Show HN: I wrote a custom assembler for CHIP-8 in C++

github.com

28 points by tack1234 5 days ago

I'm writing a custom assembler for CHIP-8 and just finished adding support for the DB/.byte directive so I can embed sprites in the ROM and finally get something drawn on the screen. It SHOULD support all OG CHIP-8 instructions, but still a ton of rough edges that I'm ironing out.

This is my first ever C++ project, just like the CHIP-8 emulator was my first ever C (and emudev) project. It's so satisfying to write assembly and have your program spit out a ROM that actually runs in the emulator you wrote.

All code is written by me (a lot of it livestreaming) as evident by the quality, no AI.

Retr0id 3 days ago

Nice, chip8 is a lot of fun. I looked at your emulator implementation too, and it looks like you have an OOB access bug in the "I" register (it can be incremented beyond the RAM array bounds). This is a very common chip8 emulator bug, which can (in some cases) be exploited to allow a malicious ROM to escape the emulator. I wrote about this here: https://www.da.vidbuchanan.co.uk/blog/bggp3.html

  • tack1234 12 hours ago

    Thanks for pointing that out, I think it was somewhere on my TODO list which miraculously got lost. I'll have a look at it!

Stevenrojas888 3 days ago

Awesome milestone! As someone diving into C and C++ right now, what was the steepest part of the learning curve for you when structuring the assembler compared to writing the emulator in C?

  • tack1234 12 hours ago

    Thanks! Well at first I did not have much structure, I just went full procedural and did what I needed to do at each step. It was an ugly, long chain of loops and switches. Then I read up about assemblers a little bit, figured out what should go into the lexing/parsing/emitting stages and split it into classes.

    The steepest part.. probably trying to do things efficiently, using string views instead of strings where relevant, references and also ranges + iterators in C++. The from_char function and how it reports errors (e.g. when the iterator it returns is the end of the provided range, there were no errors) etc.

adityamishra241 3 days ago

Really cool project. How much harder did the assembler feel compared to writing the emulator?

  • tack1234 12 hours ago

    Thank you!

    From the conceptual standpoint, it felt way clearer to me from the start. I knew I "just" had to parse some input text and figure out the instructions into which it should be turned into. The actual lexing and parsing of the source code text character by character lead me to way more errors and debugging sessions than I expected, but that's probably just me being a bit rusty.

    So I would say the emulator was harder to grasp at first, as I had no prior lower level experience, had no idea what a register or a stack pointer or a program counter is, etc.