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Show HN: Simple Gameboy Emulator

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Re: Show HN: Simple Gameboy Emulator

#11

Post by tnecniv »

deaddodo wrote: post #9
tnecniv wrote: post #6

Making a Gameboy emulator was one of my favorite and most instructive personal projects. It's worth noting that there was actual extra hardware built into the carts, so you need to implement certain features like extra RAM based on the cart ID in the ROM file.

Not the cart ID, you're looking in the cart header at byte 0147 for the actual hardware (MBC1, MBC2, MMM01, etc) functionality in conjunction with bytes 0148 (ROM size) and 0149 (RAM size) for specific bank sizing/mapping.

Yeah that's what I meant. It's been more than 5 years so I didn't remember the exact details. Thanks for the specifics!

Re: Show HN: Simple Gameboy Emulator

#12

Post by userbinator »

deaddodo wrote: post #10
rebootthebox wrote: post #8

Great! The CPU.c code is quite instructive since I'm writing a toy VM in my free time. Starring this :)

They're using a relatively simple opcode map, but most instructions are logically redundant as can be seen here: http://www.pastraiser.com/cpu/gameboy/gameboy_opcodes.html The next step in emudev, with most compiled/systems languages at least, would be to create macros (such as OP_LD or OP_ADD) that generates static instructions at compilation. Another, cleanish method, in C is to generate a 256-length function point…

The Gameboy CPU shares the same octal-structured opcode format as the Z80 from which it was derived, so it is possible to decode it even more concisely: http://www.z80.info/decoding.htm

You can see that in octal, 1xx are all moves while 2xx is all ALU ops.

Re: Show HN: Simple Gameboy Emulator

#14

Post by deaddodo »

userbinator wrote: post #12
deaddodo wrote: post #10

Earlier quoted context omitted.

They're using a relatively simple opcode map, but most instructions are logically redundant as can be seen here: http://www.pastraiser.com/cpu/gameboy/gameboy_opcodes.html The next step in emudev, with most compiled/systems languages at least, would be to create macros (such as OP_LD or OP_ADD) that generates static instructions at compilation. Another, cleanish method, in C is to generate a 256-length function point…

The Gameboy CPU shares the same octal-structured opcode format as the Z80 from which it was derived, so it is possible to decode it even more concisely: http://www.z80.info/decoding.htm You can see that in octal, 1xx are all moves while 2xx is all ALU ops.

Oh, it's definitely decodable. Most CPU's can be (though x86 post-386 wouldn't be fun), since that's what they're usually doing internally. But, with 8-bit CPU's, the opcode space is usually limited to a byte, which makes tables very doable and very efficient.

My rust-based GB emulator uses a decoder based on those very docs, in fact. Though, modified slightly since the GB's CPU only supports the CB-extended range operations.

Re: Show HN: Simple Gameboy Emulator

#16

Post by sacert »

doomlaser wrote: post #13

I love projects like these. Code looks very clear and organized. How long have you been working on it?

In total I've spent roughly a month and a half working on it but mostly only spending an hour or so every couple days on it. After understanding the architecture, the coding isn't too difficult and is immensely rewarding!

Re: Show HN: Simple Gameboy Emulator

#17

Post by sacert »

zerr wrote: post #15

Nice. Please share if this or any other of your repos help(ed) with finding new gigs/clients.

I personally just code for self interest and enjoy a challenge since my job is rather stale. I can't speak from experience but I'm sure having a portfolio relevant to the type of work you're aiming for would impress potential clients and enhance corresponding skills.

Re: Show HN: Simple Gameboy Emulator

#18

Post by pvinis »

A very nice idea. I did one for this dcpu or something like that, from a potential game from Notch (Minecraft guy).

More people should do projects like these, because many many people enjoy looking at them and studying them and contribute to them :D.

Re: Show HN: Simple Gameboy Emulator

#19

Post by nukeop »

zerr wrote: post #15

Nice. Please share if this or any other of your repos help(ed) with finding new gigs/clients.

I assure you having even a few hobby projects to show to potential employers is always beneficial. When you can talk about actual code you've written, often you don't even have to do any tests and the usual questions about the basics are skipped completely.

Re: Show HN: Simple Gameboy Emulator

#20

Post by flohofwoe »

deaddodo wrote: post #10
rebootthebox wrote: post #8

Great! The CPU.c code is quite instructive since I'm writing a toy VM in my free time. Starring this :)

They're using a relatively simple opcode map, but most instructions are logically redundant as can be seen here: http://www.pastraiser.com/cpu/gameboy/gameboy_opcodes.html The next step in emudev, with most compiled/systems languages at least, would be to create macros (such as OP_LD or OP_ADD) that generates static instructions at compilation. Another, cleanish method, in C is to generate a 256-length function point…

For Z80 emulation the most efficient method I found was indeed a big nested switch-case statement. Compilers turn this into a jump table (or several if there are gaps), but don't need the function entry/exit boilerplate if each instruction code block would be its own C function referenced through a function-pointer-table. The switch-case code is generated with a python script which uses the 'algorithmic decoding approach' described here: http://z80.info/decoding.htm

Here's what such a code-generated instruction decoder looks like: https://github.com/floooh/chips/blob/master/chips/_z80_decod... (this is for a "real" Z80 with all undocumented opcodes, so it has a lot more cases to handle than the simple Gameboy Z80 variant).

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