load program rom into proper offset in memory

1 files changed, 24 insertions, 16 deletions

diff --git a/cpu.c b/cpu.c
index 14c2ce3..c7e76b8 100644
--- a/cpu.c
+++ b/cpu.c
@@ -1,5 +1,6 @@
 #include <stdint.h>
 #include <stdio.h>
+#include <string.h>
 
 #define STATUS_UPDATE_ZERO(r)           \
         (regs.status.zero = r == 0)
@@ -41,7 +42,7 @@ enum addressing_mode {
         AM_INDIRECT_INDEXED,
 };
 
-uint8_t addrbus[0x10000];
+uint8_t memory[0x10000];
 
 /* example program taken from https://bugzmanov.github.io/nes_ebook/chapter_3_1.html */
 uint8_t program[] = { 0xa9, 0xc0, 0xaa, 0xe8, 0x00 };
@@ -82,7 +83,7 @@ lda(enum addressing_mode mode)
 
         switch (mode) {
         case AM_IMMEDIATE:      /* $A9 */
-                val = program[regs.pc++];
+                val = memory[regs.pc++];
                 printf("arg1 $%02X\n", val);
                 regs.a = val;
                 break;
@@ -119,22 +120,9 @@ void
 interpret(void)
 {
         uint8_t opcode;
-        regs.pc = 0;
-        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,
-            regs.status.decimal_mode, regs.status.brk,
-            regs.status.unused, regs.status.overflow,
-            regs.status.negative);
-        printf("A: $%02X, X: $%02X, Y: $%02X, SP: $%02X, PC: $%02X\n",
-            regs.a, regs.x, regs.y, regs.sp, regs.pc);
-
-        putchar('\n');
 
         for (;;) {
-                opcode = program[regs.pc++];
+                opcode = memory[regs.pc++];
 
                 printf("opcode: $%02X\n", opcode);
 
@@ -190,6 +178,26 @@ interpret(void)
 int
 main(void)
 {
+        if (sizeof(program) > (0x10000 - 0x8000)) {
+                fprintf(stderr, "program is too big for memory\n");
+                return 1;
+        }
+        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,
+            regs.status.decimal_mode, regs.status.brk,
+            regs.status.unused, regs.status.overflow,
+            regs.status.negative);
+        printf("A: $%02X, X: $%02X, Y: $%02X, SP: $%02X, PC: $%02X\n",
+            regs.a, regs.x, regs.y, regs.sp, regs.pc);
+
+        putchar('\n');
+
         interpret();
 
         return 0;