From 26653bc8ac93cb748b46dae933b60fa755e400b1 Mon Sep 17 00:00:00 2001
From: Daniel Jones <admin@danieljon.es>
Date: Wed, 19 Aug 2020 16:29:58 +0930
Subject: instructions and debugging features

added some more instructions, particularly calling and returning from subroutines using the stack.
also added stepping through the program using the '.' key to advance one cycle
---
 main.c | 46 +++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 39 insertions(+), 7 deletions(-)

(limited to 'main.c')

diff --git a/main.c b/main.c
index f58edf7..3b8183e 100644
--- a/main.c
+++ b/main.c
@@ -10,9 +10,11 @@
  * http://mattmik.com/files/chip8/mastering/chip8.html (lots of info)
  * http://devernay.free.fr/hacks/chip8/C8TECH10.HTM#0.1
  * http://www.emulator101.com/chip-8-sprites.html
+ * https://slack-files.com/T3CH37TNX-F3RKEUKL4-b05ab4930d?nojsmode=1
  */
 
 #define VIDEO_SCALE 5
+#define STEPPING 1 // set to 1 to step manually through program
 
 SDL_Window *window;
 SDL_Renderer *renderer;
@@ -29,6 +31,8 @@ void handle_key_up(int keycode);
 uint8_t get_chip8_key(int keycode);
 void print_registers();
 
+int step_cycle = 0;
+
 extern uint32_t video[WIDTH*HEIGHT];
 extern int draw_flag;
 extern uint8_t key[KEY_SIZE];
@@ -38,6 +42,7 @@ extern uint16_t PC;
 extern int8_t SP;
 extern uint8_t delay_timer;
 extern uint8_t sound_timer;
+extern uint16_t stack[STACK_SIZE];
 
 void
 usage(char *program)
@@ -86,13 +91,21 @@ toggle_pixel(int x, int y)
 void
 print_registers()
 {
-	puts("REGISTER DUMP");
+	puts("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);
+	// minus 2 on PC because we increment PC by 2 at the start of decoding
+	printf("PC = 0x%03X\nI = 0x%03X\nSP = 0x%02X\ndelay_timer = 0x%02X\nsound_timer = 0x%02X\n", PC-2, I, SP, delay_timer, sound_timer);
+	puts("STACK:");
+	for (int i = 0; i < 16; i++)
+	{
+		printf("[%d] = 0x%03X", i, stack[i]);
+		if (i == 7) puts("");
+	}
+	puts("");
 	puts("--------------------------");
 }
 
@@ -103,6 +116,7 @@ handle_key_down(int keycode)
 	{
 	/* detect interpreter debug keys */
 	case SDLK_p: print_registers(); break;
+	case SDLK_PERIOD: step_cycle = 1; break;
 	default: break;
 	}
 
@@ -191,7 +205,7 @@ int main(int argc, char *argv[])
 
 	init_video();
 
-	const int fps = 2;
+	const int fps = 1;
 	const int frame_delay = 1000/fps;
 	uint32_t  frame_start;
 	uint32_t frame_time;
@@ -232,12 +246,30 @@ int main(int argc, char *argv[])
 
 		// logic
 		do_quit = handle_sdl_events();
-		chip8_cycle();
+		if (STEPPING)
+		{
+			if (step_cycle == 1)
+			{
+				chip8_cycle();
+
+				if (draw_flag)
+				{
+					update_video();
+					draw_flag = 0;
+				}
 
-		if (draw_flag)
+				step_cycle = 0;
+			}
+		}
+		else
 		{
-			update_video();
-			draw_flag = 0;
+			chip8_cycle();
+
+			if (draw_flag)
+			{
+				update_video();
+				draw_flag = 0;
+			}
 		}
 
 		frame_time = SDL_GetTicks() - frame_start;
-- 
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