = readmem(pc++);

  optype_t opcode = opcodes[m];
  if (opcode.op == O_ILLEGAL || opcode.mode == A_ILLEGAL) {
#ifdef VERBOSE_CPU_ERRORS
    fprintf(stderr, "** Illegal opcode $%.2X at address $%.4x\n",
	    m,
	    pc-1);
#endif
    // Special invalid opcodes that also have arguments...
    if (m == 0x02 || m == 0x22 || m == 0x42 || m == 0x62 || m == 0x82 ||
	m == 0xC2 || m == 0xE2 || m == 0x44 || m == 0x54 || m == 0xd4 ||
	m == 0xf4) {
      pc++;
    }
    if (m == 0x5c || m == 0xdc || m == 0xfc) {
      pc += 2;
    }
    m = 0xEA; // substitute O_NOP...
    opcode = opcodes[m];
  }

  // Look at the addressing mode to determine the parameter
  uint16_t param = 0;
  uint16_t zprelParam2 = 0;
  switch (opcode.mode) {
  case A_ILLEGAL:
  default:
    // This should never happen; we're substituting NOP.
    // treat these as IMPLIED
    break;
  case A_IMP:
  case A_ACC:
    // implied: nothing to do. These have a parameter that refers to a
    // specific register or particular action to a register
    break;
  case A_IMM:
    // immediate: the next byte at PC
    param = pc++;
    break;
  case A_ABS:
    // absolute: the address referred to in the next 2 bytes at PC
    param = readmem(pc) | (readmem(pc+1) << 8);
    pc += 2;
    break;
  case A_ABX:
    // absolute indexed, based on X
    param = (readmem(pc) | (readmem(pc+1) << 8)) + x;
    pc += 2;
    break;
  case A_ABXI:
    param = (readmem(pc) | (readmem(pc+1) << 8));
    param += x;
    param = readmem(param) | (readmem(param+1) << 8);
    pc += 2;
    break;
  case A_REL:
    // relative
    param = (int8_t)readmem(pc++);
    param += pc; // do this in 2 steps b/c readmem(pc++) modifies PC
    break;
  case A_ZPREL:
    // Two params - zero page and relative.
    param = (int8_t) readmem(pc++); // a zero-page memory location
    zprelParam2 = (int8_t)readmem(pc++); // a relative branch destination
    zprelParam2 += pc;
    break;
  case A_ABI:
    // absolute indirect
    {
      uint16_t addr = readmem(pc) | (readmem(pc+1) << 8);
      pc += 2;
      param = addr;
      param = readmem(addr) | (readmem(addr+1) << 8);
    }
    break;
  case A_ZEX:
    // zero-page, indexed by X -- i.e. "ORA $44,X"
    param = (readmem(pc++) + x) & 0xFF;
    break;
  case A_ZER:
    // zero-page
    param = readmem(pc++);
    break;
  case A_ZEY:
    // zero-page, indexed by Y
    param = (readmem(pc++) + y) & 0xFF;
    break;
  case A_ABY:
    // absolute indexed, based on Y
    param = (readmem(pc) | (readmem(pc+1) << 8)) + y;
    pc += 2;
    break;
  case A_INY:
    // indirect indexed Y - refers to zero-page memory by one byte
    {
      uint8_t zpL = mmu->read(pc++);
      uint8_t zpH = zpL+1;
      param = ( mmu->read(zpL) | (mmu->read(zpH) << 8) ) + y;
    }
    break;
  case A_INX:
    {
      uint8_t zpL = mmu->read(pc++) + x;
      uint8_t zpH = zpL+1;
      param = ( mmu->read(zpL) | (mmu->read(zpH) << 8) );
    }
    break;
  case A_ZIND:
    {
      uint8_t a = mmu->read(pc);
      if (a == 0xFF) {
	// Wrap around zero-page
	param = mmu->read(0xFF) | (mmu->read(0) << 8);
      } else {
	param = mmu->read(a) | (mmu->read(a+1) << 8);
      }
      pc++;
    }
    break;
  }

  // initialize a counter for the number of cycles this run
  // (many opcodes have variable length)
  uint8_t cyclesThisStep = opcode.cycles;

