add nestest

example/nestest.cpp

@@ -0,0 +1,57 @@
+// (improperly) loads an NES ROM file to $C000 and begins execution there
+
+#include "v6502/cpu.h"
+#include "v6502/memory.h"
+#include "v6502/print.h"
+#include <fstream>
+
+class SimpleRAMBus : public MemoryBus {
+public:
+  SimpleRAMBus() { ram = new uint8_t[0x10000]; }
+  ~SimpleRAMBus() { delete[] ram; }
+
+  uint8_t read(uint16_t addr) override { return ram[addr]; }
+  void write(uint16_t addr, uint8_t data) override { ram[addr] = data; }
+
+private:
+  uint8_t *ram;
+};
+
+int main(int argc, char **argv) {
+  if (argc < 2) {
+    print("usage: {} <binary file>\nprogram will be loaded to $6000 and "
+          "executed until it reaches the end\n",
+          argv[0]);
+    return -1;
+  }
+
+  SimpleRAMBus bus;
+  CPU cpu(&bus);
+
+  uint16_t addr = 0xC000;
+  {
+    std::ifstream bin(argv[1]);
+    char byte;
+    while (bin.get(byte)) {
+      // offset by the NES file header lol
+      bus.write((addr++) - 16, byte);
+    }
+  }
+
+  bus.write(0xFFFC, 0x00);
+  bus.write(0xFFFD, 0xC0);
+
+  print("resetting CPU\n");
+  cpu.reset();
+
+  try {
+    while (true) {
+      cpu.executeStep();
+    }
+  } catch (...) {
+    print("[warn] execution ended prematurely due to a v6502 error!!!\n");
+  }
+
+  print("\n---\nresults: {:02x} {:02x}\n", bus.read(2), bus.read(3));
+  return 0;
+}

inc/v6502/print.h

@@ -3,5 +3,5 @@
 #include <format>
 #include <iostream>
 
-#define print(...) std::cout << std::format(__VA_ARGS__)
-#define print_err(...) std::cerr << std::format(__VA_ARGS__)
+#define print(...) std::cout << std::format(__VA_ARGS__) << std::flush
+#define print_err(...) std::cerr << std::format(__VA_ARGS__) << std::flush

meson.build

@@ -41,3 +41,9 @@ executable(
   'example/loadram.cpp',
   dependencies: v6502_dep,
 )
+
+executable(
+  'v6502_nestest',
+  'example/nestest.cpp',
+  dependencies: v6502_dep,
+)

src/cpu.cpp

@@ -109,13 +109,14 @@ static uint16_t getDisasmOperand(CPU *cpu, AddressingMode mode) {
   case Y_ZERO_PAGE:
   case X_ZP_INDIRECT:
   case ZP_INDIRECT_Y:
-    return cpu->bus->read(cpu->pc - 1);
+    return cpu->bus->read(cpu->pc);
   case ABSOLUTE:
   case X_ABSOLUTE:
   case Y_ABSOLUTE:
-    return cpu->readWord(cpu->pc - 2);
+  case ABSOLUTE_INDIRECT:
+    return cpu->readWord(cpu->pc);
   case RELATIVE:
-    return cpu->pc + (int8_t)(cpu->readWord(cpu->pc - 2));
+    return cpu->pc + 2 + (int8_t)(cpu->readWord(cpu->pc));
   default:
     return 0;
   }
@@ -126,14 +127,16 @@ void CPU::executeStep() {
   uint8_t opcode = fetchByte();
   if (!cpuOpcodes.contains(opcode)) {
     print_err("[v6502] unknown CPU opcode encountered: {:02x}\n", opcode);
-    std::terminate();
+    throw 1;
   }
   auto instruction = cpuOpcodes[opcode];
-  instruction.function(this, instruction.mode);
 #ifdef V6502_DEBUG
   char disasmInstruction[256] = {0};
   snprintf(disasmInstruction, 256, instruction.basicInfo.c_str(),
            getDisasmOperand(this, instruction.mode));
+#endif
+  instruction.function(this, instruction.mode);
+#ifdef V6502_DEBUG
   print_err(
       "{: <16}  A:{:02x}  X:{:02x}  Y:{:02x}  P:{:02x}  SP:{:02x}  PC:{:04x}\n",
       disasmInstruction, a, x, y, status, sp, pc);