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6502-emu
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DavidBuchanan314 2017-01-03 19:39:48 +00:00
parent d085b5699c
commit 378169d56b
9 changed files with 1213 additions and 1 deletions
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*.o
6502-emu

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6502.c Normal file
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#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "6502.h"
//#define DEBUG
int lengths[NUM_MODES]; // instruction length table, indexed by addressing mode
uint8_t * (*get_ptr[NUM_MODES])(); // addressing mode decoder table
Instruction instructions[0x100]; // instruction data table
Instruction inst; // the current instruction (used for convenience)
int jumping; // used to check that we don't need to increment the PC after a jump
/* Flag Checks */
static inline void Nflag(int8_t val)
{
SR.bits.sign = val < 0;
}
static inline void Zflag(uint8_t val)
{
SR.bits.zero = val == 0;
}
/* Stack Helpers */
static inline void stack_push(uint8_t val)
{
memory[0x100+(SP--)] = val;
}
static inline uint8_t stack_pull()
{
return memory[0x100+(++SP)];
}
/* Memory read/write wrappers */
static inline uint8_t * read_ptr()
{
return readAddr = get_ptr[inst.mode]();
}
static inline uint8_t * write_ptr()
{
return writeAddr = get_ptr[inst.mode]();
}
/* Instruction Implementations */
static void inst_ADC()
{
uint8_t operand = * read_ptr();
int tmp = A + operand + (SR.bits.carry & 1);
SR.bits.carry = tmp > 0xFF;
SR.bits.overflow = ((A^tmp)&(operand^tmp)&0x80) != 0;
A = tmp & 0xFF;
Nflag(A);
Zflag(A);
}
static void inst_AND()
{
A &= * read_ptr();
Nflag(A);
Zflag(A);
}
static void inst_ASL()
{
uint8_t tmp = * read_ptr();
SR.bits.carry = (tmp & 0x80) != 0;
tmp <<= 1;
Nflag(tmp);
Zflag(tmp);
* write_ptr() = tmp;
}
static void inst_BCC()
{
if (!SR.bits.carry) {
PC = read_ptr() - memory;
}
}
static void inst_BCS()
{
if (SR.bits.carry) {
PC = read_ptr() - memory;
}
}
static void inst_BEQ()
{
if (SR.bits.zero) {
PC = read_ptr() - memory;
}
}
static void inst_BIT()
{
uint8_t tmp = * read_ptr();
Nflag(tmp);
Zflag(tmp & A);
SR.bits.overflow = (tmp & 0x40) != 0;
}
static void inst_BMI()
{
if (SR.bits.sign) {
PC = read_ptr() - memory;
}
}
static void inst_BNE()
{
if (!SR.bits.zero) {
PC = read_ptr() - memory;
}
}
static void inst_BPL()
{
if (!SR.bits.sign) {
PC = read_ptr() - memory;
}
}
static void inst_BRK()
{
uint16_t newPC;
memcpy(&newPC, &memory[IRQ_VEC], sizeof(newPC));
PC += 2;
stack_push(PC >> 8);
stack_push(PC & 0xFF);
SR.bits.brk = 1;
stack_push(SR.byte);
SR.bits.interrupt = 1;
PC = newPC;
jumping = 1;
}
static void inst_BVC()
{
if (!SR.bits.overflow) {
PC = read_ptr() - memory;
}
}
static void inst_BVS()
{
if (SR.bits.overflow) {
PC = read_ptr() - memory;
}
}
static void inst_CLC()
{
SR.bits.carry = 0;
}
static void inst_CLD()
{
SR.bits.decimal = 0;
}
static void inst_CLI()
{
SR.bits.interrupt = 0;
}
static void inst_CLV()
{
SR.bits.overflow = 0;
}
static void inst_CMP()
{
uint8_t operand = * read_ptr();
uint8_t tmpDiff = A - operand;
Nflag(tmpDiff);
Zflag(tmpDiff);
SR.bits.carry = A >= operand;
}
static void inst_CPX()
{
uint8_t operand = * read_ptr();
uint8_t tmpDiff = X - operand;
Nflag(tmpDiff);
Zflag(tmpDiff);
SR.bits.carry = X >= operand;
}
static void inst_CPY()
{
uint8_t operand = * read_ptr();
uint8_t tmpDiff = Y - operand;
Nflag(tmpDiff);
Zflag(tmpDiff);
SR.bits.carry = Y >= operand;
}
static void inst_DEC()
{
uint8_t tmp = * read_ptr();
tmp--;
Nflag(tmp);
Zflag(tmp);
* write_ptr() = tmp;
}
static void inst_DEX()
{
X--;
Nflag(X);
Zflag(X);
}
static void inst_DEY()
{
Y--;
Nflag(Y);
Zflag(Y);
}
static void inst_EOR()
{
A ^= * read_ptr();
Nflag(A);
Zflag(A);
}
