RE: [fpu] FPU operations

["Rudolf Usselmann" <russelmann@hotmail.com>]

This should be soooo simple:

All the FPU has to provide, is status outputs. All the masking
and dealing with the status/exceptions, is done by the CPU internally.

Here is a rough outline of the FPU IOs as I presume the CPU
would want to see them:

INPUT	[31:0]	operand A
INPUT [31:0]	operand B
INPUT [3:0]		operation type (TBD, perhaps more bits ?)
OUTPUT [31:0]	result
OUTPUT		larger then ( A > B )
OUTPUT		smaller then (A < B)
OUTPUT		equal ( A == B )
OUTPUT		unordered ( A == NAN | B == NAN )
OUTPUT		overflow
OUTPUT		underflow
OUTPUT		SNAN
OUTPUT		INE
OUTPUT		INF (not sure we need this)

Damjan, did I forget something ?

Regarding pipelining:
I would suggest that all the basic functions (add, sub, mul and divide)
take the same amount of cycles to execute. I think this will make it
easier from the CPUs point of view.

Now, more complex operations (for example square root) might be variable
length execution units, that might signal to the CPU when they are done.
Damjan, can this be done with OR1K ? The CPU would have to monitor the
completion and write the result back out of order. If another operation
is trying to access the result before it is ready, the CPU would have to
stall, or enter a trap. This could be avoided by having the compiler
perform instruction sorting and scheduling. So that a instruction that
is know to take two (or more) regular instruction cycles to execute, are
not placed back to back. However, now you might need an additional write
port for the FPU register file (which I assume is kept inside the OR1K,
not the FPU).

Cheers,
rudi


>-----Original Message-----
>From: owner-fpu@opencores.org [mailto:owner-fpu@opencores.org]On Behalf
>Of Jamil Khatib
>
>
>So I have to provide exception signals on the FPU
>interface and the CPU core has to provide mask unmask
>operation and generate interrupts but what about
>rounding modes does the core need to provide the
>signals
>
>Also compare results status register?
>
>I do not know if there are more registers to have
>later
>
>Should I provide all these signals
>
>regards
>Jamil

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