Re: [openrisc] Re: PC as GPR?

["Jim Dempsey" <tapedisk@ameritech.net>]

----- Original Message -----
From: "Marko Mlinar" <markom@opencores.org>
To: <openrisc@opencores.org>
Sent: Tuesday, March 12, 2002 1:00 AM
Subject: Re: [openrisc] Re: PC as GPR?


> > Sure, MHz is not important, overall performance is important (or
> power/size
> > if you have other requirements).
> except for marketing reasons ;)
>
> > But it is hard to come from an idea to verified working HW. Not to
mention
> > all the software. If I'd know how exactly windowed register file is
> > implemented in Sparc GCC, we would have windowed register file. ;-)
> Not all windowed register files are good. For example, Sparc -- I've heard
> that thay would be happier if they would choose not to implement this
> feature, but they must do it for compatibility reasons.
> Besides normal functions you also have interrupts and system calls
> which can make windowed register file useless, if not implemented
correctly.

Look how Altera NIOS does the windowed register files. Interrupts are
quite clean as far as I can see. You have a problem only if you programmed
poorly i.e. did not leave sufficient space in register file for possible
interrupt.

Before implimenting a register file you have to look at the requirements for
the need of a register file. The overriding needs are to conserve bits in
the
instruction (keep narrow) and probably more important an efficient means
for passing arguments on a function call.

> I think we chosed the right path to implement things bit simpler, but
safer.
>
> Marko
>
>
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