static void inst_INC()
{
uint8_t tmp = * read_ptr();
tmp++;
Nflag(tmp);
Zflag(tmp);
* write_ptr() = tmp;
}
static void inst_INX()
{
X++;
Nflag(X);
Zflag(X);
}
static void inst_INY()
{
Y++;
Nflag(Y);
Zflag(Y);
}
static void inst_JMP()
{
PC = read_ptr() - memory;
jumping = 1;
}
static void inst_JSR()
{
uint16_t newPC = read_ptr() - memory;
PC += 2;
stack_push(PC >> 8);
stack_push(PC & 0xFF);
PC = newPC;
jumping = 1;
}
static void inst_LDA()
{
A = * read_ptr();
Nflag(A);
Zflag(A);
}
static void inst_LDX()
{
X = * read_ptr();
Nflag(X);
Zflag(X);
}
static void inst_LDY()
{
Y = * read_ptr();
Nflag(Y);
Zflag(Y);
}
static void inst_LSR()
{
uint8_t tmp = * read_ptr();
SR.bits.carry = tmp & 1;
tmp >>= 1;
Nflag(tmp);
Zflag(tmp);
* write_ptr() = tmp;
}
static void inst_NOP()
{
// nothing
}
static void inst_ORA()
{
A |= * read_ptr();
Nflag(A);
Zflag(A);
}
static void inst_PHA()
{
stack_push(A);
}
static void inst_PHP()
{
SR.bits.brk = 1; // this is slightly unexpected, but it's what the real hardware does.
stack_push(SR.byte);
}
static void inst_PLA()
{
A = stack_pull();
Nflag(A);
Zflag(A);
}
static void inst_PLP()
{
SR.byte = stack_pull();
SR.bits.unused = 1;
}
static void inst_ROL()
{
int tmp = (* read_ptr()) << 1;
tmp |= SR.bits.carry & 1;
SR.bits.carry = tmp > 0xFF;
tmp &= 0xFF;
Nflag(tmp);
Zflag(tmp);
* write_ptr() = tmp;
}
static void inst_ROR()
{
int tmp = * read_ptr();
tmp |= SR.bits.carry << 8;
SR.bits.carry = tmp & 1;
tmp >>= 1;
Nflag(tmp);
Zflag(tmp);
* write_ptr() = tmp;
}
static void inst_RTI()
{
SR.byte = stack_pull();
SR.bits.unused = 1;
PC = stack_pull();
PC |= stack_pull() << 8;
//PC += 1;
jumping = 1;
}
static void inst_RTS()
{
PC = stack_pull();
PC |= stack_pull() << 8;
PC += 1;
jumping = 1;
}
static void inst_SBC()
{
uint8_t operand = ~(* read_ptr()); // identical to ACD with the operand inverted
int tmp = A + operand + (SR.bits.carry & 1);
SR.bits.carry = tmp > 0xFF;
SR.bits.overflow = ((A^tmp)&(operand^tmp)&0x80) != 0;
A = tmp & 0xFF;
Nflag(A);
Zflag(A);
}
static void inst_SEC()
{
SR.bits.carry = 1;
}
static void inst_SED()
{
SR.bits.decimal = 1;
printf("DECIMAL! :(\r\n"); // TODO
}
static void inst_SEI()
{
SR.bits.interrupt = 1;
}
static void inst_STA()
{
* write_ptr() = A;
}
static void inst_STX()
{
* write_ptr() = X;
}
static void inst_STY()
{
* write_ptr() = Y;
}
static void inst_TAX()
{
X = A;
Nflag(X);
Zflag(X);
}
static void inst_TAY()
{
Y = A;
Nflag(Y);
Zflag(Y);
}
static void inst_TSX()
{
X = SP;
Nflag(X);
Zflag(X);
}
static void inst_TXA()
{
A = X;
Nflag(A);
Zflag(A);
}
static void inst_TXS()
{
SP = X;
}
static void inst_TYA()
{
A = Y;
Nflag(A);
Zflag(A);
}
/* Addressing Implementations */
uint8_t * get_IMM()
{
return &memory[PC+1];
}
uint16_t get_uint16()
{ // used only as part of other modes
uint16_t index;
memcpy(&index, get_IMM(), sizeof(index)); // hooray for optimising compilers
return index;
}
uint8_t * get_ZP()
{
return &memory[memory[PC+1]];
}
uint8_t * get_ZPX()
{
return &memory[(memory[PC+1] + X) & 0xFF];
}
uint8_t * get_ZPY()
{
return &memory[(memory[PC+1] + Y) & 0xFF];
}
uint8_t * get_ACC()
{
return &A;
}
uint8_t * get_ABS()
{
return &memory[get_uint16()];
}
uint8_t * get_ABSX()
{
return &memory[get_uint16()+X];
}
uint8_t * get_ABSY()
{
return &memory[get_uint16()+Y];
}
uint8_t * get_IND()
{
uint16_t ptr;
memcpy(&ptr, get_ABS(), sizeof(ptr));
return &memory[ptr];
}
uint8_t * get_XIND()
{
uint16_t ptr;
memcpy(&ptr, get_ZPX(), sizeof(ptr));
return &memory[ptr];
}
uint8_t * get_INDY()
{
uint16_t ptr;
memcpy(&ptr, get_ZP(), sizeof(ptr));
ptr += Y;
return &memory[ptr];
}
uint8_t * get_REL()
{
return &memory[PC + (int8_t) memory[PC+1]];
}
/* Construction of Tables */
void init_tables() // this is only done at runtime to improve code readability.
