117 lines
1.9 KiB
C
117 lines
1.9 KiB
C
#include <stdint.h>
|
|
|
|
struct cpu_flags {
|
|
uint8_t carry : 1;
|
|
uint8_t zero : 1;
|
|
uint8_t interrupt_disable : 1;
|
|
uint8_t decimal_mode : 1;
|
|
uint8_t brk : 1;
|
|
uint8_t unused : 1;
|
|
uint8_t overflow : 1;
|
|
uint8_t negative : 1;
|
|
};
|
|
|
|
struct registers {
|
|
uint8_t acc, x, y, sp;
|
|
struct cpu_flags status;
|
|
uint16_t pc;
|
|
};
|
|
struct registers regs;
|
|
|
|
enum addressing_mode {
|
|
AM_ACCUMULATOR,
|
|
AM_IMMEDIATE,
|
|
AM_ZERO_PAGE,
|
|
AM_ZERO_PAGE_X,
|
|
AM_ZERO_PAGE_Y,
|
|
AM_RELATIVE,
|
|
AM_ABSOLUTE,
|
|
AM_ABSOLUTE_X,
|
|
AM_ABSOLUTE_Y,
|
|
AM_INDIRECT,
|
|
AM_INDIRECT_X,
|
|
AM_INDIRECT_Y,
|
|
AM_INDEXED_INDIRECT,
|
|
AM_INDIRECT_INDEXED,
|
|
};
|
|
|
|
uint8_t addrbus[0x10000];
|
|
|
|
/* example program taken from https://bugzmanov.github.io/nes_ebook/chapter_3_1.html */
|
|
uint8_t program[] = { 0xa9, 0xc0, 0xaa, 0xe8, 0x00 };
|
|
|
|
void brk(void)
|
|
{
|
|
/* TODO: push regs.pc and regs.status to stack and load IRQ vector */
|
|
regs.status.brk = 1;
|
|
return;
|
|
}
|
|
|
|
void lda(enum addressing_mode mode)
|
|
{
|
|
uint8_t val;
|
|
|
|
switch (mode) {
|
|
case AM_IMMEDIATE: /* $A9 */
|
|
val = program[regs.pc++];
|
|
regs.acc = val;
|
|
break;
|
|
case AM_ZERO_PAGE: /* $A5 */
|
|
case AM_ZERO_PAGE_X: /* $B5 */
|
|
case AM_ABSOLUTE: /* $AD */
|
|
case AM_ABSOLUTE_X: /* $BD */
|
|
case AM_ABSOLUTE_Y: /* $B9 */
|
|
case AM_INDIRECT_X: /* $A1 */
|
|
case AM_INDIRECT_Y: /* $B1 */
|
|
/* TODO */
|
|
break;
|
|
default:
|
|
return;
|
|
}
|
|
|
|
regs.status.zero = regs.acc == 0;
|
|
regs.status.negative = (regs.acc & (1 << 7)) != 0;
|
|
}
|
|
|
|
void interpret(void)
|
|
{
|
|
uint8_t opcode;
|
|
regs.pc = 0;
|
|
|
|
for (;;) {
|
|
opcode = program[regs.pc++];
|
|
|
|
switch (opcode) {
|
|
case 0x00:
|
|
brk();
|
|
return;
|
|
case 0xa9:
|
|
lda(AM_IMMEDIATE);
|
|
break;
|
|
case 0xa5:
|
|
lda(AM_ZERO_PAGE);
|
|
break;
|
|
case 0xb5:
|
|
lda(AM_ZERO_PAGE_X);
|
|
break;
|
|
case 0xad:
|
|
lda(AM_ABSOLUTE);
|
|
break;
|
|
case 0xbd:
|
|
lda(AM_ABSOLUTE_X);
|
|
break;
|
|
case 0xb9:
|
|
lda(AM_ABSOLUTE_Y);
|
|
break;
|
|
case 0xa1:
|
|
lda(AM_INDIRECT_X);
|
|
break;
|
|
case 0xb1:
|
|
lda(AM_INDIRECT_Y);
|
|
break;
|
|
default:
|
|
break;
|
|
}
|
|
}
|
|
}
|