Some improvements

Fix minor bugs (pong, pong2 now works)
Add fontset support
Still outstanding bugs: pong doesn't keep track of score, aliens reappear.

main.c

@@ -5,7 +5,7 @@
 #include <time.h>
 #include <SDL2/SDL.h>
 
-#define DEBUG 1
+#define DEBUG 0
 #define TDISP 0
 
 typedef uint8_t		byte;
@@ -23,6 +23,26 @@ typedef struct {
 	byte stack_pointer;
 } emulator_state_s;
 
+byte chip8_fontset[80] =
+{ 
+  0xF0, 0x90, 0x90, 0x90, 0xF0, // 0
+  0x20, 0x60, 0x20, 0x20, 0x70, // 1
+  0xF0, 0x10, 0xF0, 0x80, 0xF0, // 2
+  0xF0, 0x10, 0xF0, 0x10, 0xF0, // 3
+  0x90, 0x90, 0xF0, 0x10, 0x10, // 4
+  0xF0, 0x80, 0xF0, 0x10, 0xF0, // 5
+  0xF0, 0x80, 0xF0, 0x90, 0xF0, // 6
+  0xF0, 0x10, 0x20, 0x40, 0x40, // 7
+  0xF0, 0x90, 0xF0, 0x90, 0xF0, // 8
+  0xF0, 0x90, 0xF0, 0x10, 0xF0, // 9
+  0xF0, 0x90, 0xF0, 0x90, 0x90, // A
+  0xE0, 0x90, 0xE0, 0x90, 0xE0, // B
+  0xF0, 0x80, 0x80, 0x80, 0xF0, // C
+  0xE0, 0x90, 0x90, 0x90, 0xE0, // D
+  0xF0, 0x80, 0xF0, 0x80, 0xF0, // E
+  0xF0, 0x80, 0xF0, 0x80, 0x80  // F
+};
+
 byte keymap[0x10] = { 	SDL_SCANCODE_X, SDL_SCANCODE_1, SDL_SCANCODE_2, SDL_SCANCODE_3, SDL_SCANCODE_Q, 
 						SDL_SCANCODE_W, SDL_SCANCODE_E, SDL_SCANCODE_A, SDL_SCANCODE_S, SDL_SCANCODE_D, 
 						SDL_SCANCODE_Z, SDL_SCANCODE_C, SDL_SCANCODE_4, SDL_SCANCODE_R, SDL_SCANCODE_F, SDL_SCANCODE_V };
@@ -41,6 +61,7 @@ int main(int argc, char **argv) {
 	
 	emulator_state_s state = { 0 }; // initialize everything to 0
 	state.program_counter = 0x200; // chip8 programs start at 0x200 address space
+	memcpy(state.memory, chip8_fontset, 80);
 	
 	FILE *f = fopen(argv[1], "rb");
 	fread(&state.memory[0x200], 0x1000 - 0x200, 1, f); // see above comment
@@ -75,7 +96,7 @@ int main(int argc, char **argv) {
 		// SDL_PumpEvents();
 		const byte *keystate = SDL_GetKeyboardState(NULL);
 
-		for (int i = 0; i < 0x10 && !press; ++i) {
+		for (int i = 0; i < 0x10; ++i) {
 			if (keystate[keymap[i]]){
 				state.input[i] = 1;
 				press = 1;
@@ -103,12 +124,10 @@ int main(int argc, char **argv) {
 			case 0x0000: // screen clear or function return 
 				if (opcode == 0x00E0){
 					memset(state.screen, 0, 64 * 32);
-					SDL_SetRenderDrawColor(renderer, 0, 0, 0, 0xFF);
-					SDL_RenderClear(renderer);
 					if (DEBUG) printf("Clearing screen ");
 				}
 				
