From 68ecbce235295884ea2f593f1dbe5cc76683e5cc Mon Sep 17 00:00:00 2001
From: vin <git@vineetk.net>
Date: Sun, 9 Jun 2024 22:15:21 +0530
Subject: [PATCH] add opcode_mem() function to return memory address and not
 pre-peek

Of course all of these opcodes need to be tested later...
---
 cpu.c | 70 ++++++++++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 55 insertions(+), 15 deletions(-)

diff --git a/cpu.c b/cpu.c
index 076e887..65e60d4 100644
--- a/cpu.c
+++ b/cpu.c
@@ -67,7 +67,7 @@ peek16(uint16_t addr)
 static void
 memwrite(uint16_t addr, uint8_t byte)
 {
-	memory[addr] = b;
+	memory[addr] = byte;
 }
 
 static void
@@ -125,6 +125,50 @@ opcode_arg(enum addressing_mode mode)
 	return val;
 }
 
+static uint16_t
+opcode_mem(enum addressing_mode mode)
+{
+	uint8_t arg;
+	uint16_t val;
+
+	if (mode != AM_ABS && mode != AM_ABS_X && mode != AM_ABS_Y)
+		arg = peek(regs.pc++);
+	else
+		arg = peek16(regs.pc), regs.pc += 2;
+
+	switch (mode) {
+	case AM_ZP:
+		val = arg % 256;
+		break;
+	case AM_ZP_X:
+		val = (arg + regs.x) % 256;
+		break;
+	case AM_ZP_Y:
+		val = (arg + regs.y) % 256;
+		break;
+	case AM_ABS:
+		val = arg;
+		break;
+	case AM_ABS_X:
+		val = arg + regs.x;
+		break;
+	case AM_ABS_Y:
+		val = arg + regs.y;
+		break;
+	case AM_IND_X:
+		val = peek((arg + regs.x) % 256) + peek((arg + regs.x + 1) % 256) * 256;
+		break;
+	case AM_IND_Y:
+		val = peek(arg) + peek((arg + 1) % 256) * 256 + regs.y;
+		break;
+	default:
+		fprintf(stderr, "INVALID ADDRESSING MODE\n");
+		abort();
+	}
+
+	return val;
+}
+
 static void
 adc(uint8_t arg)
 {
@@ -313,9 +357,9 @@ cpy(uint8_t arg)
 }
 
 static void
-dec(uint8_t arg)
+dec(uint16_t mem)
 {
-/* TODO: complete this */
+	memwrite(mem, peek(mem) - 1);
 }
 
 static void
@@ -839,19 +883,19 @@ interpret(void)
 			cycles += 2;
 			break;
 		case 0x18:
-			clc(opcode_arg(AM_ACC));
+			clc();
 			cycles += 2;
 			break;
 		case 0xd8:
-			cld(opcode_arg(AM_ACC));
+			cld();
 			cycles += 2;
 			break;
 		case 0x58:
-			cli(opcode_arg(AM_ACC));
+			cli();
 			cycles += 2;
 			break;
 		case 0xb8:
-			clv(opcode_arg(AM_ACC));
+			clv();
 			cycles += 2;
 			break;
 		case 0xc9:
@@ -914,24 +958,20 @@ interpret(void)
 			cpy(opcode_arg(AM_ABS));
 			cycles += 4;
 			break;
-		case 0x3a:
-			dec(opcode_arg(AM_ACC));
-			cycles += 2;
-			break;
 		case 0xc6:
-			dec(opcode_arg(AM_ZP));
+			dec(opcode_mem(AM_ZP));
 			cycles += 5;
 			break;
 		case 0xd6:
-			dec(opcode_arg(AM_ZP_X));
+			dec(opcode_mem(AM_ZP_X));
 			cycles += 6;
 			break;
 		case 0xce:
-			dec(opcode_arg(AM_ABS));
+			dec(opcode_mem(AM_ABS));
 			cycles += 6;
 			break;
 		case 0xde:
-			dec(opcode_arg(AM_ABS_X));
+			dec(opcode_mem(AM_ABS_X));
 			cycles += 7;
 			break;
 		case 0xca: