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From: mikem@uhccux.uhcc.hawaii.edu (Mike Morton)
Newsgroups: comp.sys.mac.programmer
Subject: MacsBug crash when interrupting 68881 operations
Keywords: MacsBug 68881 float math-chip
Message-ID: <4168@uhccux.uhcc.hawaii.edu>
Date: 24 Jun 89 00:28:01 GMT
Organization: University of Hawaii
Lines: 43


MacsBug seems to have problems interrupting certain math-chip operations.
I run this LightspeedC program with 68881 code enabled:
    main ()
    { register float a = 0, b = 0;

      while (1)
        a += b;
    }

When I press the interrupt switch, I get:
    Spurious Interrupt or Uninitialized Interrupt Vector at 003ED970.
    *** MacsBug caused the exception ***

The code which crashes looks like this (comments are my own, natch):
    TST.B  -$071B(A5)            ; have a math-chip installed?
    BEQ.S  *+12                  ; no: skip
    LEA    -$023A(A5), A0        ; yes: find a place to...
    FMOVEM FP0-FP7, (A0)         ; ...store the math registers
   *LEA    -$0246(A5), A0        ; and a place for...
    FMOVEM FPCR/FPSR/FPIAR, (A0) ; ...weird regs
The crash is at the starred location.  At this point I usually get Bus
Errors, etc., if I try to continue.

LSC doesn't put a and b into registers unless I declare them double,
in which case the problem goes away.  The code LSC emits for the
float case looks like:
 l: FMOVE.S -4(A6), FP0
    FADD.S  -8(A6), FP0
    FMOVE.S FP0, -8(A6)
    BRA.S   l

My guess is that when there's a lot of traffic between the math chip
and memory, interrupts can leave the chip in a state which doesn't
like FMOVEMs.  Any comments from MacsBug folks at Apple?  From '881
hackers?

Boring details: SE/30m 4Mb, System 6.0.3, Macsbug 6.1, no inits.

Ever try to find anagrams for "MacsBug"?

 -- Mike Morton // P.O. Box 11378, Honolulu, HI  96828, (808) 676-6966 HST
      Internet: mikem@uhccux.uhcc.hawaii.edu
    (anagrams): Mr. Machine Tool; Ethical Mormon; Chosen Immortal; etc.{