{
/* Instruction Lengths */
lengths[ACC] = 1;
lengths[ABS] = 3;
lengths[ABSX] = 3;
lengths[ABSY] = 3;
lengths[IMM] = 2;
lengths[IMPL] = 1;
lengths[IND] = 3;
lengths[XIND] = 2;
lengths[INDY] = 2;
lengths[REL] = 2;
lengths[ZP] = 2;
lengths[ZPX] = 2;
lengths[ZPY] = 2;
/* Addressing Modes */
get_ptr[ACC] = get_ACC;
get_ptr[ABS] = get_ABS;
get_ptr[ABSX] = get_ABSX;
get_ptr[ABSY] = get_ABSY;
get_ptr[IMM] = get_IMM;
get_ptr[IND] = get_IND;
get_ptr[XIND] = get_XIND;
get_ptr[INDY] = get_INDY;
get_ptr[REL] = get_REL;
get_ptr[ZP] = get_ZP;
get_ptr[ZPX] = get_ZPX;
get_ptr[ZPY] = get_ZPY;
/* Instructions */
instructions[0x00] = (Instruction) {"BRK impl", inst_BRK, IMPL};
instructions[0x01] = (Instruction) {"ORA X,ind", inst_ORA, XIND};
instructions[0x02] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x03] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x04] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x05] = (Instruction) {"ORA zpg", inst_ORA, ZP};
instructions[0x06] = (Instruction) {"ASL zpg", inst_ASL, ZP};
instructions[0x07] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x08] = (Instruction) {"PHP impl", inst_PHP, IMPL};
instructions[0x09] = (Instruction) {"ORA #", inst_ORA, IMM};
instructions[0x0A] = (Instruction) {"ASL A", inst_ASL, ACC};
instructions[0x0B] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x0C] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x0D] = (Instruction) {"ORA abs", inst_ORA, ABS};
instructions[0x0E] = (Instruction) {"ASL abs", inst_ASL, ABS};
instructions[0x0F] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x10] = (Instruction) {"BPL rel", inst_BPL, REL};
instructions[0x11] = (Instruction) {"ORA ind,Y", inst_ORA, INDY};
instructions[0x12] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x13] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x14] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x15] = (Instruction) {"ORA zpg,X", inst_ORA, ZPX};
instructions[0x16] = (Instruction) {"ASL zpg,X", inst_ASL, ZPX};
instructions[0x17] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x18] = (Instruction) {"CLC impl", inst_CLC, IMPL};
instructions[0x19] = (Instruction) {"ORA abs,Y", inst_ORA, ABSY};
instructions[0x1A] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x1B] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x1C] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x1D] = (Instruction) {"ORA abs,X", inst_ORA, ABSX};
instructions[0x1E] = (Instruction) {"ASL abs,X", inst_ASL, ABSX};
instructions[0x1F] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x20] = (Instruction) {"JSR abs", inst_JSR, ABS};
instructions[0x21] = (Instruction) {"AND X,ind", inst_AND, XIND};
instructions[0x22] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x23] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x24] = (Instruction) {"BIT zpg", inst_BIT, ZP};
instructions[0x25] = (Instruction) {"AND zpg", inst_AND, ZP};
instructions[0x26] = (Instruction) {"ROL zpg", inst_ROL, ZP};
instructions[0x27] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x28] = (Instruction) {"PLP impl", inst_PLP, IMPL};
instructions[0x29] = (Instruction) {"AND #", inst_AND, IMM};
instructions[0x2A] = (Instruction) {"ROL A", inst_ROL, ACC};
instructions[0x2B] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x2C] = (Instruction) {"BIT abs", inst_BIT, ABS};
instructions[0x2D] = (Instruction) {"AND abs", inst_AND, ABS};
instructions[0x2E] = (Instruction) {"ROL abs", inst_ROL, ABS};
instructions[0x2F] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x30] = (Instruction) {"BMI rel", inst_BMI, REL};
instructions[0x31] = (Instruction) {"AND ind,Y", inst_AND, INDY};
instructions[0x32] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x33] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x34] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x35] = (Instruction) {"AND zpg,X", inst_AND, ZPX};
instructions[0x36] = (Instruction) {"ROL zpg,X", inst_ROL, ZPX};
instructions[0x37] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x38] = (Instruction) {"SEC impl", inst_SEC, IMPL};
instructions[0x39] = (Instruction) {"AND abs,Y", inst_AND, ABSY};
instructions[0x3A] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x3B] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x3C] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x3D] = (Instruction) {"AND abs,X", inst_AND, ABSX};
instructions[0x3E] = (Instruction) {"ROL abs,X", inst_ROL, ABSX};
