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Message
From: Damjan Lampret<lampret@o...>
Date: Mon Dec 29 17:20:39 CET 2003
Subject: [openrisc] FPU
Hi All,I'm integrating a FPU into OR1200. Openrisc architecture defines FP
instructions however it doesn't defines some additional results of FP insns.
For example FP insns can produce the following flags: not a number,
underflow, overflow, invalid result, infinity, divide by zero etc. Question
is, should this flags be available to the user or not? If yes, how should
they be available? Also FPU supports different rounding modes. Two control
bits can control rounding mode. I see two ways:
1) SR register
2) FPU specific SPR
For result flags I prefer FPU SPR because SR doesn't have to be modified by
adding these special flags. However to control FPU rounding modes if FPU SPR
is used, and there are different user applications running that use
different rounding modes, then FPU SPR would have to be saved by context
switching code. So in a way using SR for rounding control bits would already
save FPU rounding mode by current context switching code.... Also should
some FP result flags cause an exception - for example divide by zero flag?
Which exception should happen? Also keep in mind that there are probably two
scenarios: 1) Embedded applications where application software can run also
in supervisor mode and could easily switch between different rounding modes
etc. And 2) Linux type of application code where there is a big difference
between user application code running only in user mode and OS running in
supervisor mode - in this case user code can or should be able to set
rounding modes or not - SR can onyl be written in supervisor mode... By the
way how can application SW under Linux/I386 change rounding modes?
So what do you think is the best way to incorporate FP support? Remember
maybe result flags are not needed, and only one rounding mode is enough?
I copy Stefano who developed the FPU and has a lot of experience with the
rounding modes etc. I remember Stefano you once told me that only one
rounding mode is usually used and the other two and rarely used.
regards,
Damjan
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