tykel / mash16 Goto Github PK

I was upgrading my own emulator to add support for the Chip16 v1.3 instructions, and the new division instructions seemed ambiguous to me, specially because behavior around negative operands tends to be problematic, and because there were both MOD and REM instructions without a clear difference between them. Not even the sign of the division operation is not even clearly defined for C89 and C++03, unless you use the div() function (See https://en.wikipedia.org/wiki/Modulo_operation), and information about what the differences between MOD and REM should be online is pretty sparse.

So I wrote a Test ROM and tested it in the two most famous emulators, mash16 and RefChip16. I found that the DIV and REM operations were implemented consistently between the two emulators (they were just the regular results of the / and % operator you get with GCC). However, I noticed that there was a different behavior on those two emulators on the MOD operation when the sign of the divisor was negative (the tests labeled test43 and test44 in the test ROM).

There is a pretty detailed write-up on the Test ROM header on the exact behavior implemented by both emulators. While the Chip16 reference is vague about how the instructions should behave, in my opinion I think that mash16's behavior is much harder to justify, both because it's different than other implementations of the modulo such as Haskell's one which are designed around certain useful mathematical properties, and also because behavior like -2 MOD -4 = -6 seems pretty unintuitive.

If you want to play around with it, the Test ROM is also in .c16 format here. It should display a green screen if the MOD behavior is the same as RefChip16. Currently it displays a red screen with Mash16.

Would be happy to see your thoughts about this!

The specification says that CLS should clear the foreground, and set the background to 0.
The latter is not done as of now.

Currently console output is redirected to stdout.txt/stderr.txt on Windows.

Users require output in the console window for interactivity.

Fix: See the SDL wiki: a cleaner solution may be to call freopen() with stdout.

Building currently fails, due to inline declarations with initialisation.

Fix: Move all variable declarations to the beginning of their scopes.

Audio crackling noticeable in some programs.

Boing.c16 presents this problem (Linux, Alsa).

Currently an unsupported Makefile is included in the repository, which does not even compile properly.

Either remove it, or fix it.

Changing the sample rate with --audio-sample-rate modifies the tone of the output audio -- which is undesired behaviour.