instructions[0x3F] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x40] = (Instruction) {"RTI impl", inst_RTI, IMPL};
instructions[0x41] = (Instruction) {"EOR X,ind", inst_EOR, XIND};
instructions[0x42] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x43] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x44] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x45] = (Instruction) {"EOR zpg", inst_EOR, ZP};
instructions[0x46] = (Instruction) {"LSR zpg", inst_LSR, ZP};
instructions[0x47] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x48] = (Instruction) {"PHA impl", inst_PHA, IMPL};
instructions[0x49] = (Instruction) {"EOR #", inst_EOR, IMM};
instructions[0x4A] = (Instruction) {"LSR A", inst_LSR, ACC};
instructions[0x4B] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x4C] = (Instruction) {"JMP abs", inst_JMP, ABS};
instructions[0x4D] = (Instruction) {"EOR abs", inst_EOR, ABS};
instructions[0x4E] = (Instruction) {"LSR abs", inst_LSR, ABS};
instructions[0x4F] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x50] = (Instruction) {"BVC rel", inst_BVC, REL};
instructions[0x51] = (Instruction) {"EOR ind,Y", inst_EOR, INDY};
instructions[0x52] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x53] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x54] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x55] = (Instruction) {"EOR zpg,X", inst_EOR, ZPX};
instructions[0x56] = (Instruction) {"LSR zpg,X", inst_LSR, ZPX};
instructions[0x57] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x58] = (Instruction) {"CLI impl", inst_CLI, IMPL};
instructions[0x59] = (Instruction) {"EOR abs,Y", inst_EOR, ABSY};
instructions[0x5A] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x5B] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x5C] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x5D] = (Instruction) {"EOR abs,X", inst_EOR, ABSX};
instructions[0x5E] = (Instruction) {"LSR abs,X", inst_LSR, ABSX};
instructions[0x5F] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x60] = (Instruction) {"RTS impl", inst_RTS, IMPL};
instructions[0x61] = (Instruction) {"ADC X,ind", inst_ADC, XIND};
instructions[0x62] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x63] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x64] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x65] = (Instruction) {"ADC zpg", inst_ADC, ZP};
instructions[0x66] = (Instruction) {"ROR zpg", inst_ROR, ZP};
instructions[0x67] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x68] = (Instruction) {"PLA impl", inst_PLA, IMPL};
instructions[0x69] = (Instruction) {"ADC #", inst_ADC, IMM};
instructions[0x6A] = (Instruction) {"ROR A", inst_ROR, ACC};
instructions[0x6B] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x6C] = (Instruction) {"JMP ind", inst_JMP, IND};
instructions[0x6D] = (Instruction) {"ADC abs", inst_ADC, ABS};
instructions[0x6E] = (Instruction) {"ROR abs", inst_ROR, ABS};
instructions[0x6F] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x70] = (Instruction) {"BVS rel", inst_BVS, REL};
instructions[0x71] = (Instruction) {"ADC ind,Y", inst_ADC, INDY};
instructions[0x72] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x73] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x74] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x75] = (Instruction) {"ADC zpg,X", inst_ADC, ZPX};
instructions[0x76] = (Instruction) {"ROR zpg,X", inst_ROR, ZPX};
instructions[0x77] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x78] = (Instruction) {"SEI impl", inst_SEI, IMPL};
instructions[0x79] = (Instruction) {"ADC abs,Y", inst_ADC, ABSY};
instructions[0x7A] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x7B] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x7C] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x7D] = (Instruction) {"ADC abs,X", inst_ADC, ABSX};
instructions[0x7E] = (Instruction) {"ROR abs,X", inst_ROR, ABSX};
instructions[0x7F] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x80] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x81] = (Instruction) {"STA X,ind", inst_STA, XIND};
instructions[0x82] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x83] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x84] = (Instruction) {"STY zpg", inst_STY, ZP};
instructions[0x85] = (Instruction) {"STA zpg", inst_STA, ZP};
instructions[0x86] = (Instruction) {"STX zpg", inst_STX, ZP};
instructions[0x87] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x88] = (Instruction) {"DEY impl", inst_DEY, IMPL};
