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add adc and actually fix peek/16

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Vineet K 2024-06-08 17:41:06 +05:30
parent bd85b80ba7
commit 1ac30a026b
1 changed files with 82 additions and 2 deletions
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84
cpu.c
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@ -1,7 +1,10 @@
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX(a, b) ((a > b) ? a : b)
#define STATUS_UPDATE_ZERO(r) \
(regs.status.zero = r == 0)
#define STATUS_UPDATE_NEGATIVE(r) \
@ -47,18 +50,70 @@ uint8_t memory[0x16000];
uint8_t program[] = { 0xa9, 0xc0, 0xaa, 0xe8, 0x00 };
uint8_t
peek(uint8_t addr)
peek(uint16_t addr)
{
return memory[addr];
}
uint8_t
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
adc(enum addressing_mode mode)
{
uint8_t arg, val;
uint16_t sum; // 16-bit sum makes it easier to determine carry flag
if (mode != AM_ABS && mode != AM_ABS_X && mode != AM_ABS_Y)
arg = peek(regs.pc++);
else
arg = peek16(regs.pc++);
switch (mode) {
case AM_IMM: /* $69 */
val = arg;
break;
case AM_ZP: /* $65 */
val = peek(arg % 256);
break;
case AM_ABS: /* $6D */
val = peek16(arg);
break;
case AM_ZP_X: /* $75 */
val = peek((arg + regs.x) % 256);
break;
case AM_ABS_X: /* $7D */
val = peek16(arg + regs.x);
break;
case AM_ABS_Y: /* $79 */
val = peek16(arg + regs.y);
break;
case AM_IND_X: /* $61 */
val = peek(peek((arg + regs.x) % 256) + peek((arg + regs.x + 1) % 256) * 256);
break;
case AM_IND_Y: /* $71 */
val = peek(peek(arg) + peek((arg + 1) % 256) * 256 + regs.y);
break;
default:
fprintf(stderr, "INVALID ADC ADDRESSING MODE\n");
abort();
return;
}
sum = regs.a + val + regs.status.carry;
regs.a = sum & 0xFF;
regs.status.carry = sum > 0xFF;
/* overflow flag formula: https://stackoverflow.com/a/29224684 */
regs.status.overflow = ~(regs.a ^ arg) & (regs.a ^ sum) & 0x80;
STATUS_UPDATE_ZERO(regs.a);
STATUS_UPDATE_NEGATIVE(regs.a);
}
void
brk(void)
{
@ -150,6 +205,30 @@ interpret(void)
case 0x00:
brk();
return;
case 0x61:
adc(AM_IND_X);
break;
case 0x65:
adc(AM_ZP);
break;
case 0x69:
adc(AM_IMM);
break;
case 0x6D:
adc(AM_ABS);
break;
case 0x71:
adc(AM_IND_Y);
break;
case 0x75:
adc(AM_ZP_X);
break;
case 0x79:
adc(AM_ABS_Y);
break;
case 0x7D:
adc(AM_ABS_X);
break;
case 0xa1:
lda(AM_IND_X);
break;
@ -216,6 +295,7 @@ main(void)
fprintf(stderr, "program is too big for memory\n");
return 1;
}
memcpy(memory + 0x8000, program, sizeof(program));
regs.pc = 0x8000;