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Message
From: Victor<victor.lopez@a...>
Date: Fri May 14 17:15:36 CEST 2004
Subject: [openrisc] What information should be pushed to the Stack for
Call Instruction?
Hello, I guess you are not talking either about the OpenRISC in this
question so the answer to your questions depends very much on what do
you want your processor to be able to do. But in any case I guess you
should look for its being able to resume execution perfectly, if your
way of jumping is through a "call" instruction which obviously has to
save the return address it is up to you if it should save any other
thing. OpenRISC does save the Status Register in case of exceptions,
for instance, and as such l.rfe instruction puts back the PC and SR
prior to the exception. Usually, it is up to the software to save
things like the other registers, but that is defined by the "calling
convention" which you should define for your processor. If we are
talking about your status register then you should decide if you want
it to be saved or not, it depends on the way you execute exceptions and
such, because you can always have the software doing it for you...
Everything is up to you, but a good rule of thumb is: the simpler you
make it in hardware the more complex will be the software. A RISC takes
many more instructions than a CISC to execute some algorithm but the
thing is that the RISC is so fast because of its simpleness that it
outdoes the CISC. I would recommend you to think about an architecture
and then start "working against it" by translating programs, by hand,
into your assembler code and "executing on paper" that code so you can
see if it is really worth it or if you should better modify certain
things, but always have in mind what the purpose of your processor is
going to be... I mean, if you want to play a tiny Linux on it then you
should start looking at its internals before even thinking about
condition codes. That is... maybe for a custom application it is not
mandatory to check for, let's say Overflow in operations (with this I
am also answering your other email) and maybe you don't even need to
set flags like the carry after an addition but if you are planning to
run something more serious... then it is better to start reading some
more literature... Anyways in order to get right opinions I think it
would be better if you could explain a bit more your questions, your
other email of today I couldn't understand completely what you meant,
but maybe it was my fault ;). Best regards and good luck,
Victor Lopez
> Dear Helper,
>
> When "call" instruction is executed (or earlier decoded), I know
that the return address should be pushed to a stack (stack-register)
if more that a call will be needed.
> My question is do I also have to push the condition code register (N,
Z,V, and C) and then pop them again with the return address when return
from call (rfc) instruction is executed or I just push the return
address (the PC of the call + 2 for 16-bit processor).
> I am thinking to push only the return address to the stack-register
during call and I push the "ra" and CCR flags for interrupt only.
>
> Many thanks,
> /Ben
> UEC, IS
>
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