instructions[0x89] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x8A] = (Instruction) {"TXA impl", inst_TXA, IMPL};
instructions[0x8B] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x8C] = (Instruction) {"STY abs", inst_STY, ABS};
instructions[0x8D] = (Instruction) {"STA abs", inst_STA, ABS};
instructions[0x8E] = (Instruction) {"STX abs", inst_STX, ABS};
instructions[0x8F] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x90] = (Instruction) {"BCC rel", inst_BCC, REL};
instructions[0x91] = (Instruction) {"STA ind,Y", inst_STA, INDY};
instructions[0x92] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x93] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x94] = (Instruction) {"STY zpg,X", inst_STY, ZPX};
instructions[0x95] = (Instruction) {"STA zpg,X", inst_STA, ZPX};
instructions[0x96] = (Instruction) {"STX zpg,Y", inst_STX, ZPY};
instructions[0x97] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x98] = (Instruction) {"TYA impl", inst_TYA, IMPL};
instructions[0x99] = (Instruction) {"STA abs,Y", inst_STA, ABSY};
instructions[0x9A] = (Instruction) {"TXS impl", inst_TXS, IMPL};
instructions[0x9B] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x9C] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x9D] = (Instruction) {"STA abs,X", inst_STA, ABSX};
instructions[0x9E] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0x9F] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xA0] = (Instruction) {"LDY #", inst_LDY, IMM};
instructions[0xA1] = (Instruction) {"LDA X,ind", inst_LDA, XIND};
instructions[0xA2] = (Instruction) {"LDX #", inst_LDX, IMM};
instructions[0xA3] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xA4] = (Instruction) {"LDY zpg", inst_LDY, ZP};
instructions[0xA5] = (Instruction) {"LDA zpg", inst_LDA, ZP};
instructions[0xA6] = (Instruction) {"LDX zpg", inst_LDX, ZP};
instructions[0xA7] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xA8] = (Instruction) {"TAY impl", inst_TAY, IMPL};
instructions[0xA9] = (Instruction) {"LDA #", inst_LDA, IMM};
instructions[0xAA] = (Instruction) {"TAX impl", inst_TAX, IMPL};
instructions[0xAB] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xAC] = (Instruction) {"LDY abs", inst_LDY, ABS};
instructions[0xAD] = (Instruction) {"LDA abs", inst_LDA, ABS};
instructions[0xAE] = (Instruction) {"LDX abs", inst_LDX, ABS};
instructions[0xAF] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xB0] = (Instruction) {"BCS rel", inst_BCS, REL};
instructions[0xB1] = (Instruction) {"LDA ind,Y", inst_LDA, INDY};
instructions[0xB2] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xB3] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xB4] = (Instruction) {"LDY zpg,X", inst_LDY, ZPX};
instructions[0xB5] = (Instruction) {"LDA zpg,X", inst_LDA, ZPX};
instructions[0xB6] = (Instruction) {"LDX zpg,Y", inst_LDX, ZPY};
instructions[0xB7] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xB8] = (Instruction) {"CLV impl", inst_CLV, IMPL};
instructions[0xB9] = (Instruction) {"LDA abs,Y", inst_LDA, ABSY};
instructions[0xBA] = (Instruction) {"TSX impl", inst_TSX, IMPL};
instructions[0xBB] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xBC] = (Instruction) {"LDY abs,X", inst_LDY, ABSX};
instructions[0xBD] = (Instruction) {"LDA abs,X", inst_LDA, ABSX};
instructions[0xBE] = (Instruction) {"LDX abs,Y", inst_LDX, ABSY};
instructions[0xBF] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xC0] = (Instruction) {"CPY #", inst_CPY, IMM};
instructions[0xC1] = (Instruction) {"CMP X,ind", inst_CMP, XIND};
instructions[0xC2] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xC3] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xC4] = (Instruction) {"CPY zpg", inst_CPY, ZP};
instructions[0xC5] = (Instruction) {"CMP zpg", inst_CMP, ZP};
instructions[0xC6] = (Instruction) {"DEC zpg", inst_DEC, ZP};
instructions[0xC7] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xC8] = (Instruction) {"INY impl", inst_INY, IMPL};
instructions[0xC9] = (Instruction) {"CMP #", inst_CMP, IMM};
instructions[0xCA] = (Instruction) {"DEX impl", inst_DEX, IMPL};
instructions[0xCB] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xCC] = (Instruction) {"CPY abs", inst_CPY, ABS};
instructions[0xCD] = (Instruction) {"CMP abs", inst_CMP, ABS};
instructions[0xCE] = (Instruction) {"DEC abs", inst_DEC, ABS};
instructions[0xCF] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xD0] = (Instruction) {"BNE rel", inst_BNE, REL};
instructions[0xD1] = (Instruction) {"CMP ind,Y", inst_CMP, INDY};
