From 7245191e86e4c8da9fe18788e3daae9b30005812 Mon Sep 17 00:00:00 2001
From: vin <git@vineetk.net>
Date: Tue, 4 Jun 2024 15:54:09 +0530
Subject: [PATCH] add peek(), peek16(), and finish lda()

---
 cpu.c | 64 +++++++++++++++++++++++++++++++++++++++++++----------------
 1 file changed, 47 insertions(+), 17 deletions(-)

diff --git a/cpu.c b/cpu.c
index c7e76b8..770206d 100644
--- a/cpu.c
+++ b/cpu.c
@@ -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,