decoupled timers from interpreter tick rate

CMakeLists.txt

@@ -5,5 +5,5 @@ SET(CMAKE_BUILD_TYPE Debug)
 add_subdirectory(submodules/sdl2)
 
 SET(CMAKE_EXPORT_COMPILE_COMMANDS ON)
-add_executable(chocochip8 main.cpp Interpreter.cpp DisplaySDL.cpp BuzzerSDL.cpp)
+add_executable(chocochip8 main.cpp Interpreter.cpp DisplaySDL.cpp BuzzerSDL.cpp CountdownTimerSDL.cpp)
 target_link_libraries(chocochip8 SDL2::SDL2-static)

CountdownTimerSDL.cpp

@@ -0,0 +1,18 @@
+#include "CountdownTimerSDL.hpp"
+
+CountdownTimerSDL::CountdownTimerSDL(unsigned frequency):
+  mDesiredFrequency{frequency},
+  mSDLFrequency{SDL_GetPerformanceFrequency()},
+  mStartTime{0},
+  mStartValue{0} {}
+
+void CountdownTimerSDL::set(unsigned value) {
+  mStartTime = SDL_GetPerformanceCounter();
+  mStartValue = value;
+}
+
+unsigned CountdownTimerSDL::get() const {
+  Uint64 elapsedTime = SDL_GetPerformanceCounter() - mStartTime;
+  Uint64 elapsedTicks = (elapsedTime * mDesiredFrequency) / mSDLFrequency;
+  return elapsedTicks >= mStartValue ? 0 : mStartValue - elapsedTicks;
+}

CountdownTimerSDL.hpp

@@ -0,0 +1,17 @@
+#pragma once
+
+#include <SDL2/SDL_timer.h>
+
+#include "Peripherals.hpp"
+
+class CountdownTimerSDL : public chocochip8::CountdownTimer {
+public:
+  CountdownTimerSDL(unsigned frequency); 
+  void set(unsigned value) override;
+  unsigned get() const override;
+private:
+  const Uint64 mDesiredFrequency;
+  const Uint64 mSDLFrequency;
+  Uint64 mStartTime;
+  Uint64 mStartValue;
+};

DisplaySDL.cpp

@@ -1,8 +1,13 @@
 #include "DisplaySDL.hpp"
+#include "Peripherals.hpp"
 
+#include <SDL2/SDL_surface.h>
 #include <stdexcept>
 
-DisplaySDL::DisplaySDL(int w, int h, uint32_t fgColor, uint32_t bgColor) {
+DisplaySDL::DisplaySDL(int w, int h, uint32_t fgColor, uint32_t bgColor):
+  mpFramebuffer{std::make_unique<Framebuffer>()},
+  mpDisplayState{std::make_unique<Framebuffer>()},
+  mDoClear{true} {
   // Create SDL Window
   mpWindow = SDL_CreateWindow(
     "ChocoChip-8",
@@ -35,43 +40,69 @@ DisplaySDL::~DisplaySDL() {
   SDL_DestroyWindow(mpWindow);
 }
 
-void DisplaySDL::updateWindow() const {
+void DisplaySDL::clear() {
+  for(auto &scanline : *mpFramebuffer) {
+    scanline.reset();
+  }
+  mDoClear = true;
+}
+
+int DisplaySDL::blit(const chocochip8::Scanline &spriteScanline, int y) {
+  using chocochip8::Scanline;
+  Scanline &targetScanline = mpFramebuffer->at(y);
+  bool collision = (spriteScanline & targetScanline).any();
+  targetScanline ^= spriteScanline;
+  return collision;
+}
+
+void DisplaySDL::updateWindow(bool doWindowUpdate) const {
+  using chocochip8::Scanline;
   SDL_Surface *pSurface = SDL_GetWindowSurface(mpWindow);
-  SDL_LockSurface(pSurface);
+  if(mDoClear) {
+    for(auto &scanline : *mpDisplayState) {
+      scanline.reset();
+    }
+    SDL_FillRect(pSurface, NULL, mBgColor);
+    mDoClear = false;
+    doWindowUpdate = true;
+  }
 
