riscv/fastcpu: UMul/IMul versus their description

[neuschaefer]

In js/worker/riscv/fastcpu.js, there are the two functions UMul and IMul. UMul is documented as a signedsigned multiplication and IMul as unsignedunsigned multiplication. However, their names suggest that it's in fact the other way around (UMul sounds like unsigned multiplication).

Which function does what?

[andrakis]

If I understand you correctly, you want to know whether jor1k is doing what the (incorrect?) documentation says, or the "correct" behavior that occurs in a real device.

According to the imul section, it's been implemented as per existing real devices, not what the documentation says.

[neuschaefer]

I think a was a little tired when I wrote the above :)

I'm talking about fastcpu.js in the (experimental?) RISC-V code. And by documentation I mean the comment in the third line of these functions.

By reading some more of the code, I found out that the mulh instruction (signedsigned) uses UMul and the mulhu instruction (unsignedunsigned) uses IMul. So the comments are correct, but the names are wrong (IMHO, at least).

[s-macke]

I think the labeling is wrong. But to check the multiplication functions was on my list anyhow. The official riscv test cases (https://github.com/riscv/riscv-tests) are fine with the implementation.

[s-macke]

Problem solved. The multiplication instructions still need a thorough check https://github.com/s-macke/jor1k/commit/3ce20cb17a720e0cda56288e5d00ef5d159071c3