-				else if (opcode = 0x00EE) {
+				else if (opcode == 0x00EE) {
 					state.program_counter = state.stack[--state.stack_pointer];
 					if (DEBUG) printf("Returning to %x ", state.stack[state.stack_pointer]);
 				}
@@ -161,14 +180,14 @@ int main(int argc, char **argv) {
 
 			case 0x6000: // set reg to constant
 				reg = (opcode & 0x0F00) >> 8;
-				state.registers[reg] = opcode & 0x00FF;
+				state.registers[reg] = (byte) opcode;
 				if (DEBUG) printf("Assigning %hx to %i", state.registers[reg], reg);
 
 				break;
 
 			case 0x7000: // add constant to reg
 				reg = (opcode & 0x0F00) >> 8;
-				state.registers[reg] += opcode & 0x00FF;
+				state.registers[reg] += (byte) opcode;
 				if (DEBUG) printf("Add %hi to reg %i (%hi)", opcode & 0x00FF, reg, state.registers[reg]);
 				
 				break;
@@ -203,23 +222,23 @@ int main(int argc, char **argv) {
 						unsigned int add = state.registers[first] + state.registers[second];
 						if (add > 255) state.registers[0xF] = 1;
 						else state.registers[0xF] = 0;
-						state.registers[first] = add & 0x00FF;
+						state.registers[first] = (byte) add;
 						if (DEBUG) printf("ADD register %i to %i (%hx) ", second, first, state.registers[first]);
 						if (DEBUG) printf("VF is %hx", state.registers[0xF]);
 						break;
 
 					case 5: // X = X - Y, VF = X - Y < 0
 						state.registers[first] -= state.registers[second];
-						if (state.registers[first] > state.registers[second]) state.registers[0xF] = 1;
-						else state.registers[0xF] = 0;
+						if (state.registers[first] > state.registers[second]) state.registers[0xF] = 0;
+						else state.registers[0xF] = 1;
 						if (DEBUG) printf("SUB register %i from %i (%hx) ", second, first, state.registers[first]);
 						if (DEBUG) printf("VF is %hx", state.registers[0xF]);
 						break;
 
 					case 6: // set least significant bit of X in VF, bitshift right
-						state.registers[0xF] = state.registers[first] & 0x000F;
+						state.registers[0xF] = state.registers[first] & 0b00000001;
 						state.registers[first] /= 2;
-						if (DEBUG) printf("RS register %i (%hx) ", first, second, state.registers[first]);
+						if (DEBUG) printf("RS register %i (%hx) ", first, state.registers[first]);
 						if (DEBUG) printf("VF is %hx", state.registers[0xF]);
 						break;
 
@@ -232,9 +251,9 @@ int main(int argc, char **argv) {
 						break;
 
 					case 0xE:; // set msot significant bit of X in VF, bitshift left
-						state.registers[0xF] = (state.registers[first] & 0xF000) >> 12;
+						state.registers[0xF] = (state.registers[first] & 0b10000000) >> 7;
 						state.registers[first] *= 2;
-						if (DEBUG) printf("LS register %i (%hx) ", first, second, state.registers[first]);
+						if (DEBUG) printf("LS register %i (%hx) ", first, state.registers[first]);
 						if (DEBUG) printf("VF is %hx", state.registers[0xF]);
 						break;
 
@@ -285,10 +304,13 @@ int main(int argc, char **argv) {
 					for (int xpos = 0; xpos < 8; ++xpos)
 					{
 						int ypos = y + line;
-						if (ypos > 63) ypos -= 64;
-						if (!(data & (1 << xpos))) continue;
-						if (state.screen[x + (7 - xpos)][ypos]) state.registers[0xF] = 1;
-						state.screen[x + (7 - xpos)][ypos] ^= 1;
+						byte mask = 1 << 7 - xpos;
+						if (ypos > 31) ypos -= 32;
+						if (!(data & mask)) continue;
+						if (state.screen[x + xpos][ypos]){
+							state.registers[0xF] = 1;
+						}
+						state.screen[x + xpos][ypos] ^= 1;
 					}
 				}
 
@@ -348,7 +370,8 @@ int main(int argc, char **argv) {
 						break;
 
 					case 0x29:
-						break; // fontset not implemented yet
+						state.address_I = state.registers[reg] * 5;
+						break;
 
 					case 0x33:;
 						int value = state.registers[reg];
@@ -358,12 +381,12 @@ int main(int argc, char **argv) {
 						break;
 
 					case 0x55:
-						for (int i = 0; i < (reg + 1); ++i)
+						for (int i = 0; i <= reg; ++i)
 							state.memory[state.address_I + i] = state.registers[i];
 						break;
 
 					case 0x65:
-						for (int i = 0; i < (reg + 1); ++i)
+						for (int i = 0; i <= reg + 1; ++i)
 							state.registers[i] = state.memory[state.address_I + i];
 						break;