  // Then look at the opcode type to perform the operations necessary
  switch (opcode.op) {
  case O_ILLEGAL:
  default:
    // This should never happen; we're trapping O_ILLEGAL above and substituting O_NOP.
#ifdef EXIT_ON_ILLEGAL
    printf("Programming error: unhandled opcode $%X\n", m);
    exit(0);
#endif
    break;
  case O_CLD:
    FLAG(F_D, 0);
    break;
  case O_LDX:
    x = readmem(param);
    SETNZX;
    break;
  case O_TXS:
    sp = x;
    break;
  case O_LDA:
    a = readmem(param);
    SETNZA;
    break;
  case O_STA:
    writemem(param, a);
    break;
  case O_JSR:
    pushS16(pc-1);
    pc = param;
    break;
  case O_RTS:
    pc = popS16()+1;
    break;
  case O_PHA:
    pushS8(a);
    break;
  case O_INX:
    x++;
    SETNZX;
    break;
  case O_BNE:
    if (!(flags & F_Z)) {
#ifdef TESTHARNESS
      if (pc == param+2) {
	printf("CPU halt (BNE busy loop)\n");
	exit(0);
      }
#endif
      pc = param;
      cyclesThisStep++;
    }
    break;
  case O_BVS:
    if (flags & F_V) {
      pc = param;
      cyclesThisStep++;
    }
    break;
  case O_BRK:
    brk();
    break;
  case O_PHP:
    pushS8(flags | F_B);
    break;
  case O_DEY:
    y--;
    SETNZY;
    break;
  case O_CMP:
    {
      uint16_t tmp = a - readmem(param);
      FLAG(F_C, tmp < 0x100);
      FLAG(F_Z, !(tmp & 0xFF));
      FLAG(F_N, tmp & 0x80);
    }
    break;
  case O_BEQ:
    if (flags & F_Z) {
#ifdef TESTHARNESS
      if (pc == param+2) {
        printf("CPU halt (BEQ busy loop)\n");
        exit(0);
      }
#endif
      pc = param;
      cyclesThisStep++;
    }
    break;
  case O_TXA:
    a = x;
    SETNZA;
    break;
  case O_TYA:
    a = y;
    SETNZA;
    break;
  case O_ASL_ACC:
    FLAG(F_C, a & 0x80);
    a <<= 1;
    SETNZA;
    break;
  case O_ORA:
    a |= readmem(param);
    SETNZA;
    break;
  case O_JMP:
#ifdef TESTHARNESS
    if (param == pc-3) {
      printf("CPU halt (JMP busy loop)\n");
      exit(0);
    }
#endif
    pc = param;
    break;
  case O_DEX:
    x--;
    SETNZX;
    break;
  case O_LDY:
    y = readmem(param);
    SETNZY;
    break;
  case O_NOP:
  case O_WAI:
    break;
  case O_TAX:
    x = a;
    SETNZX; 
    break;
  case O_BPL:
    if (!(flags & F_N)) {
      pc = param;
      cyclesThisStep++;
    }    
    break;
  case O_CLC:
    FLAG(F_C, 0);
    break;
  case O_EOR:
    a ^= readmem(param);
    SETNZA;
    break;
  case O_CPY:
    {
      uint16_t tmp = y - readmem(param);
      FLAG(F_C, tmp < 0x100);
      FLAG(F_Z, !(tmp & 0xFF));
      FLAG(F_N, tmp & 0x80);
    }
    break;
  case O_TSX:
    x = sp;
    SETNZX;
    break;
  case O_LSR_ACC:
    FLAG(F_C, a & 0x01);
    a >>= 1;
    SETNZA;
    break;
  case O_BCC:
    if (!(flags & F_C)) {
      pc = param;
      cyclesThisStep++;
    }
    break;
  case O_PLA:
    a = popS8();
    SETNZA;
    break;
  case O_AND:
    a &= readmem(param);
    SETNZA;
    break;
  case O_CPX:
    {
      uint16_t tmp = x - readmem(param);
      FLAG(F_C, tmp < 0x100);
      FLAG(F_Z, !(tmp & 0xFF));
      FLAG(F_N, tmp & 0x80);
    }
    break;
  case O_BCS:
    if (flags & F_C) {
      pc = param;
      cyclesThisStep++;
    }
    break;
  case O_BMI:
    if (flags & F_N) {
      pc = param;
      cyclesThisStep++;
    }
    break;
  case O_TAY:
    y = a;
    SETNZY;
    break;
  case O_PLP:
    flags = popS8();
    FLAG(F_UNK, 1); // ??
    break;
  case O_BVC:
    if (!(flags & F_V)) {
      pc = param;
      cyclesThisStep++;
    }
    break;
  case O_INY:
    y++;
    SETNZY;
    break;
  case O_STX:
    writemem(param, x);
    break;
  case O_RTI:
    flags = popS8();
    pc = popS16();
    break;
  case O_SEC:
    FLAG(F_C, 1);
    break;
  case O_CLI:
    FLAG(F_I, 0);
    break;
  case O_SEI:
    FLAG(F_I, 1);
    break;
  case O_SED:
    FLAG(F_D, 1);
    break;
  case O_CLV:
    FLAG(F_V, 0);
    break;
  case O_STY:
    writemem(param, y);
    break;
  case O_BIT:
    { 
      uint8_t m = readmem(param);
      uint8_t v = a & m;
      FLAG(F_Z, v == 0);
      if (opcode.mode != A_IMM) {
	FLAG(F_N, (v & 0x80) | (m & 0x80));
	FLAG(F_V, m & 0x40);
      }
      //      status = (status & 0x3F) | (uint8_t)(m & 0xC0);
    }
    break;
  case O_TRB:
    {
      uint8_t m = readmem(param);
      uint8_t v = a & m;
      m &= ~a;
      writemem(param, m);
      FLAG(F_Z, v == 0);
    }
    break;
  case O_TSB:
    {
      uint8_t m = readmem(param);
      uint8_t v = a & m;
      m |= a;
      writemem(param, m);
      FLAG(F_Z, v == 0);
    }
    break;
  case O_ROL_ACC:
    {
      uint8_t v = a << 1;
      if (flags & F_C)
	v |= 0x01;
      FLAG(F_C, a & 0x80);
      a = v;
      SETNZA;
    }
    break;
  case O_ROR_ACC:
    {
      uint8_t v = a >> 1;
      if (flags & F_C)
	v |= 0x80;
      FLAG(F_C, a & 0x01);
      a = v;
      SETNZA;
    }
    break;
  case O_ASL:
    { 
      uint8_t v = readmem(param);
      FLAG(F_C, v & 0x80);
      v <<= 1;

