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#include <stdint.h>
#include <stdio.h>
#define STATUS_UPDATE_ZERO(r) \
(regs.status.zero = r == 0)
#define STATUS_UPDATE_NEGATIVE(r) \
(regs.status.negative = ((r & (1 << 7)) > 0))
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 a, 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)
{
/* $00 */
/* TODO: push regs.pc and regs.status to stack and load IRQ vector */
regs.status.brk = 1;
return;
}
void
tax(void)
{
/* $AA */
regs.x = regs.a;
STATUS_UPDATE_ZERO(regs.x);
STATUS_UPDATE_NEGATIVE(regs.x);
}
void
inx(void)
{
/* $E8 */
regs.x++;
STATUS_UPDATE_ZERO(regs.x);
STATUS_UPDATE_NEGATIVE(regs.x);
}
void
lda(enum addressing_mode mode)
{
uint8_t val;
switch (mode) {
case AM_IMMEDIATE: /* $A9 */
val = program[regs.pc++];
printf("arg1 $%02X\n", val);
regs.a = val;
break;
case AM_ZERO_PAGE: /* $A5 */
/* TODO */
break;
case AM_ZERO_PAGE_X: /* $B5 */
/* TODO */
break;
case AM_ABSOLUTE: /* $AD */
/* TODO */
break;
case AM_ABSOLUTE_X: /* $BD */
/* TODO */
break;
case AM_ABSOLUTE_Y: /* $B9 */
/* TODO */
break;
case AM_INDIRECT_X: /* $A1 */
/* TODO */
break;
case AM_INDIRECT_Y: /* $B1 */
/* TODO */
break;
default:
return;
}
STATUS_UPDATE_ZERO(regs.a);
STATUS_UPDATE_NEGATIVE(regs.a);
}
void
interpret(void)
{
uint8_t opcode;
regs.pc = 0;
regs.status.unused = 1;
printf("Initial State:\n");
printf("status: %i%i%i%i%i%i%i%i\n", regs.status.carry,
regs.status.zero, regs.status.interrupt_disable,
regs.status.decimal_mode, regs.status.brk,
regs.status.unused, regs.status.overflow,
regs.status.negative);
printf("A: $%02X, X: $%02X, Y: $%02X, SP: $%02X, PC: $%02X\n",
regs.a, regs.x, regs.y, regs.sp, regs.pc);
putchar('\n');
for (;;) {
opcode = program[regs.pc++];
printf("opcode: $%02X\n", opcode);
switch (opcode) {
case 0x00:
brk();
return;
case 0xa1:
lda(AM_INDIRECT_X);
break;
case 0xa5:
lda(AM_ZERO_PAGE);
break;
case 0xa9:
lda(AM_IMMEDIATE);
break;
case 0xaa:
tax();
break;
case 0xad:
lda(AM_ABSOLUTE);
break;
case 0xb1:
lda(AM_INDIRECT_Y);
break;
case 0xb5:
lda(AM_ZERO_PAGE_X);
break;
case 0xb9:
lda(AM_ABSOLUTE_Y);
break;
case 0xbd:
lda(AM_ABSOLUTE_X);
break;
case 0xe8:
inx();
break;
default:
printf("opcode $%02X not implemented\n", opcode);
break;
}
printf("status: %i%i%i%i%i%i%i%i\n", regs.status.carry,
regs.status.zero, regs.status.interrupt_disable,
regs.status.decimal_mode, regs.status.brk,
regs.status.unused, regs.status.overflow,
regs.status.negative);
printf("A: $%02X, X: $%02X, Y: $%02X, SP: $%02X, PC: $%02X\n",
regs.a, regs.x, regs.y, regs.sp, regs.pc);
}
}
int
main(void)
{
interpret();
return 0;
}
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