From 1ac30a026b7877ca6b295c661d91b20eaddb01c9 Mon Sep 17 00:00:00 2001
From: vin <git@vineetk.net>
Date: Sat, 8 Jun 2024 17:41:06 +0530
Subject: [PATCH] add adc and actually fix peek/16

---
 cpu.c | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 82 insertions(+), 2 deletions(-)

diff --git a/cpu.c b/cpu.c
index f018a44..08c6119 100644
--- a/cpu.c
+++ b/cpu.c
@@ -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;