      FLAG(F_N, v & 0x80);
      FLAG(F_Z, v == 0);
      writemem(param, v);
    }
    break;
  case O_LSR:
    {
      uint8_t v = readmem(param);
      FLAG(F_C, v & 0x01);
      v >>= 1;
      FLAG(F_N, 0);
      FLAG(F_Z, v == 0);
      writemem(param, v);
    }
    break;
  case O_ROL:
    {
      uint8_t m = readmem(param);
      uint8_t v = m << 1;
      if (flags & F_C)
	v |= 0x01;
      FLAG(F_C, m & 0x80);
      FLAG(F_N, v & 0x80);
      FLAG(F_Z, v == 0);
      writemem(param, v);
    }
    break;
  case O_ROR:
    {
      uint8_t m = readmem(param);
      uint8_t v = m >> 1;
      if (flags & F_C)
	v |= 0x80;
      FLAG(F_C, m & 0x01);
      FLAG(F_N, v & 0x80);
      FLAG(F_Z, v == 0);
      writemem(param, v);
    }
    break;
  case O_INC:
    {
      uint8_t v = mmu->read(param) + 1;
      FLAG(F_N, v & 0x80);
      FLAG(F_Z, v == 0);
      writemem(param, v);
    }
    break;
  case O_DEC:
    {
      uint8_t v = mmu->read(param) - 1;
      FLAG(F_N, v & 0x80);
      FLAG(F_Z, v == 0);
      writemem(param, v);
    }
    break;
  case O_DCP:
    // not a real opcode; one of the 65c02 side-effect "illegal" opcodes
    {
      uint8_t v = mmu->read(param) - 1;
      FLAG(F_N, v & 0x80);
      FLAG(F_Z, v == 0);
      writemem(param, v);

      uint16_t tmp = a - v;
      FLAG(F_C, tmp < 0x100);
      FLAG(F_Z, !(tmp & 0xFF));
      FLAG(F_N, tmp & 0x80);
    }
    break;
  case O_SBC:
    {
      uint8_t B = readmem(param) ^ 0xFF;
      uint8_t Cin = (flags & F_C);
      uint8_t Cout, Vout;
      int16_t Aout;

      if ((flags & F_D) == 0) {
	// Binary mode: same as ADC
	Aout = a + B + Cin;
	Vout = (a ^ Aout) & (B ^ Aout) & 0x80;
	Cout = (Aout >= 0x100) ? 1 : 0;
	a = Aout & 0xFF;
      } else {
	// Decimal mode
	cyclesThisStep++;

	Aout = (a & 0x0F) + (B & 0x0F) + Cin;
	if (Aout < 0x10) {
	  Aout = (Aout - 0x06) & 0x0F;
	}
	Aout = Aout + (a & 0xF0) + (B & 0xF0);
	Vout = (a ^ Aout) & (B ^ Aout) & 0x80;
	if (Aout < 0x100) {
	  Aout = (Aout + 0xa0) & 0xFF;
	}
	Cout = (Aout >= 0x100) ? 1 : 0;

