CHIP-8 test suite

A collection of ROM images with tests that will aid you in developing your own CHIP-8, SCHIP or XO-CHIP interpreter (or "emulator")

Table of contents

• Introduction
• Available tests
  • CHIP-8 splash screen
  • IBM logo
  • Corax+ opcode test
  • Flags test
  • Quirks test
  • Keypad test
• Contributing
• Community response 😄

Introduction

I found it hard to find reliable sources on what is the right behaviour and what is not, especially with the subtle differences between the original "Cosmac VIP" CHIP-8 and the HP84's SCHIP (or "superchip"). Now that I have written and ported a couple of interpreters as well as a few programs and games for the platform, I thought it was time to put that knowledge into code.

If you're having issues with your interpreter, you can find help in the EmuDev discord channel #chip-8. Every test has a clearly visible version number, which will help people to diagnose your problems if you share a screenshot. If you discover a problem with this test ROM itself, feel free to file an issue or open a pull request. It's open source, licensed under the GPLv3, and you're welcome to contribute.

Available tests

CHIP-8 splash screen

• Download ROM (source code available here)
• Run this ROM in Octo to see what's supposed to happen

The first test is a very simple splash screen. It is very similar to the often used IBM ROM, but it is actually a bit easier to get running. First, it doesn't use the "add value to register" opcode (7XNN) and second it only draws aligned sprites to the screen (the X coordinate is always a multiple of 8). So if you use an array of bytes for your display buffer, you don't have to shift the bits of the sprite to align with the buffer.

Run the ROM for 39 cycles to see this splash screen on the display. This first test can tell you if you're interpreting these opcodes properly:

• 00E0 - Clear the screen
• 6xnn - Load normal register with immediate value
• Annn - Load index register with immediate value
• Dxyn - Draw sprite to screen (only aligned)

If you run the ROM for more than 39 cycles, it will enter an endless loop. If that also works as expected, you've also correctly interpreted the jump opcode:

• 1nnn - Jump

IBM logo

• Download ROM (source code available here)
• Run this ROM in Octo to see what's supposed to happen

The second test is the classic IBM ROM. If the splash screen works as expected, this test should be pretty easy to pass, but it is a rite of passage for any CHIP-8 developer.

I did not write this ROM, it's probably decades old and I'm not sure who wrote it so I can't really credit them. The code in this suite is a re-implementation in Octo-mnemonics by my own hand, but it compiles to the exact same bytes as the original (except for the version number).

Run the ROM for 20 cycles to see the IBM logo on the display. If you can see the IBM logo, you are properly interpreting these opcodes:

• 00E0 - Clear the screen
• 6xnn - Load normal register with immediate value
• Annn - Load index register with immediate value
• 7xnn - Add immediate value to normal register
• Dxyn - Draw sprite to screen (un-aligned)

If you run the ROM for more than 20 cycles, it will enter an endless loop. If that also works as expected, you've also correctly interpreted the jump opcode:

• 1nnn - Jump

Corax+ opcode test

• Download ROM (source code available here)
• Run this ROM in Octo to see what's supposed to happen

This ROM is an adaptation of another famous one, the CHIP-8 test rom written by corax89. Many people use corax89's ROM to prove to themselves that their interpreter runs as it should. However, there are a couple of minor issues with the ROM and corax89 doesn't really seem to be maintaining this ROM anymore.

So I've taken the liberty to fix those issues, add a few new tests and make some cosmetic changes for this test suite.

The codes on the screen correspond to the functioning of these opcodes:

3xnn    2nnn    8xy4    Fx55
4xnn    00EE    8xy5    Fx33
5xy0    8xy0    8xy7    Fx1E
7xnn    8xy1    8xy6    Registers
9xy0    8xy2    8xyE
1nnn    8xy3    Fx65

If you see a checkmark, they're at least somewhat functional in the happy path. If you see a cross you can be sure that you have an issue with that opcode.

