[Libre-soc-isa] [Bug 968] document shift-and-add instruction

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=968

--- Comment #9 from Andrey Miroshnikov <andrey at technepisteme.xyz> ---
(In reply to Luke Kenneth Casson Leighton from comment #8)
> the word descriptioj makes no sense at all

Yes, true.

> now try this:
> 
>    wben sm is zero, the contents of register RB are multiplied by two,
>    added to the contents of register RA, and the result stored in RT.

Changed the descriptions:
https://git.libre-soc.org/?p=libreriscv.git;a=commitdiff;h=2ee653d030e00ffd1e1a93644088e6f2fee59650

> shadduw is "unsigned word" btw

This is not obvious to me (I'm probably not understanding). Not sure what the
purpose of this instruction is.

The pseudocode of shadd doesn't indicate that it is **signed**, therefore why
have shadduw?

Second line of shadduw pseudocode:
n <- (RB)[XLEN/2:XLEN-1]  # btw we agreed on not using XLEN in RFCs for now?

With XLEN=64, this would be (RB)[32:63], or the lower half (MSB ordering?).
Why ignore bits (RB)[0:31]?

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