instructions[0xD2] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xD3] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xD4] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xD5] = (Instruction) {"CMP zpg,X", inst_CMP, ZPX};
instructions[0xD6] = (Instruction) {"DEC zpg,X", inst_DEC, ZPX};
instructions[0xD7] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xD8] = (Instruction) {"CLD impl", inst_CLD, IMPL};
instructions[0xD9] = (Instruction) {"CMP abs,Y", inst_CMP, ABSY};
instructions[0xDA] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xDB] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xDC] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xDD] = (Instruction) {"CMP abs,X", inst_CMP, ABSX};
instructions[0xDE] = (Instruction) {"DEC abs,X", inst_DEC, ABSX};
instructions[0xDF] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xE0] = (Instruction) {"CPX #", inst_CPX, IMM};
instructions[0xE1] = (Instruction) {"SBC X,ind", inst_SBC, XIND};
instructions[0xE2] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xE3] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xE4] = (Instruction) {"CPX zpg", inst_CPX, ZP};
instructions[0xE5] = (Instruction) {"SBC zpg", inst_SBC, ZP};
instructions[0xE6] = (Instruction) {"INC zpg", inst_INC, ZP};
instructions[0xE7] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xE8] = (Instruction) {"INX impl", inst_INX, IMPL};
instructions[0xE9] = (Instruction) {"SBC #", inst_SBC, IMM};
instructions[0xEA] = (Instruction) {"NOP impl", inst_NOP, IMPL};
instructions[0xEB] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xEC] = (Instruction) {"CPX abs", inst_CPX, ABS};
instructions[0xED] = (Instruction) {"SBC abs", inst_SBC, ABS};
instructions[0xEE] = (Instruction) {"INC abs", inst_INC, ABS};
instructions[0xEF] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xF0] = (Instruction) {"BEQ rel", inst_BEQ, REL};
instructions[0xF1] = (Instruction) {"SBC ind,Y", inst_SBC, INDY};
instructions[0xF2] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xF3] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xF4] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xF5] = (Instruction) {"SBC zpg,X", inst_SBC, ZPX};
instructions[0xF6] = (Instruction) {"INC zpg,X", inst_INC, ZPX};
instructions[0xF7] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xF8] = (Instruction) {"SED impl", inst_SED, IMPL};
instructions[0xF9] = (Instruction) {"SBC abs,Y", inst_SBC, ABSY};
instructions[0xFA] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xFB] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xFC] = (Instruction) {"???", inst_NOP, IMPL};
instructions[0xFD] = (Instruction) {"SBC abs,X", inst_SBC, ABSX};
instructions[0xFE] = (Instruction) {"INC abs,X", inst_INC, ABSX};
instructions[0xFF] = (Instruction) {"???", inst_NOP, IMPL};
}
void reset_cpu()
{
A = 0;
X = 0;
Y = 0;
SP = 0xFF;
SR.byte = 0;
SR.bits.interrupt = 1;
SR.bits.unused = 1;
memcpy(&PC, &memory[RST_VEC], sizeof(PC));
}
int load_rom(char * filename)
{ // TODO allow more flexible loading
memset(memory, 0, sizeof(memory)); // clear ram first
FILE * fp = fopen(filename, "r");
if (fp == NULL) {
printf("Error: could not open file\n");
return -1;
}
if (!fread(&memory[0xC000], 0x4000, 1, fp)) {
printf("Error: ROM file too short.\n");
return -1;
}
fclose(fp);
return 0;
}
void step_cpu()
{
inst = instructions[memory[PC]];
#ifdef DEBUG
printf("PC=%04X OPCODE=%02X: %s\r\n", PC, memory[PC], inst.mnemonic);
printf("A=%02X X=%02X Y=%02X SR=%02X SP=%02X\r\n", A, X, Y, SR.byte, SP);
/* dump memory for analysis (slows down emulation significantly) */
FILE * fp = fopen("memdump", "w");
fwrite(&memory, sizeof(memory), 1, fp);
fclose(fp);
#endif
jumping = 0;
inst.function();
if (jumping == 0) PC += lengths[inst.mode];
}

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#include <stdint.h>
#define CPU_FREQ 4000000
#define STEP_DURATION 10000000 // 10ms
#define ONE_SECOND 1e9
#define NUM_MODES 13
#define NMI_VEC 0xFFFA
#define RST_VEC 0xFFFC
#define IRQ_VEC 0xFFFE
uint8_t memory[1<<16];
uint8_t A;
uint8_t X;
uint8_t Y;
uint16_t PC;
uint8_t SP; // points to first empty stack location
void * readAddr;
void * writeAddr;
struct StatusBits{
int carry:1; // bit 0
int zero:1;
int interrupt:1;
int decimal:1;
int brk:1; // "break" is a reserved word :(
int unused:1;
int overflow:1;
int sign:1; // bit 7
};
union StatusReg { // this means we can access the status register as a byte, or as individual bits.
