Support for variable length instructions

[Rodrigodd]

I saw the instruction decoding feature in Surfer, and as I am working with simulation and emulation of the Game Boy processor, I thought it would be cool to implement decoding for it.

However, after taking a quick look at the TOML spec and the crate API, it does not appear to support variable-length instructions, which Game Boy instructions are. They vary from 1 to 3 bytes (see the optable here, if you're curious). They are similar to the i8080 and the Z80 instruction sets.

Does this crate intend to eventually support variable-length instructions?

[FRoith]

It's not a priority at the moment, since it was created to be used primarily in the context of Surfer, and signals are always expected to be the same length, but it is something that is on my list.

However a way to implement it right now, is to create 3 toml files, one for each subset of instruction length, and then load all three into the decoder at the same time, and call "decode" on the three potential instruction lengths, only one of which should return a decoded "Ok()" value.

[Rodrigodd]

Actually, I did not give enough thought for this feature, because retrieving the instructions from the simulation signals is non-trivial. It would need to somehow retrieve a stream of bytes that the micro-controller reads, and then parse that.

A more reasonable approach is to really add some kind of 24-bit debug signal to the simulation that stores the entire of the last decoded instructions, and run the decoding on that. I guess this crate would already support that, assuming it can ignore one or two bytes of the input.

However a way to implement it right now, is to create 3 toml files, one for each subset of instruction length, and then load all three into the decoder at the same time, and call "decode" on the three potential instruction lengths, only one of which should return a decoded "Ok()" value.

I assume your suggestion is about using this crate as a library, and not on Surfer itself. But my intention was to get this on Surfer.

[FRoith]

I assume your suggestion is about using this crate as a library, and not on Surfer itself. But my intention was to get this on Surfer.

True.
I've actually done something similar with the RISC-V toml files I've created, since RISC-V has compressed instructions, that are 16 bit wide instead of the normal 32. See toml/RV32C-lower.toml for the expanded version to 32 bit for surver vs toml/RV32C.toml for the standard 16 bit one.