disassemble simple 6502 instructions

6502.c

@@ -1,3 +0,0 @@
-#include <stdio.h>
-
-int main() { printf("Hello, World!\n"); }

build.sh

@@ -1,4 +1,4 @@
 #!/bin/bash
 set -xe
 cc -O3 -o chip8 chip8.c -L/usr/local/lib/libraylib.a -lraylib -lm
-cc -O3 -o 6502 6502.c
+cc -O3 -o mos6502 mos6502.c

chip8.c

@@ -205,15 +205,6 @@ void chip8_disassemble(CHIP8 *c, size_t ins_count) {
 }
 
 void chip8_step(CHIP8 *c) {
-  do {
-    if (c->delay_timer > 0) {
-      c->delay_timer--;
-    }
-    if (c->sound_timer > 0) {
-      c->sound_timer--;
-      // TODO: buzzer
-    }
-
   READ_INS();
 
   switch ((ins >> 12) & 0xF) {
@@ -397,7 +388,6 @@ void chip8_step(CHIP8 *c) {
   default:
     BAD_INS();
   }
-  } while (0);
 }
 
 void chip8_free(CHIP8 c) {
@@ -426,6 +416,14 @@ int main(int argc, char *argv[]) {
   SetTargetFPS(60);
 
   while (!WindowShouldClose()) {
+    if (c.delay_timer > 0) {
+      c.delay_timer--;
+    }
+    if (c.sound_timer > 0) {
+      c.sound_timer--;
+      // TODO: buzzer
+    }
+
     for (int i = 0; i < 25; i++) {
       chip8_step(&c);
     }

mos6502.c

@@ -0,0 +1,82 @@
+// https://www.masswerk.at/6502/6502_instruction_set.html
+// https://tutorial-6502.sourceforge.io/specification/opcodes/
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#define READ_ADDR()                                                            \
+  uint16_t low = m->memory[m->pc++];                                           \
+  uint16_t high = m->memory[m->pc++];                                          \
+  uint16_t addr = (high << 8) | low
+
+typedef struct MOS6502 {
+  uint8_t *memory;
+  uint16_t pc;
+  uint8_t A;
+  uint8_t X;
+  uint8_t Y;
+  uint8_t SP;
+  uint8_t P;
+} MOS6502;
+
+MOS6502 mos6502_create() {
+  MOS6502 m = {0};
+  m.pc = 0x600;
+  m.memory = malloc(1 << 16);
+  return m;
+}
+
+void mos6502_disassemble(MOS6502 *m, size_t ins_count) {
+  for (size_t i = 0; i < ins_count; i++) {
+    uint8_t op = m->memory[m->pc++];
+    switch (op) {
+    case 0x00: {
+      printf("BRK\n");
+    } break;
+    case 0x4C: {
+      READ_ADDR();
+      printf("JMP $%X\n", addr);
+    } break;
+    case 0x8D: {
+      READ_ADDR();
+      printf("STA $%X\n", addr);
+    } break;
+    case 0xA2: {
+      printf("LDX #$%X\n", m->memory[m->pc++]);
+    } break;
+    case 0xBD: {
+      READ_ADDR();
+      printf("LDA $%X,X\n", addr);
+    } break;
+    case 0xE8: {
+      printf("INX\n");
+    } break;
+    case 0xF0: {
+      printf("BEQ %d\n", m->memory[m->pc++]);
+    } break;
+    default:
+      fprintf(stderr, "unrecognized opcode: %x\n", op);
+      exit(1);
+    }
+  }
+}
+
+int main(int argc, char *argv[]) {
+  if (argc < 2) {
+    fprintf(stderr, "Usage: %s <path>\n", argv[0]);
+    return 1;
+  }
+
+  MOS6502 m = mos6502_create();
+
+  uint8_t buffer[4000] = {0};
+  FILE *f = fopen(argv[1], "rb");
+  size_t n = fread(buffer, 1, 4000, f);
+  fclose(f);
+
+  for (size_t i = 0; i < n; i++) {
+    m.memory[0x600 + i] = buffer[i];
+  }
+
+  mos6502_disassemble(&m, 7);
+}