Here's a rough description of what these opcodes should do to pass the tests:

• 3xnn - if vX == nn, skip next opcode
• 4xnn - if vX != nn, skip next opcode
• 5xy0 - if vX == vY, skip next opcode
• 7xnn - vX += nn
• 9xy0 - if vX != vY, skip next opcode
• 1nnn - jump nnn (goto)
• 2nnn - call nnn (subroutine)
• 00EE - return from subroutine
• 8xy0 - vX = vY
• 8xy1 - vX |= vY
• 8xy2 - vX &= vY
• 8xy3 - vX ^= vY
• 8xy4 - vX += vY
• 8xy5 - vX -= vY
• 8xy7 - vX = vY - vX
• 8xy6 - vX = vY >> 1 or vX = vX >> 1 depending on quirks
• 8xyE - vX = vY << 1 or vX = vX << 1 depending on quirks
• Fx65 - load registers v0 - vX from memory starting at i
• Fx55 - save registers v0 - vX to memory starting at i
• Fx33 - store binary-coded decimal representation of vX to memory at i, i + 1 and i + 2
• Fx1E - i += vX
• Registers - The v0 - vF registers should be 8 bits wide. This tests to see if it can overflow your registers.

If you are having trouble figuring out how each opcode is supposed to behave, check out Tobias' guide or Gulrak's table of opcodes (but make sure you only check "CHIP-8" if you are just starting out).

Flags test

• Download ROM (source code available here)
• Run this ROM in Octo to see what's supposed to happen

This test checks to see if your math operations function properly on some given set of input values. But more importantly: it checks to see if you set the flag register vF properly when running those opcodes, and if you don't mess up vF too early (when vF is used as one of the operands). This is often an issue as the flags are pretty unintuitive and fairly hard to debug.

When running this test you should see the above on your screen. Each code on the screen corresponds to an opcode, and shows if the output value is correct (first checkmark), if the flag in vF is correct (second checkmark), if vF can be used as the vY input (third checkmark) and if vF can be used as the vX input (fourth checkmark, where present). If you see a cross instead of a checkmark in any of these spots, you have an issue in your interpreter logic.

First, a note on the third and fourth checkmarks: these check to see if an instruction where vF is one of the operands (like v0 += vF / 0x80F4) works as expected. It's easy to make the mistake of setting the vF register first, and then performing the mathematical operation. If you do that, however, vF will not hold the right value anymore at calculation time and your maths will be off when using that register as an input.

The top part (that starts with "HAPPY" for "happy path") checks the behaviour of the following opcodes, in the case where we don't expect an overflow, carry or shifted out bit:

HAPPY  8xy1   8xy2
8xy3   8xy4   8xy5
8xy6   8xy7   8xyE

• 8xy1 - vX |= vY
• 8xy2 - vX &= vY
• 8xy3 - vX ^= vY
• 8xy4 - vX += vY
• 8xy5 - vX -= vY
• 8xy6 - vX = vY >> 1 or vX = vX >> 1 depending on quirks
• 8xy7 - vX = vY - vX
• 8xyE - vX = vY << 1 or vX = vX << 1 depending on quirks

The bottom part (that starts with "CARRY") checks behaviour of the following opcodes, in the case that there is an overflow, carry or shifted out bit:

CARRY  8xy4   8xy5
8xy6   8xy7   8xyE

• 8xy4 - vX += vY
• 8xy5 - vX -= vY
• 8xy6 - vX = vY >> 1 or vX = vX >> 1 depending on quirks
• 8xy7 - vX = vY - vX
• 8xyE - vX = vY << 1 or vX = vX << 1 depending on quirks

The last row (that starts with "OTHER") checks that the opcode Fx1E (i += vX) properly adds the value of register vX to the index register, first for a regular register and then when using vF as the vX input register. For this test, only the value is checked because overflow of the index register is not really defined in CHIP-8 and this opcode is not supposed to influence the flag register.

See this article or this article for more information on the arithmetic operations and the flags.

Quirks test

• Download ROM (source code available here)
• Run this ROM in Octo to see what's supposed to happen

CHIP-8, SCHIP and XO-CHIP have subtle differences in the way they interpret the bytecode. We often call these differences quirks. This test detects which quirks your interpreter implements, and if those quirks match the platform you're trying to target. This is one of the hardest parts to "get right" and often a reason why "some games work, but some don't".

The menu

The test asks you to choose the platform you are targeting:

You can press any of the numbers 1 to 3 on the CHIP-8 keypad to jump to the corresponding test.

Alternatively, you can move the cursor up and down with CHIP-8 keys E and F and select an item with A. This feature mainly exists so people implementing interpreters for platforms with limited input devices (like a game controller) can map their buttons to those CHIP-8 keys and have an intuitive interface too.

If you want to repeat a test often or even automate them, having to use the graphical menu just gets in the way. In that case, you can force this ROM to select a specific platform by loading a value between 1 and 3 into memory at the address 0x1FF (512).

