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add peek(), peek16(), and finish lda()

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This commit is contained in:
Vineet K 2024-06-04 15:54:09 +05:30
parent 10567e0be8
commit 7245191e86
1 changed files with 47 additions and 17 deletions
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64
cpu.c
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@ -42,11 +42,25 @@ enum addressing_mode {
AM_INDIRECT_INDEXED,
};
uint8_t memory[0x10000];
/* 64K address space, 16bit words */
uint8_t memory[0x16000];
/* example program taken from https://bugzmanov.github.io/nes_ebook/chapter_3_1.html */
uint8_t program[] = { 0xa9, 0xc0, 0xaa, 0xe8, 0x00 };
uint8_t
peek(uint16_t addr)
{
return memory[addr];
}
uint16_t
peek16(uint16_t addr)
{
/* bytes are stored in little-endian (low then high) */
return (uint16_t)memory[addr] | ((uint16_t)memory[addr + 1] << 8);
}
void
brk(void)
{
@ -79,39 +93,43 @@ inx(void)
void
lda(enum addressing_mode mode)
{
uint8_t val;
uint8_t arg, val;
arg = peek(regs.pc++);
switch (mode) {
case AM_IMMEDIATE: /* $A9 */
val = memory[regs.pc++];
printf("arg1 $%02X\n", val);
regs.a = val;
val = arg;
break;
case AM_ZERO_PAGE: /* $A5 */
/* TODO */
break;
case AM_ZERO_PAGE_X: /* $B5 */
/* TODO */
val = peek(arg % 256);
break;
case AM_ABSOLUTE: /* $AD */
/* TODO */
val = peek(arg);
break;
case AM_ZERO_PAGE_X: /* $B5 */
val = peek((arg + regs.x) % 256);
break;
case AM_ABSOLUTE_X: /* $BD */
/* TODO */
val = peek(arg + regs.x);
break;
case AM_ABSOLUTE_Y: /* $B9 */
/* TODO */
val = peek(arg + regs.y);
break;
case AM_INDIRECT_X: /* $A1 */
/* TODO */
val = peek(peek((arg + regs.x) % 256) + peek((arg + regs.x + 1) % 256) * 256);
break;
case AM_INDIRECT_Y: /* $B1 */
/* TODO */
val = peek(peek(arg) + peek((arg + 1) % 256) * 256 + regs.y);
break;
default:
regs.pc--;
return;
}
printf("arg1 $%02X\n", arg);
regs.a = val;
STATUS_UPDATE_ZERO(regs.a);
STATUS_UPDATE_NEGATIVE(regs.a);
}
@ -122,7 +140,7 @@ interpret(void)
uint8_t opcode;
for (;;) {
opcode = memory[regs.pc++];
opcode = peek(regs.pc++);
printf("opcode: $%02X\n", opcode);
@ -175,9 +193,23 @@ interpret(void)
}
}
/* https://www.nesdev.org/wiki/CPU_power_up_state */
void
cpu_init(void)
{
regs.a = regs.x = regs.y = 0;
regs.pc = 0xFFFC;
regs.sp = 0xFD;
memset(&regs.status, 0, sizeof(regs.status));
regs.status.unused = 1;
}
int
main(void)
{
cpu_init();
if (sizeof(program) > (0x10000 - 0x8000)) {
fprintf(stderr, "program is too big for memory\n");
return 1;
@ -185,8 +217,6 @@ main(void)
memcpy(memory + 0x8000, program, sizeof(program));
regs.pc = 0x8000;
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,