6502-sim/cpu.c

#include <assert.h>

#include "cpu.h"
#include "opcodes.h"

extern void reset6502(void);
extern void step6502(void);

uint8_t cpu_memory[65536] = {
  [0xfffc] = 0x0,
  [0xfffd] = 0x0,
};

uint8_t read6502(uint16_t address)
{
  return cpu_memory[address];
}

void write6502(uint16_t address, uint8_t value)
{
  cpu_memory[address] = value;
}

extern uint16_t pc;
extern uint8_t sp, a, x, y, status;

cpu_history_t cpu_history[65536] = { 0 };
int cpu_history_len;
int cpu_history_ix;

void cpu_get_state(cpu_state_t * state) {
  *state = (cpu_state_t){
    .a = a,
    .y = y,
    .x = x,
    .pc = pc,
    .sp = sp,
    .status = status,
  };
}

static inline void update_history(void)
{
  instruction_t instruction = decode_ins[cpu_memory[pc]];
  addressing_mode_t addressing_mode = addressing_modes[instruction.mode];

  if (cpu_history_len < 65536)
    cpu_history_len++;

  cpu_history[cpu_history_ix] = (cpu_history_t){
    .state = (cpu_state_t){
      .a = a,
      .y = y,
      .x = x,
      .pc = pc,
      .sp = sp,
      .status = status,
    },
    .pc_mem = { cpu_memory[pc], cpu_memory[pc+1], cpu_memory[pc+2] },
    .value = (addressing_mode.func)(pc),
  };

  cpu_history_ix = (cpu_history_ix + 1) & 0xffff;
}

void cpu_reset(void)
{
  reset6502();

  cpu_history_len = 0;
  cpu_history_ix = 0;
}

void cpu_step(void)
{
  update_history();
  step6502();
}

static uint8_t * mem = cpu_memory;

uint16_t absolute(uint16_t _pc)
{
  uint16_t effective = ((uint16_t)mem[_pc + 2] << 8) | ((uint16_t)mem[_pc + 1] << 0);

  return effective;
}

uint16_t absolute_indexed_indirect(uint16_t _pc)
{
  uint16_t effective = (((uint16_t)mem[_pc + 2] << 8) | ((uint16_t)mem[_pc + 1] << 0))
                     + x;

  uint16_t indirect = ((uint16_t)mem[effective + 1] << 8)
                    | ((uint16_t)mem[effective + 0] << 0);

  return indirect;
}

uint16_t absolute_indexed_with_x(uint16_t _pc)
{
  uint16_t effective = (((uint16_t)mem[_pc + 2] << 8) | ((uint16_t)mem[_pc + 1] << 0))
                     + x;

  return effective;
}

uint16_t absolute_indexed_with_y(uint16_t _pc)
{
  uint16_t effective = (((uint16_t)mem[_pc + 2] << 8) | ((uint16_t)mem[_pc + 1] << 0))
                     + y;

  return effective;
}

uint16_t absolute_indirect(uint16_t _pc)
{
  uint16_t effective = (((uint16_t)mem[_pc + 2] << 8) | ((uint16_t)mem[_pc + 1] << 0))
                     + y;

  uint16_t indirect = ((uint16_t)mem[effective + 1] << 8)
                    | ((uint16_t)mem[effective + 0] << 0);

  return indirect;
}

uint16_t accumulator(uint16_t _pc)
{
  return a;
}

uint16_t immediate(uint16_t _pc)
{
  return (uint8_t)mem[_pc + 1];
}

uint16_t implied(uint16_t _pc)
{
  return 0; // ???
}

uint16_t program_counter_relative(uint16_t _pc)
{
  uint16_t effective = _pc + 2 + ((int8_t)mem[_pc + 1]);

  return effective;
}

uint16_t stack(uint16_t _pc)
{
  uint16_t effective = (0x01 << 8) | (sp << 0);

  return effective;
}

uint16_t zero_page(uint16_t _pc)
{
  uint16_t effective = (0x00 << 8) | ((uint16_t)mem[_pc + 1] << 0);

  return effective;
}

uint16_t zero_page_indexed_indirect(uint16_t _pc)
{
  uint8_t base = x + ((uint8_t)mem[_pc + 1] << 0);

  uint16_t effective = (0x00 << 8) | base;

  uint16_t indirect = ((uint16_t)mem[effective + 1] << 8)
                    | ((uint16_t)mem[effective + 0] << 0);

  return indirect;
}

uint16_t zero_page_indexed_with_x(uint16_t _pc)
{
  uint8_t base = x + ((uint8_t)mem[_pc + 1] << 0);

  uint16_t effective = (0x00 << 8) | base;

  return effective;
}

uint16_t zero_page_indexed_with_y(uint16_t _pc)
{
  uint8_t base = y + ((uint8_t)mem[_pc + 1] << 0);

  uint16_t effective = (0x00 << 8) | base;

  return effective;
}

uint16_t zero_page_indirect(uint16_t _pc)
{
  uint8_t base = ((uint8_t)mem[_pc + 1] << 0);

  uint16_t effective = (0x00 << 8) | base;

  uint16_t indirect = ((uint16_t)mem[effective + 1] << 8)
                    | ((uint16_t)mem[effective + 0] << 0);

  return indirect;
}

uint16_t zero_page_indirect_indexed_with_y(uint16_t _pc)
{
  uint8_t base = ((uint8_t)mem[_pc + 1] << 0);

  uint16_t indirect = ((uint16_t)mem[base + 1] << 8)
                    | ((uint16_t)mem[base + 0] << 0);

  uint16_t effective = indirect + y;

  return effective;
}

addressing_mode_t addressing_modes[16] = {
  [A]     = { .alen = 2, .olen = 2, .func = absolute },
  [AII]   = { .alen = 2, .olen = 2, .func = absolute_indexed_indirect },
  [AIX]   = { .alen = 2, .olen = 2, .func = absolute_indexed_with_x },
  [AIY]   = { .alen = 2, .olen = 2, .func = absolute_indexed_with_y },
  [AI]    = { .alen = 2, .olen = 2, .func = absolute_indirect },
  [ACC]   = { .alen = 0, .olen = 1, .func = accumulator },
  [IMM]   = { .alen = 1, .olen = 1, .func = immediate },
  [I]     = { .alen = 0, .olen = 0, .func = implied },
  [R]     = { .alen = 1, .olen = 2, .func = program_counter_relative },
  [S]     = { .alen = 0, .olen = 2, .func = stack },
  [ZP]    = { .alen = 1, .olen = 2, .func = zero_page },
  [ZPII]  = { .alen = 1, .olen = 2, .func = zero_page_indexed_indirect },
  [ZPX]   = { .alen = 1, .olen = 2, .func = zero_page_indexed_with_x },
  [ZPY]   = { .alen = 1, .olen = 2, .func = zero_page_indexed_with_y },
  [ZPI]   = { .alen = 1, .olen = 2, .func = zero_page_indirect },
  [ZPIY]  = { .alen = 1, .olen = 2, .func = zero_page_indirect_indexed_with_y },
};