FP arithmetic on Kernel Mode - Done, thank you. :)

NTDEV
1

Thank you, Elyas.
I did searched the newsgroup archives you gently pointed me out. As
I suspected, this is not a peaceful question. If anyone’s interested,
here’s what I’ve learned and concluded about the subject:

  1. Floating point operations in kernel-mode doesn’t make great sense.

  2. If sense is not to be called here, you should be aware that FP
    support in KM is very limited.

  3. In fact, there’s no guarantee that NT 4.0 and prior releases support
    FP in KM.

  4. If you’re using Win2K the FP restriction has been lifted: you may
    use FP math in KM. But it’s best to stick with integer math.

  5. If you really, really insist on using FP math then:

a) Save the FP state (KeSaveFloatingPointState)
b) Do your (single) FP operation
c) Restore the FP state (KeRestoreFloatingPointState)

  1. Use integer math in KM and pass those silly FP operations up to
    user mode level.

OK, that’s it. I’ll have to resign myself and stick with the last (6)
“brilliant” bottomline… :)))

Miguel Monteiro
xxxxx@criticalsoftware.com
www.criticalsoftware.com

“Humour and love are God’s answers
to Human weaknesses”

On Monday, June 18, 2001 8:31 PM “Eliyas Yakub” wrote:

Please search the newsgroup archive
(http://groups.google.com/groups?hl=en&lr=&safe=off&group=comp.os.ms-wi
ndows.programmer.nt).
This has been discussed several times before.

Windows beta2 DDK documentation has new section called “Using Floating
Point or MMX in a WDM Driver”. Read that for more info.

-Eliyas

—Original Message—
On Friday, June 15, 2001 11:32 AM I (Miguel Monteiro) wrote:

Hi,
Using FP arithmetic on KM seems a hedious task. :frowning:
Strange as it might seem, suppose such task is needed on a km driver.
I could:

  1. Use KeSaveFloatingPointState to save the FP context in a resident
    (e.g. in the device extension) KFLOATING_SAVE variable;
  2. Perform the FP operation (e.g. a DOUBLE division);
  3. Restore the FP context, using KeRestoreFloatingPointState.

This scheme works fine for a single operation (A), but what if I need
to
perform a sequence of FP operations:

save(A), restore(A), save(B), restore(B) = Ok
save(A), save(B), save(C) … restore(C), restore(B), restore(A) = NOT
Ok… :frowning:

(When dividing, one possible solution would be to use integer
arithmetic
instead, “expanding” the divident (SHL), perform the division and then
“shrink” the result (SHR) again. But I’m not sure if the results are
reliable…)

Does anyone know how the “community” has been handling/solving the
FP problem on KM drivers? Any tips/points/links are mostly appreciated.

Thank you,

Miguel Monteiro
xxxxx@criticalsoftware.com
www.criticalsoftware.com

“Humour and love are God’s answers
to Human weaknesses”

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