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

1 files 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,