Re: [f-cpu] 15 MIPS FC0 emulator

[Michael Riepe <michael@stud.uni-hannover.de>]

On Sat, Dec 07, 2002 at 11:19:26PM +0100, Yann Guidon wrote:

> before making it fast, what about making it acurate ?
> what about the problem with signed saturations ?

Before anything can be made accurate, we got to fix the manual.  I looked
at 0.2.7 today, and it seems to move away from reality more and more...

In particular, the description of the double shift operations is still
wrong. Instruction names have changed without notice (when did we decide
to rename `shiftli' to `shiftil', and who introduced `cor' and `cand'?),
and there are instructions that we never agreed on (e.g. cload/cstore).
To cut a long story short: it's all a big mess.

I suggest that you (all of you) don't add any new instructions unless
you prove that you can implement them - in VHDL, not in C++! You guys
add more and more stuff to the manual, but it's only science fiction :(

We also badly need to clean things up. I found an ancient bug today -
one that has been there before I joined the project. Look at the examples
for `byterev': the lower part of the result is the byte-reversed least
significant chunk of the operand - and the upper part is *copied*?
That's impossible. With the old `partial write' semantics, it should
remain unchanged, and with the current semantics, it should be zero.

[...]
> >PS: There should be specified whether ADD saturation is signed or not !
> >  
> As far as i remember, it is unsigned.

Yep. It was cheaper to implement (there only is an overflow in one
direction), and is IMHO also more useful than signed saturation.

-- 
 Michael "Tired" Riepe <Michael.Riepe@stud.uni-hannover.de>
 "All I wanna do is have a little fun before I die"
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