	B = readmem(param);
	int8_t AL = (a & 0x0F) - (B & 0x0F) + (Cin - 1);
	Aout = a - B + Cin - 1;
	if (Aout < 0) {
	  Aout = Aout - 0x60;
	}
	if (AL < 0) {
	  Aout = Aout - 0x06;
	}

	a = Aout & 0xFF;
      }

      FLAG(F_C, Cout);
      FLAG(F_V, Vout);
      SETNZA;
    }
    break;

  case O_ADC:
    {
      uint8_t B = readmem(param);
      uint8_t Cin = (flags & F_C);
      uint8_t Cout, Vout;
      uint16_t Aout;

      if ((flags & F_D) == 0x00) {
	// Simple binary mode
	Aout = a + B + Cin;
	Vout = (a ^ Aout) & (B ^ Aout) & 0x80;
	Cout = (Aout >= 0x100) ? 1 : 0;
	a = Aout & 0xFF;
      } else {
	// Decimal mode
	cyclesThisStep++;
	Aout = (a & 0x0F) + (B & 0x0F) + Cin;
	int tmpOverflow = 0;
	if (Aout >= 0x0A) {
	  tmpOverflow = 0x10;
	  Aout = (Aout + 0x06) & 0x0F;
	}
	Aout = Aout | (a & 0xF0);
	Aout = Aout + (B & 0xF0) + tmpOverflow;

	Vout = 0;
	if ( ((a ^ B) & 0x80) == 0) {
	  if (((a ^ Aout) & 0x80)) {
	    Vout = 1;
	  }
	}

	if (Aout >= 0xA0) {
	  Aout = Aout + 0x60;
	  Cout = 1;
	} else {
	  Cout = 0;
	}

	a = Aout & 0xFF;
      }

      FLAG(F_C, Cout);
      FLAG(F_V, Vout);
      SETNZA;
    }
    break;
  case O_PHX:
    pushS8(x);
    break;
  case O_PHY:
    pushS8(y);
    break;
  case O_PLY:
    y = popS8();
    SETNZY;
    break;
  case O_PLX:
    x = popS8();
    SETNZX;
    break;
  case O_BRA:
    pc = param;
    break;
  case O_BBR:
    {
      // The bit to test is encoded in the opcode [m].
      uint8_t btt = 1 << ((m >> 4) & 0x07);
      uint8_t v = readmem(param); // zero-page memory location to test
      if (!(v & btt)) {
	pc = zprelParam2;
      }
    }
    break;
  case O_BBS:
    {
      // The bit to test is encoded in the opcode [m].
      uint8_t btt = 1 << ((m >> 4) & 0x07);
      uint8_t v = readmem(param); // zero-page memory location to test
      if (v & btt) {
	pc = zprelParam2;
      }
    }
    break;
  case O_INC_ACC:
    a = a + 1;
    a &= 0xFF;
    SETNZA;
    break;
  case O_DEC_ACC:
    a = a - 1;
    a &= 0xFF;
    SETNZA;
    break;
  case O_STZ:
    writemem(param, 0x00);
    break;
  case O_RMB:
    {
      // The bit to test is encoded in the opcode [m].
      uint8_t btt = 1 << ((m >> 4) & 0x07);
      writemem(param, readmem(param) & ~btt);
    }
    break;
  case O_SMB:
    {
      // The bit to test is encoded in the opcode [m].
      uint8_t btt = 1 << ((m >> 4) & 0x07);
      writemem(param, readmem(param) | btt);
    }
    break;
  }

  // And finally update our executed cycle count with the runtime
  cycles += cyclesThisStep;

  return cyclesThisStep;
}

uint8_t Cpu::X()
{
  return x;
}

uint8_t Cpu::Y()
{
  return y;
}

uint8_t Cpu::A()
{
  return a;
}

uint16_t Cpu::PC()
{
  return pc;
}

uint8_t Cpu::SP()
{
  return sp;
}

uint8_t Cpu::P()
{
  return flags;
}


void Cpu::pushS8(uint8_t b)
{
  writemem(0x100 + sp, b);
  sp--; // may underflow (does this at startup)
}

void Cpu::pushS16(uint16_t w)
{
  pushS8((w >> 8) & 0xFF); 
  pushS8(w & 0xFF);
}

uint8_t Cpu::popS8()
{
  sp++; // may overflow
  return readmem(0x100 + sp);
}

uint16_t Cpu::popS16()
{
  uint8_t lsb = popS8();
  uint8_t msb = popS8();

  return (msb << 8) | lsb;
}

void Cpu::realtime()
{
  realtimeProcessing = true;
}