-  int fx, fy; // ChocoChip8 Framebuffer coordinates
-  int sx, sy; // SDL Surface coordinates
-  int lx, ly; // Last-used ChocoChip8 Framebuffer coordinates
-  Uint32 lc;  // Last-used SDL color
+  for(int y = 0; y < mpFramebuffer->size(); y++) {
+    Scanline &rNewScanline = (*mpFramebuffer)[y];
+    Scanline &rOldScanline = (*mpDisplayState)[y];
+    if(rNewScanline == rOldScanline) {
+      // Skip scanlines that haven't changed since the last update
+      continue;
+    }
 
-  // Fill the entire SDL surface, one pixel at a time
-  lx = -1;
-  ly = -1;
-  for(sy = 0; sy < pSurface->h; sy++) {
-    for(sx = 0; sx < pSurface->w; sx++) {
-      // Map SDL surface coordinates to Chocochip8 Framebuffer coordinates
-      fx = sx * (double(chocochip8::gcWidth) / pSurface->w);
-      fy = sy * (double(chocochip8::gcHeight) / pSurface->h);
-
-      // Reuse color if this screen pixel maps to the same ChocoChip8 pixel as the last
-      if(fx != lx || fy != ly) {
-        lx = fx;
-        ly = ly;
-        // Read Chocochip8 Framebuffer and choose appropriate color,
-        // note that the MSB of an scanline's bitset is the leftmost pixel.
-        lc = (mpFramebuffer->at(fy)[(chocochip8::gcWidth - 1) - fx] ? mFgColor : mBgColor);
+    for(int x = 0; x < rNewScanline.size(); x++) {
+      bool isSet = rNewScanline._Unchecked_test(x);
+      bool wasSet = rOldScanline._Unchecked_test(x);
+      if(isSet == wasSet) {
+        continue;
       }
 
-      // Convert (x, y) indexes into SDL Surface pixel array index
-      Uint32 *pPixel = static_cast<Uint32*>(static_cast<void*>(
-          static_cast<char*>(pSurface->pixels)
-          + sy * pSurface->pitch
-          + sx * pSurface->format->BytesPerPixel
-      ));
-      *pPixel = lc;
+      // Map framebuffer pixel to SDL surface rectangle,
+      // note that the MSB of an scanline's bitset is the leftmost pixel.
+      int z = (chocochip8::gcWidth - 1) - x;
+      int x1 = (      z * pSurface->w) / chocochip8::gcWidth;
+      int x2 = ((z + 1) * pSurface->w) / chocochip8::gcWidth;
+      int y1 = (      y * pSurface->h) / chocochip8::gcHeight;
+      int y2 = ((y + 1) * pSurface->h) / chocochip8::gcHeight;
+      SDL_Rect rect;
+      rect.x = x1;
+      rect.y = y1;
+      rect.w = x2 - x1;
+      rect.h = y2 - y1;
+
+      Uint32 color = (isSet ? mFgColor : mBgColor);
+      SDL_FillRect(pSurface, &rect, color);
+      doWindowUpdate = true;
     }
   }
 
-  SDL_UnlockSurface(pSurface);
-  SDL_UpdateWindowSurface(mpWindow);
+  if(doWindowUpdate) {
+    *mpDisplayState = *mpFramebuffer;
+    SDL_UpdateWindowSurface(mpWindow);
+  }
 }

DisplaySDL.hpp

@@ -2,17 +2,25 @@
 
 #include "Peripherals.hpp"
 
+#include <array>
 #include <cstdint>
+#include <memory>
 #include <SDL2/SDL_video.h>
 
 class DisplaySDL : public chocochip8::Display {
 public:
   DisplaySDL(int w, int h, uint32_t fgCol = 0xffffff, uint32_t bgCol = 0x000000);
   ~DisplaySDL() override;
-  void updateWindow() const;
+  void clear() override;
+  int blit(const chocochip8::Scanline &scanline, int y) override;
+  void updateWindow(bool forceWindowUpdate = false) const;
 
 private:
+  using Framebuffer = std::array<chocochip8::Scanline, chocochip8::gcHeight>;
+  std::unique_ptr<Framebuffer> mpFramebuffer;
+  mutable std::unique_ptr<Framebuffer> mpDisplayState;
   SDL_Window *mpWindow;
   Uint32 mFgColor;
   Uint32 mBgColor;
+  mutable bool mDoClear;
 };

