What is it?

Vinegar is a CHIP-8 and SCHIP 'emulator'/interpreter.

CHIP-8 was a simple language interpreter found on some early home computers, and has a very small instruction set. The machines it ran on had a 64x32 resolution display; they also had a simple hex keypads (a 4x4 grid of keys 0 to F) for input, which might explain the odd key layout this program uses.

CHIP-8 was revisited in the 1990s thanks to an interpeter for the HP48 calculator. The instruction set was extended and the screen resolution doubled to 128x64, but is fully backwards compatible with the original CHIP-8. This is the "SUPER-CHIP" (SCHIP) part of this interpreter.

It is easy to find programs in CHIP-8 or SCHIP, and using the command script file included you can easily convert the files to a format that can be sent to your calculator.

Features

This program lets you run most CHIP-8/SCHIP programs. It makes more sense to list the limitations, rather than the features...

• No 1802 emulation (the PC CHIP-8 emulators I've used ignore this as well).
• Mapping 128x64 and 64x32 to a 96x64 screen is tricky; all attempts at stretching/squashing were very ugly. Because of this, the CHIP-8 mode is small and centred and the SCHIP mode is cropped.
• Lack of proper sound hardware means that when a beep is sounded, the simulation freezes briefly, and long tones are stuttery.
• I have no idea what the HP48 flags are meant to do. You can copy registers V0 to V7 to and from them in Vinegar, so if a program uses them as temporary storage then that's all well and good.
• As the CHIP-8 display is handled at a 1:1 resolution, not stretched to 2:1, half-pixel scrolls don't work.

Usage

This package comes with 3 binaries; one for TI-83, and two for TI-83 Plus. Send the one that best matches your calculator model and shell to your calculator using your linking program.

You won't be able to use this interpreter without having a few CHIP-8 programs installed. I've included a rather large number of them in the ROMs subfolder of this package. Note that ones with (CHIP-8) in their name are CHIP-8 programs, and that ones with (SCHIP) in their name are SCHIP programs.

If you run the program, you should be able to see the installed program in a list, a bit like Ion's main view. Highlight the program you wish to start, then press 2nd or Enter to run it.

The key layout is a little odd; here is a CHIP-8 keypad and a TI-83 keypad for comparison.

The other keys for this program are:

The settings screen (Alpha) is very useful when it comes to setting the speed of the simulation.

The various options can be summarised thusly:

• Instruction delay - the length of the delay after each instruction. This can be useful to throttle games that run too quickly.
• Frame skip - the number of instructions executed per frame.
• Always refresh LCD - refresh the LCD, regardless of whether a sprite was drawn or not. Disabling this makes the simulation faster, but makes the timing more inconsistent.
• Invert colours - draw as white-on-black, rather than black-on-white.
• Enable sound - when enabled, you can plug a pair of headphones via an adapter into the calculator's link port and hear the CHIP-8 beep.
• Ignore errors - if the simulation encounters an error (see below), do not display a prompt - just carry on. For some games, this doesn't appear to have an effect, but certain games depend on an unsupported instruction and will "crash" inside the simulation.

More simply, the fastest possible simulation is to have an Instruction delay of 0, a Frame skip of 255, and Always refresh LCD and sound disabled. The slowest is to have a delay of 255, a skip of 0, LCD refreshing and sound on.

Errors

CHIP-8 is a high-level low-level language, and as such can potentially muck up whatever it is you are running it on.

Vinegar checks the stack level, data pointer, current instruction and program counter whenever it's about to use them for something potentially dangerous. If it senses that something is wrong, you'll end up with a screen like the following:

The three reported value are program counter, opcode and index register. The different errors are:

• Instruction - the current instruction is not recognised or supported.
• Stack - the stack has underflowed or overflowed. It is suggested that implementations support 16 levels; Vinegar has 64, so I hope that's enough.
• PC - the program counter has ambled out of the ~3.5KB allowed memory space.
• Data - the program was about to copy data to memory outside of the allowed memory space.

You can disable this error screen on the settings page; beware that this will cause some simulated programs to 'crash' (there should be no danger to your calculator).

Converting

To convert a "plain" CHIP-8 program to the format suitable for the calculator, go to Convert folder in this package.

1. Download Brass and copy the EXE file into the Convert folder.
2. Copy your CHIP-8 program into the Convert folder.
3. Go to the command prompt, browse to the folder then type "Convert <filename>", where <filename> is the filename of your CHIP-8 program.

It should spit out two files; an 8xp for the TI-83 Plus and an 83p for the TI-83.

Thanks

• Joe Wingbermuehle - the sprite routines are mutilated version of his Ion sprite routines.
• David Winter - CHIP-8/SCHIP documentation.
• hap - large font is from his Fish 'N' Chips emulator, the ROMs are also the ones bundled with said emulator.