[parisc-linux] floating point exception error

[David Huggins-Daines]

Ah, well, on further study, the documentation
(http://puffin.external.hp.com/docs/pcxl2_ers.pdf) explains all, for
the 7300LC at least.

However, if we intend to support PA2.0, then we will need to have some
kind of documentation on what parts of the FPU architecture the
various PA2.0 chips implement.

> So, are unordered comparisons just not handled by the 7300LC?  Which

According to the docs:

    4.3 Unimplemented Exception/Trap

    The only kind of exception generated by the PA7300LC floating point is
    the unimplemented exception.  It is always signaled with a delayed
    floating­point exception trap.  The unimplemented trap is raised
    instead of the overflow, un- derflow, division by zero, invalid and
    inexact traps.

Well, that's just great :P

> I have no idea why the exception shows up in FR1L instead of in FR0R

According to the docs:

    4.3.6 Exception Registers

    The excepting flop will be in exception register 2.  PA­RISC 1.1
    exception codes are used.  Exception registers 1, 3, 4, 5, 6, and 7
    may be loaded or stored, but hardware will never place an excepting
    flop in them.

    Exception register 2 is guaranteed to retain its contents as long as
    the T bit remains set and continuing thereafter until a flop is
    executed, or until it is explicitly cleared by software with a load.

Okay, that makes life easier, at least.  I'm really fearful of what
the 8500/8600 will do though.  You'd think that since they execute out
of order they'd be able to provide some sort of precise trapping..

> It looks like we are going to need some floating-point completion
> support in the kernel, which evidently is going to involve walking the
> trap shadow and other fun things I thought you only had to do on DEC
> machines ;)

Well, I guess not on 7300LC at least, given that only FR1L (except2)
is used.  So that's a relief.  I'll see if I can write a rudimentary
completion handler today that should at least enable us to build awk
and so forth.

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