Interpreter.cpp

@@ -44,13 +44,14 @@ constexpr uint8_t gcvLowResFontData[80] = {
   0xF0, 0x80, 0xF0, 0x80, 0x80
 };
 
-Interpreter::Interpreter(unsigned ticksPerSecond, Display &display, Buzzer &buzzer, Keypad &keypad):
+Interpreter::Interpreter(Display &display, Buzzer &buzzer, Keypad &keypad, CountdownTimer &delayTimer, CountdownTimer &soundTimer):
   mvMemory(scMemorySize),
   mCallStack{},
   mrDisplay{display},
   mrBuzzer{buzzer},
   mrKeypad{keypad},
-  mcTicksPerSecond{ticksPerSecond},
+  mrDelayTimer(delayTimer),
+  mrSoundTimer(soundTimer),
   mvSpecialReg{},
   mvReg{},
   mIsHighResMode{false} {
@@ -87,9 +88,7 @@ void Interpreter::tick() {
   case 0x0000: // 0NNN - call machine language routine
     switch(inst) {
     case 0x00E0: // clear display
-      for(auto &scanline : *mrDisplay.mpFramebuffer) {
-        scanline.reset();
-      }
+      mrDisplay.clear();
       break;
     case 0x00EE: // return from subroutine
       mvSpecialReg[SR_PC] = mCallStack.top();
@@ -145,16 +144,16 @@ void Interpreter::tick() {
   case 0xF000: // several unique instructions
     switch(inst & 0xF0FF) {
     case 0xF007: // FX07 - read timer register
-      mvReg[iRegDst] = (mvSpecialReg[SR_T1] * scTimerFreq + (mcTicksPerSecond - 1)) / mcTicksPerSecond;
+      mvReg[iRegDst] = mrDelayTimer.get();
       break;
     case 0xF015: // FX15 - set timer register
-      mvSpecialReg[SR_T1] = (mcTicksPerSecond * mvReg[iRegDst]) / scTimerFreq;
+      mrDelayTimer.set(mvReg[iRegDst]);
       break;
     case 0xF018: // FX18 - set sound timer register
-      if(mvSpecialReg[SR_T2] == 0 && mvReg[iRegDst] != 0) {
+      if(mvReg[iRegDst] != 0) {
         mrBuzzer.on();
       }
-      mvSpecialReg[SR_T2] = (mcTicksPerSecond * mvReg[iRegDst]) / scTimerFreq;
+      mrSoundTimer.set(mvReg[iRegDst]);
       break;
     case 0xF01E: // FX1E - add to I
       mvSpecialReg[SR_I] += mvReg[iRegDst];
@@ -184,15 +183,8 @@ void Interpreter::tick() {
     break;
   }
 
-  // decrement timers
-  if(mvSpecialReg[SR_T1] > 0) {
-    mvSpecialReg[SR_T1] -= 1;
-  }
-  if(mvSpecialReg[SR_T2] > 0) {
-    mvSpecialReg[SR_T2] -= 1;
-    if(mvSpecialReg[SR_T2] == 0) {
-      mrBuzzer.off();
-    }
+  if(mrSoundTimer.get() == 0) {
+    mrBuzzer.off();
   }
 }
 
@@ -292,12 +284,10 @@ void Interpreter::executeDraw(uint8_t x, uint8_t y, uint8_t n) {
 
     // we draw one scanline per sprite row in high-res mode, but twice in low-res mode
     for(int j = 0; j < (mIsHighResMode ? 1 : 2); j++) {
-      Scanline &targetScanline = mrDisplay.mpFramebuffer->at(y + i * (mIsHighResMode ? 1 : 2) + j);
-      if((targetScanline & spriteScanline) != 0) {
+      // blit the sprite bitset into the screen
+      if(mrDisplay.blit(spriteScanline, y + i * (mIsHighResMode ? 1 : 2) + j) != 0) {
         collisionCount += 1;
       }
-      // blit the sprite bitset into the screen
-      targetScanline ^= spriteScanline;
     }
 
     // update state flags

Interpreter.hpp

@@ -16,7 +16,6 @@ private:
 public:
   constexpr static sreg_t scLowRestFontAddr = 0x0000;
   constexpr static sreg_t scResetVector = 0x0200;
-  constexpr static sreg_t scTimerFreq = 60;
   constexpr static size_t scMemorySize = 4096;
 
   enum {
@@ -26,7 +25,7 @@ public:
   };
 
   enum {
-    SR_PC, SR_I, SR_T1, SR_T2,
+    SR_PC, SR_I, /*SR_T1, SR_T2,*/
     SR_COUNT
   };
 
