From d64874fc4e50f22ecb5aa669abfe5d5a7931255a Mon Sep 17 00:00:00 2001
From: Daniel Jones <admin@danieljon.es>
Date: Wed, 19 Aug 2020 01:51:16 +0930
Subject: begin implementing instructions

screen clearing and loading bytes into registers implemented currently
---
 chip8.c | 72 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 chip8.h |  2 ++
 main.c  | 35 +++++++++++++++++++++++++++++---
 3 files changed, 105 insertions(+), 4 deletions(-)

diff --git a/chip8.c b/chip8.c
index 9933096..1d8de29 100644
--- a/chip8.c
+++ b/chip8.c
@@ -77,15 +77,17 @@ chip8_init()
 	//TODO: add whole alphabet, then can write a rom to write strings or something
 	for (uint8_t i = 0; i < FONT_BYTE_SIZE; i++)
 	{
-		memory[ + i] = chip8_fontset[i];
+		memory[0x0 + i] = chip8_fontset[i];
 	}
 
+	/*
 	uint8_t sinv[] = {0xBA, 0x7C, 0xD6, 0xFE, 0x54, 0xAA};
 	// temp
 	for (uint8_t i = 0; i <= FONT_WIDTH; i++)
 	{
 		memory[PROGRAM_START+i] = sinv[i];
 	}
+	*/
 }
 
 void
@@ -121,8 +123,76 @@ chip8_draw_sprite(int startx, int starty, uint16_t mem, uint8_t size)
 	}
 }
 
+void
+chip8_beep()
+{
+	// TODO: holy shit this is terrible, do something better
+	puts("\a");
+}
+
+void
+unknown_opcode(uint16_t bad_opcode)
+{
+	fprintf(stderr, "unknown opcode: 0x%02X at 0x%02X\n", bad_opcode, PC);
+}
 void
 chip8_cycle()
 {
+	/*
+	 * fetch, decode, execute
+	*/
 
+	/*
+	 * the opcode is a 2 byte value, while our memory is 1 byte
+	 * so to get our opcode we need to combine 2 bytes located at PC and PC+1
+	 * we right shift the first byte by 8 (1 byte) to place it in the high byte
+	 * and we store the second byte in the lower byte
+	 */
+	opcode = (memory[PC] << 8) | memory[PC+1];
+
+	uint16_t nnn = opcode & 0x0FFF; 	// lowest 12 bits
+	uint8_t n = opcode & 0x000F; 		// lowest 4 bits
+	uint8_t x = (opcode >> 8) & 0x000F;	// lower 4 bits of the high byte, we discard the low byte by right shifting it out
+	uint8_t y = (opcode >> 4) & 0x000F;	// upper 4 bits of the low byte, so we need to discard the lower 4 bits
+	uint8_t kk = opcode & 0x00FF;		// lowest 8 bits
+
+	switch (opcode & 0xF000)
+	{
+	/*
+	 * decode highest 4 bits
+	 * the highest 4 bits contains the instruction
+	 */
+	case 0x0000:
+	{
+		switch (kk)
+		{
+		case 0x00E0: /* cls (clear screen) */
+			puts("clear");
+			memset(video, 0, (WIDTH*HEIGHT) * sizeof(uint32_t));
+			draw_flag = 1;
+			break;
+		case 0x00EE: /* ret (return from subroutine) */
+			break;
+		default: unknown_opcode(opcode);
+		}
+		break;
+	}
+	case 0x6000: /* LD Vx, byte (load byte in register x) */
+	{
+		V[x] = kk;
+		break;
+	}
+
+	default: unknown_opcode(opcode);
+	}
+
+	/* timers */
+	if (delay_timer > 0)
+		delay_timer--;
+	if (sound_timer > 0)
+	{
+		chip8_beep();
+		sound_timer--;
+	}
+	PC += 2;
 }
diff --git a/chip8.h b/chip8.h
index 6779567..6d59ea8 100644
--- a/chip8.h
+++ b/chip8.h
@@ -25,5 +25,7 @@ int load_rom();
 void chip8_init();
 void chip8_draw_sprite(int startx, int starty, uint16_t mem, uint8_t size);
 void chip8_cycle();
+void chip8_beep();
+void unknown_opcode(uint16_t bad_opcode);
 
 #endif
diff --git a/main.c b/main.c
index 9a81c31..f58edf7 100644
--- a/main.c
+++ b/main.c
@@ -27,10 +27,17 @@ int handle_sdl_events();
 void handle_key_down(int keycode);
 void handle_key_up(int keycode);
 uint8_t get_chip8_key(int keycode);
+void print_registers();
 
 extern uint32_t video[WIDTH*HEIGHT];
 extern int draw_flag;
 extern uint8_t key[KEY_SIZE];
+extern uint8_t V[REGISTER_COUNT];
+extern uint16_t I;
+extern uint16_t PC;
+extern int8_t SP;
+extern uint8_t delay_timer;
+extern uint8_t sound_timer;
 
 void
 usage(char *program)
@@ -76,9 +83,29 @@ toggle_pixel(int x, int y)
 		video[WIDTH*y+x] = 0;
 }
 
+void
+print_registers()
+{
+	puts("REGISTER DUMP");
+	puts("--------------------------");
+	for (int reg = 0; reg < REGISTER_COUNT; reg++)
+	{
+		printf("V[0x%01X] = 0x%02X\n", reg, V[reg]);
+	}
+	printf("PC = 0x%03X\nI = 0x%03X\nSP = 0x%02X\ndelay_timer = 0x%02X\nsound_timer = 0x%02X\n", PC, I, SP, delay_timer, sound_timer);
+	puts("--------------------------");
+}
+
 void
 handle_key_down(int keycode)
 {
+	switch (keycode)
+	{
+	/* detect interpreter debug keys */
+	case SDLK_p: print_registers(); break;
+	default: break;
+	}
+
 	uint8_t k = get_chip8_key(keycode);
 	if (k > 0xF) return; /* unknown key */
 	key[k] = 1;
@@ -154,17 +181,17 @@ int main(int argc, char *argv[])
 		exit(EXIT_FAILURE);
 	}
 
+	chip8_init();
+
 	if (!load_rom(argv[1]))
 	{
 		fprintf(stderr, "cannot start interpreter\n");
 		exit(EXIT_FAILURE);
 	}
 
-	chip8_init();
-
 	init_video();
 
-	const int fps = 60;
+	const int fps = 2;
 	const int frame_delay = 1000/fps;
 	uint32_t  frame_start;
 	uint32_t frame_time;
@@ -189,6 +216,7 @@ int main(int argc, char *argv[])
 	chip8_draw_sprite(30, 9, 0xE*5, 0x5);
 	chip8_draw_sprite(35, 9, 0xf*5, 0x5);
 
+	/*
 	chip8_draw_sprite(0, 20, 0x200, 0x6);
 	chip8_draw_sprite(8, 20, 0x200, 0x6);
 	chip8_draw_sprite(16, 20, 0x200, 0x6);
@@ -196,6 +224,7 @@ int main(int argc, char *argv[])
 
 	chip8_draw_sprite(61, 25, 0x200, 0x6);
 	chip8_draw_sprite(50, 30, 0x200, 0x6);
+	*/
 
 	while(!do_quit)
 	{
-- 
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