Performance tune

Performance tune and PA2 bugfix. Supporting ICC.

nemu/src/device/vga.cc

@@ -37,11 +37,13 @@ static void init_vga_impl() {
 }
 
 void update_screen() {
+#ifndef DISABLE_MMIO
   if(window == nullptr) init_vga_impl();
   SDL_ErrorCheck(SDL_UpdateTexture(texture, NULL, vmem, SCREEN_W * sizeof(vmem[0][0])));
   SDL_ErrorCheck(SDL_RenderClear(renderer));
   SDL_ErrorCheck(SDL_RenderCopy(renderer, texture, NULL, NULL));
   SDL_RenderPresent(renderer);
+#endif
 }
 
 void init_vga() {