struct StatusBits bits;
uint8_t byte;
};
union StatusReg SR;
typedef enum {
ACC,
ABS,
ABSX,
ABSY,
IMM,
IMPL,
IND,
XIND,
INDY,
REL,
ZP,
ZPX,
ZPY
} Mode;
typedef struct {
char * mnemonic;
void (*function)();
Mode mode;
} Instruction;
Instruction instructions[0x100];
void init_tables();
void reset_cpu();
int load_rom(char * filename);
void step_cpu();

55
6850.c Normal file
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#include <stdio.h>
#include <unistd.h>
#include <sys/poll.h>
#include <stdlib.h>
#include "6502.h"
#include "6850.h"
int n;
void init_uart() {
memory[DATA_ADDR] = 0;
uart_SR.byte = 0;
uart_SR.bits.TDRE = 1; // we are always ready to output data
uart_SR.bits.RDRF = 0;
incomingChar = 0;
}
int stdin_ready() {
struct pollfd fds;
fds.fd = 0; // stdin
fds.events = POLLIN;
return poll(&fds, 1, 0) == 1; // timeout = 0
}
void step_uart() {
if (writeAddr == &memory[DATA_ADDR]) {
putchar(memory[DATA_ADDR]);
fflush(stdout);
writeAddr = NULL;
} else if (readAddr == &memory[DATA_ADDR]) {
uart_SR.bits.RDRF = 0;
readAddr = NULL;
}
/* update input register if empty */
if ((n++ % 10000) == 0) { // polling stdin every cycle is performance intensive. This is a bit of a dirty hack.
if (!uart_SR.bits.RDRF && stdin_ready()) { // the real hardware has no buffer. Remote the RDRF check for more accurate emulation.
if (read(0, &incomingChar, 1) != 1) {
printf("Warning: read() returned 0\n");
}
if (incomingChar == 0x18) { // CTRL+X
printf("\r\n");
exit(0);
}
uart_SR.bits.RDRF = 1;
}
}
memory[DATA_ADDR] = incomingChar;
memory[CTRL_ADDR] = uart_SR.byte;
}

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6850.h Normal file
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#define CTRL_ADDR 0xA000
#define DATA_ADDR 0xA001
struct UartStatusBits{
int RDRF:1; // bit 0
int TDRE:1;
int DCD:1;
int CTS:1;
int FE:1;
int OVRN:1;
int PE:1;
int IRQ:1; // bit 7
};
union UartStatusReg {
struct UartStatusBits bits;
uint8_t byte;
};
union UartStatusReg uart_SR;
uint8_t incomingChar;
void init_uart();
void step_uart();

22
Makefile Normal file
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CC=gcc
CFLAGS=-Wall -Ofast -std=gnu99
all: 6502-emu
6502-emu: main.o 6502.o 6850.o
$(CC) -o 6502-emu main.c 6502.o 6850.o
main.o: main.c
$(CC) $(CFLAGS) -c main.c
6502.o: 6502.c
$(CC) $(CFLAGS) -c 6502.c
6850.o: 6850.c
$(CC) $(CFLAGS) -c 6850.c
clean:
rm *.o 6502-emu
test: 6502-emu
./6502-emu examples/ehbasic.rom

81
README.md
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@ -1,2 +1,81 @@
# 6502-emu
A simple 6502 emulator, with I/O via a 6850 UART.
A relatively simple and bug-free 6502 emulator. It is capable of running ehBasic.