The test

The test will now run through a couple of steps, which you will see on the screen as a loading progress bar followed by some artifacts. After about three seconds, you should see this screen:

The screen shows you if the following quirks are detected as active ("on", "off" or an error) and if that matches your chosen target platform (a checkmark or a cross).

• vF reset - The AND, OR and XOR opcodes (8xy1, 8xy2 and 8xy3) reset the flags register to zero. Test will show E1 if the AND and OR tests don't behave the same and E2 if the AND and XOR tests don't behave the same.
• Memory - The save and load opcodes (Fx55 and Fx65) increment the index register. More information here and here.
• Display wait - Drawing sprites to the display waits for the vertical blank interrupt, limiting their speed to max 60 sprites per second. More information here. Test will show LOW if the number of cycles per frame is too low for the test to be deterministic (this is not necessarily an error, but a suggestion to rerun the test with a higher number of cycles per frame).
• Clipping - Sprites drawn at the bottom edge of the screen get clipped instead of wrapping around to the top of the screen. When clipping is off, the test checks if sprites get rendered at the right coordinates on the other side of the screen. This also tests that sprites drawn at coordinates of x > 63 and/or y > 31 wrap around to x % 64 and y % 32. More information here. Test will show E1 if the clipping is inconsistent in different dimensions or wrapping to the wrong coordinates and E2 if sprites don't wrap around as expected.
• Shifting - The shift opcodes (8xy6 and 8xyE) only operate on vX instead of storing the shifted version of vY in vX (more information here). Test will show E1 if the shift opcodes behave differently.
• Jumping - The "jump to some address plus v0" instruction (Bnnn) doesn't use v0, but vX instead where X is the highest nibble of nnn (more information here)

Note that you need timer support for this test to run.

See this excellent table by Gulrak for an overview of all the known quirks for the relatively popular CHIP-8 versions. See this website for a lot more information about all the historical versions of the platform and the different quirks among them.

Keypad test

• Download ROM (source code available here)
• Run this ROM in Octo to see what's supposed to happen

This test allows you to test all three CHIP-8 key input opcodes. It shows you a little menu, asking which opcode you would like to test:

You can press any of the numbers 1 to 3 on the CHIP-8 keypad to jump to the corresponding test.

Alternatively, you can move the cursor up and down with CHIP-8 keys E and F and select an item with A. This feature mainly exists so people implementing interpreters for platforms with limited input devices (like a game controller) can map their buttons to those CHIP-8 keys and have an intuitive interface too.

If you want to repeat a test often or even automate them, having to use the graphical menu just gets in the way. In that case, you can force this ROM to select a specific platform by loading a value between 1 and 3 into memory at the address 0x1FF (512).

1. Ex9E DOWN

Ex9E skips the next instruction if the key indicated in vX is currently pressed. In the test, when you press a key, the corresponding value lights up on the screen.

Pressing keys 1 and 6

2. ExA1 UP

ExA1 skips the next instruction if the key indicated in vX is currently not pressed. In the test, when you are not pressing a key, the corresponding value lights up on the screen.

Pressing keys 1 and 6

3. Fx0A GETKEY

Fx0A waits for a key press and returns the pressed key in vX.

The test asks you to press a key on the CHIP-8 keypad. When you do, it checks for two issues that are easy to accidentally introduce when implementing this opcode. If all is well, you should be seeing a checkmark and "all good" on the screen:

Otherwise, you can get either of these errors:

• NOT HALTING - Your implementation immediately returns the value of any currently pressed keys in vX, instead of halting the interpreter until a key is pressed (note that this needs timer support to be accurate)
• NOT RELEASED - Your implementation doesn't wait for the pressed key to be released before resuming

See this article for more information.

Contributing

Do you find an issue in this test suite that you think you can fix? Feel free to submit a PR! Here's how to build the project, assuming you have Nodejs and NPM installed:

git clone [email protected]:Timendus/chip8-test-suite.git
cd chip8-test-suite
npm install

# Build all the tests in `src/tests/` to `bin/`:
npm start

# Build a specific test:
npm run build-logo
npm run build-ibm
npm run build-corax
npm run build-flags
npm run build-quirks
npm run build-keypad

Note that the npm scripts use the MacOS command pbcopy to copy the resulting Octo source file to the clipboard. Depending on your OS this may not work properly. Edit package.json and remove this part from the end of scripts -> build-test if you get errors:

 && cat bin/${TEST}.8o | pbcopy

Community response 😄