@@ -35,7 +34,7 @@ public:
   };
 
 public:
-  Interpreter(unsigned ticksPerSecond, Display &display, Buzzer &buzzer, Keypad &keypad);
+  Interpreter(Display &display, Buzzer &buzzer, Keypad &keypad, CountdownTimer &delayTimer, CountdownTimer &soundTimer);
   void tick();
   void loadProgram(char const* data, size_t count, size_t where = scResetVector);
 
@@ -49,7 +48,8 @@ private:
   Display &mrDisplay;
   Buzzer &mrBuzzer;
   Keypad &mrKeypad;
-  const unsigned mcTicksPerSecond;
+  CountdownTimer &mrDelayTimer;
+  CountdownTimer &mrSoundTimer;
   sreg_t mvSpecialReg[SR_COUNT];
   reg_t mvReg[R_COUNT];
   bool mIsHighResMode;

Peripherals.hpp

@@ -1,15 +1,12 @@
 #pragma once
 
-#include <array>
 #include <bitset>
-#include <memory>
 
 namespace chocochip8 {
 
   constexpr size_t gcWidth = 128;
   constexpr size_t gcHeight = 64;
   using Scanline = std::bitset<gcWidth>;
-  using Framebuffer = std::array<Scanline, gcHeight>;
 
   enum class Key {
     KEY_0, KEY_1, KEY_2, KEY_3,
@@ -20,11 +17,9 @@ namespace chocochip8 {
 
   class Display {
   public:
-    friend class Interpreter;
-    Display(): mpFramebuffer{std::make_unique<Framebuffer>()} {}
     virtual ~Display() = default;
-  protected:
-    std::unique_ptr<Framebuffer> mpFramebuffer;
+    virtual int blit(const Scanline& spriteScanline, int y) = 0;
+    virtual void clear() = 0;
   };
 
   class Buzzer {
@@ -40,4 +35,11 @@ namespace chocochip8 {
     virtual bool isKeyPressed(Key key) = 0;
   };
 
+  class CountdownTimer {
+  public:
+    virtual ~CountdownTimer() = default;
+    virtual void set(unsigned value) = 0;
+    virtual unsigned get() const = 0;
+  };
+
 }; // namespace chocochip8

main.cpp

@@ -6,11 +6,11 @@
 #include <SDL2/SDL.h>
 
 #include "BuzzerSDL.hpp"
+#include "CountdownTimerSDL.hpp"
 #include "DisplaySDL.hpp"
 #include "Interpreter.hpp"
 #include "Peripherals.hpp"
-#include "SDL_events.h"
-#include "SDL_timer.h"
+#include "SDL2/SDL_events.h"
 
 int main(int argc, char* argv[]) {
   if (SDL_Init(SDL_INIT_EVERYTHING) < 0) {
@@ -23,9 +23,13 @@ int main(int argc, char* argv[]) {
   };
 
   BuzzerSDL buzzer(440);
-  DisplaySDL display(1280, 640);
+  int N = 4;
+  DisplaySDL display(128*N, 64*N);
   TestKeypad keypad;
-  chocochip8::Interpreter chip8(90, display, buzzer, keypad);
+  CountdownTimerSDL delayTimer(60);
+  CountdownTimerSDL soundTimer(60);
+  CountdownTimerSDL displayTimer(20);
+  chocochip8::Interpreter chip8(display, buzzer, keypad, delayTimer, soundTimer);
 
   auto rom = std::vector<char>();
   auto romfile = std::ifstream(argv[1] != NULL ? argv[1] : "/dev/stdin", std::ios_base::in | std::ios_base::binary);
@@ -45,12 +49,13 @@ int main(int argc, char* argv[]) {
       }
     }
     chip8.tick();
-    display.updateWindow();
-    SDL_Delay(1000.0 / 60);
+    if(displayTimer.get() == 0) {
+      display.updateWindow();
+      displayTimer.set(1);
+    }
   }
   
   buzzer.off();
   SDL_Quit();
   return 0;
 }
-