Many 6502 emulators make extensive use of macros, which I personally think makes
code harder to understand. At the other extreme, some emulators implement each
addressing mode of each instruction separately, which is a big waste of time and
makes the code harder to maintain.
I chose a middle-ground, and used lookup tables for instruction decoding and address
decoding. This also gets rid of the common "giant opcode switch statement" (which
can be seen in my CHIP8 emulator project).
### Usage Example:
```
[david@D-ARCH 6502-emu]$ ./6502-emu examples/ehbasic.rom
Enhanced 6502 BASIC 2.22 (c) Lee Davison
[C]old/[W]arm ?
Memory size ?
40191 Bytes free
Enhanced BASIC 2.22
Ready
10 X1=59
15 Y1=21
20 I1=-1.0
23 I2=1.0
26 R1=-2.0
28 R2=1.0
30 S1=(R2-R1)/X1
35 S2=(I2-I1)/Y1
40 FOR Y=0 TO Y1
50 I3=I1+S2*Y
60 FOR X=0 TO X1
70 R3=R1+S1*X
73 Z1=R3
76 Z2=I3
80 FOR N=0 TO 30
90 A=Z1*Z1
95 B=Z2*Z2
100 IF A+B>4.0 GOTO 130
110 Z2=2*Z1*Z2+I3
115 Z1=A-B+R3
120 NEXT N
130 PRINT CHR$(63-N);
140 NEXT X
150 PRINT
160 NEXT Y
170 END
RUN
??????>>>>>===============<<<<<<;;;:7143;<<<<====>>>>>>>>>>>
?????>>>================<<<<<<<;;;984+18:;;<<<<=====>>>>>>>>
????>>>===============<<<<<<<;;::85 )/:;;;;<<=====>>>>>>>
???>>===============<<<<<<;:9999875 689::::;<<=====>>>>>
??>>=============<<<<;;;;::7/ '3 56446;<======>>>>
??>===========<<<;;;;;;:::863 +8:;<======>>>
?>========<<<;6::::::::997! &89;<<======>>
?====<<<<<;;;:83567.678874 ,:<<=======>
?=<<<<<<;;;;:986' /4 +:<<<======>
?<<<<<<;;::8675( ( 9;<<<======>
?;;:999:88460 '9:;<<<======>
?;;:999:88460 '9:;<<<======>
?<<<<<<;;::8675( ( 9;<<<======>
?=<<<<<<;;;;:986' /4 +:<<<======>
?====<<<<<;;;:83567.678874 ,:<<=======>
?>========<<<;6::::::::997! &89;<<======>>
??>===========<<<;;;;;;:::863 +8:;<======>>>
??>>=============<<<<;;;;::7/ '3 56446;<======>>>>
???>>===============<<<<<<;:9999875 689::::;<<=====>>>>>
????>>>===============<<<<<<<;;::85 )/:;;;;<<=====>>>>>>>
?????>>>================<<<<<<<;;;984+18:;;<<<<=====>>>>>>>>
??????>>>>>===============<<<<<<;;;:7143;<<<<====>>>>>>>>>>>
Ready
```

BIN
examples/ehbasic.rom Normal file
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#include <stdio.h>
#include <stdlib.h>
#include <termios.h>
#include <time.h>
#include "6502.h"
#include "6850.h"
struct termios oldTermios;
void step_delay()
{
struct timespec req, rem;
req.tv_sec = 0;
req.tv_nsec = STEP_DURATION;
nanosleep(&req, &rem);
}
void run_cpu()
{
for (;;) {
// CPU timing is currently very far from being cycle-accurate
for (int i = 0; i < (CPU_FREQ / (ONE_SECOND / STEP_DURATION)); i++) {
step_cpu();
step_uart();
}
step_delay(); // remove this for more speed
}
}
void restore_stdin()
{
tcsetattr(0, TCSANOW, &oldTermios);
}
void raw_stdin()
{
struct termios newTermios;
tcgetattr(0, &oldTermios);
newTermios = oldTermios;
cfmakeraw(&newTermios);
tcsetattr(0, TCSANOW, &newTermios);
atexit(restore_stdin);
}
int main(int argc, char *argv[])
{
if (argc != 2) {
printf("Usage: %s file.rom\n", argv[0]);
printf("The first 16k of \"file.rom\" is loaded into the last 16k of memory.\n");
return EXIT_FAILURE;
}
if (load_rom(argv[1]) != 0) {
printf("Error loading \"%s\".\n", argv[1]);
return EXIT_FAILURE;
}
raw_stdin(); // allow individual keystrokes to be detected
init_tables();
init_uart();
reset_cpu();
run_cpu();
return EXIT_SUCCESS;
}