CHIP-8 delay timer in C++

Question

I'm writing a CHIP-8 emulator in C++, which has two timers. If these timers are non-zero, they are decremented by 1 at 60Hz. I'm still getting the hang of C++, so I wanted to see if I'm doing this the Right Way.

Here's the header file:

#include <chrono>

struct TimedRegister
{
  unsigned char value;
  std::chrono::time_point<std::chrono::high_resolution_clock> last_write;

  TimedRegister();
  void set(const unsigned char new_value);
  void decrement();
};

And this is the the implementation:

#include "chip8.hpp"

#include <chrono>

TimedRegister::TimedRegister()
  : value(0)
{}

void
TimedRegister::set(const unsigned char new_value)
{
  value = new_value;
  last_write = std::chrono::high_resolution_clock::now();
}

void
TimedRegister::decrement()
{
  if (value == 0)
    return;

  const auto now = std::chrono::high_resolution_clock::now();
  if (std::chrono::duration_cast<std::chrono::milliseconds>(now - last_write)
        .count() >= 1000 / 60) {
    --value;
    last_write = now;
  }
}

The decrement function is called from the main emulator loop like this:

while (!chip8.halted) {
  chip8.delay_timer.decrement();
}

They seem to work fine, but I'm not sure if I'm getting away with using a heavy-handed, brute-force approach.

This is somewhat unrelated, but ideally you should be updating the timers while refreshing the screen, like the original interpreter does. — 12Me21
– 12Me21, Commented Mar 14, 2018 at 19:39

ratchet freak · Accepted Answer · 2018-03-14 17:25:53Z

7

When doing emulation you should disconnect from the real world and only provide a abstract view to a world.

In other words don't use a real time clock instead simulate the timer by linking it to the emulated clockspeed. For example if you pick 8 MHz you would have 133 333 instructions executed per decrement of each timer.

This will allow you to pause and fast forward when debugging programs.

answered Mar 14, 2018 at 17:25

ratchet freak

13k22 silver badges46 bronze badges

\$\begingroup\$ For Chip8, the timer should be cued off display refresh. If memory serves, vintage Chip8 implementations run roughly 1800 cycles (900 two-cycle instructions) between frames, but some operations like draw sprite wait for the next refresh. \$\endgroup\$

supercat
– supercat

2018-03-14 22:01:31 +00:00
Commented Mar 14, 2018 at 22:01

Add a comment |

Toby Speight · Accepted Answer · 2018-03-14 18:24:16Z

Since you keep a note of last_write, you don't need to keep updating the actual value.

Instead, implement a read() overload that computes the value as seen by the emulated system:

unsigned char TimedRegister::read()
{
    if (!value)
        return 0;

    auto now = std::chrono::high_resolution_clock::now();
    auto elapsed = std::chrono::duration_cast<Chip8::Tick>(now - last_write);
    if (elapsed.count() > value)
        // assign to value, so we don't look at clock next time
        return value = 0;

    return value - elapsed.count();
}

I'd consider moving away from std::chrono::high_resolution_clock for this - you really want your own emulated-system clock, which you can advance under program control. That will make both unit-testing and debugging easier.

yuri · Accepted Answer · 2018-03-14 18:26:13Z

Some minor observations:

Make the code easier to read
Long lines including std::chrono::high_resolution_clock and std::chrono::milliseconds might be a good choice for applying the using directive:
using high_res_clock = std::chrono::high_resolution_clock;
You can then use high_res_clock instead of the unwieldy old term.
Use compiler warnings
Always enable as many warning flags as possible. That way the compiler can help you eliminate possible errors. In this case you should initialize last_write just like you do with value.
class VS. struct
I'm unsure how this ties into the rest of your code but I was taught to use struct for PODs and class if you have methods. Just remember that in classes everything is private by default unlike in structs where it's the opposite.

Stack Exchange Network

CHIP-8 delay timer in C++

3 Answers 3

You must log in to answer this question.

Hot Network Questions

CHIP-8 delay timer in C++

3 Answers 3

You must log in to answer this question.

Related

Hot Network Questions