Limit instructions per second (#50)

* Add tick counter for execution speed control

* Update README table of contents

README.md

@@ -18,7 +18,7 @@ An Octo compatible XO Chip, Super Chip, and Chip 8 emulator.
 4. [Running](#running)
    1. [Running a ROM](#running-a-rom)
    2. [Screen Scale](#screen-scale)
-   3. [Execution Delay](#execution-delay)
+   3. [Instructions Per Second](#instructions-per-second)
    4. [Quirks Modes](#quirks-modes)
       1. [Shift Quirks](#shift-quirks) 
       2. [Index Quirks](#index-quirks)
@@ -207,18 +207,14 @@ scale is 64 x 32):
 The command above will scale the window so that it is 10 times the normal
 size.
 
-### Execution Delay
+### Instructions Per Second
 
-You may also wish to experiment with the `--delay` switch, which instructs
-the emulator to add a delay to every operation that is executed. For example,
+The `--ticks` switch will limit the number of instructions per second that the 
+emulator is allowed to run. By default, the value is set to 1,000. Minimum values 
+are 200. Use this switch to adjust the running time of ROMs that execute too quickly. 
+For simplicity, each instruction is assumed to take the same amount of time.
 
-    python yac8e.py /path/to/rom/filename --delay 10
-
-The command above will add a 10 ms delay to every opcode that is executed.
-This is useful for very fast computers (note that it is difficult to find
-information regarding opcode execution times, as such, I have not attempted
-any fancy timing mechanisms to ensure that instructions are executed in a
-set amount of time).
+    python yac8e.py /path/to/rom/filename --ticks 2000   
 
 ### Quirks Modes
 

chip8/cpu.py

@@ -84,7 +84,8 @@ def __init__(
             jump_quirks=False,
             clip_quirks=False,
             logic_quirks=False,
-            mem_size="64K"
+            mem_size="64K",
+            max_ticks=1000,
     ):
         """
         Initialize the Chip8 CPU. The only required parameter is a screen
@@ -98,6 +99,7 @@ def __init__(
         :param clip_quirks: enables screen clipping quirks
         :param logic_quirks: enables logic quirks
         :param mem_size: sets the maximum memory available "4K" or "64K"
+        :param max_ticks: sets the maximum allowable operations per second
         """
         self.last_pc = 0x0000
         self.last_op = "None"
@@ -109,6 +111,11 @@ def __init__(
         self.index = 0
         self.rpl = [0] * NUM_REGISTERS
 
+        self.tick_counter = 0
+        self.max_ticks = max_ticks
+        if self.max_ticks < 200:
+            self.max_ticks = 200
+
         self.pitch = 64
         self.playback_rate = 4000
         self.audio_pattern_buffer = [0] * 16
@@ -226,6 +233,9 @@ def execute_instruction(self, operand=None):
         :param operand: the operand to execute
         :return: returns the operand executed
         """
+        if self.tick_counter > self.max_ticks:
+            return None
+
         self.last_pc = self.pc
         if operand:
             self.operand = operand
@@ -236,6 +246,7 @@ def execute_instruction(self, operand=None):
             self.pc += 2
         operation = (self.operand & 0xF000) >> 12
         self.operation_lookup[operation]()
+        self.tick_counter += 1
         return self.operand
 
     def execute_logical_instruction(self):
@@ -1249,6 +1260,7 @@ def reset(self):
         self.sound_playing = False
         self.sound_waveform = None
         self.bitplane = 1
+        self.tick_counter = 0
 
     def load_rom(self, filename, offset=PROGRAM_COUNTER_START):
         """
@@ -1269,6 +1281,8 @@ def decrement_timers(self):
         """
         Decrement both the sound and delay timer.
         """
+        self.tick_counter = 0
+
         self.delay -= 1 if self.delay > 0 else 0
         self.sound -= 1 if self.delay > 0 else 0
         if self.sound > 0 and not self.sound_playing:

chip8/emulator.py

@@ -22,6 +22,7 @@ def main_loop(args):
     :param args: the parsed command-line arguments
     """
     delay_timer_event = 24
+    max_ticks = int(args.ticks / 60)
 
     screen = Chip8Screen(
         scale_factor=args.scale,
@@ -39,6 +40,7 @@ def main_loop(args):
         clip_quirks=args.clip_quirks,
         logic_quirks=args.logic_quirks,
         mem_size=args.mem_size,
+        max_ticks=max_ticks
     )
     cpu.load_rom(FONT_FILE, 0)
     cpu.load_rom(args.rom)
@@ -47,8 +49,6 @@ def main_loop(args):
     pygame.time.set_timer(delay_timer_event, 17)
 
     while cpu.running:
-        pygame.time.wait(args.op_delay)
-
         if not cpu.awaiting_keypress:
             cpu.execute_instruction()
 

yac8e.py

@@ -33,9 +33,8 @@ def parse_arguments():
         "--scale", help="the scale factor to apply to the display "
         "(default is 5)", type=int, default=5, dest="scale")
     parser.add_argument(
-        "--delay", help="sets the CPU operation to take at least "
-        "the specified number of milliseconds to execute (default is 1)",
-        type=int, default=1, dest="op_delay")
+        "--ticks", help="how many instructions per second are allowed",
+        type=int, default=1000, dest="ticks")
     parser.add_argument(
         "--shift_quirks", help="Enable shift quirks",
         action="store_true", dest="shift_quirks"