0x8XY4, 0x8XY5, 0xDXYN: Avoid bugs when Vx = Vf or Vy = Vf

leonmavr · Aug 17, 2024 · 0bfc7a9 · 0bfc7a9
1 parent 2e25b55
commit 0bfc7a9
Showing 1 changed file with 8 additions and 5 deletions.
diff --git a/src/chip8.cpp b/src/chip8.cpp
@@ -265,7 +265,7 @@ void Chip8::Exec(const opcode_t& opc) {
 #define EXEC_INSTRUCTION \
  /* assembly , condition , instruction(s) */ \
  X("ERR" , (prefix == 0x0 && nnn == 0x0ee && SP == 0) || \
- ((prefix == 0x1 || prefix == 0x2) && nnn < 2) || \
+ ((prefix == 0x1 || prefix == 0x2) && nnn == 0) || \
  (prefix == 0x2 && SP >= stack_.size() - 1) \
  , ERROR_POINTER(); ) \
  X("CLS" , prefix == 0x0 && nnn == 0x0e0 , Cls();) \
@@ -282,8 +282,8 @@ void Chip8::Exec(const opcode_t& opc) {
  X("OR Vx Vy" , prefix == 0x8 && n == 0x1 , Vx |= Vy;) \
  X("AND Vx Vy" , prefix == 0x8 && n == 0x2 , Vx &= Vy;) \
  X("XOR Vx Vy" , prefix == 0x8 && n == 0x3 , Vx ^= Vy;) \
- X("ADD Vx Vy" , prefix == 0x8 && n == 0x4 , uint16_t sum = Vx + Vy; Vf = sum > 0xFF; Vx = sum & 0xFF;) \
- X("SUB Vx Vy" , prefix == 0x8 && n == 0x5 , Vf = Vx >= Vy; Vx = (Vx - Vy) & 0xFF) \
+ X("ADD Vx Vy" , prefix == 0x8 && n == 0x4 , uint16_t sum = Vx + Vy; Vx = sum & 0xFF; Vf = sum > 0xFF;) \
+ X("SUB Vx Vy" , prefix == 0x8 && n == 0x5 , Vf = Vx >= Vy; Vx = (Vx - Vy) & 0xFF;) \
  X("SHR Vx Vy" , prefix == 0x8 && n == 0x6 , Vf = Vx & 0x1; Vx >>= 1;) \
  X("SUBN Vx Vy" , prefix == 0x8 && n == 0x7 , Vf = Vy >= Vx; Vx = (Vy - Vx) & 0xFF;) \
  X("SHL Vx Vy" , prefix == 0x8 && n == 0xe , Vf = (Vx >> 7) & 0x1; Vx = (Vx << 1) & 0xFF;) \
@@ -294,13 +294,16 @@ void Chip8::Exec(const opcode_t& opc) {
  X("RND Vx nn" , prefix == 0xc , Vx = rng(seed) & nn;) \
  X("DRW Vx Vy n", prefix == 0xd, \
  do { \
+ /* Vx or Vy could be Vf so store them */ \
+ const auto x0 = Vx; \
+ const auto y0 = Vy; \
  Vf = 0; \
  for (uint8_t row = 0; row < n; row++) { \
  uint8_t sprite = ram_[I + row]; \
  for (uint8_t col = 0; col < 8; col++) { \
  if ((sprite & 0x80) != 0) { \
- size_t x = (Vx + col) % COLS; \
- size_t y = (Vy + row) % ROWS; \
+ size_t x = (x0 + col) % COLS; \
+ size_t y = (y0 + row) % ROWS; \
  size_t index = y * COLS + x; \
  if (frame_buffer_[index] == 1) \
  Vf = 1; \