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PDP8: Fix crash when using EP multiply

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When performing floating point multiplication, the prior code overwrote
an additional word of the floating point fraction with zeros. This is
harmless with standard FP, as the floating variables always have space
for EP-length vars. When doing an EP multiply, this causes a word on
the stack to be zeroed. For the latest Raspbian release, this causes a
segfault as there's no padding past that var on the stack.

This fix, which has been tested against the original crashing code plus
validated using the FPP-8 diagnostics, avoids the overwrite.
This commit is contained in:
Rick Murphy 2022-01-06 18:53:03 -08:00 • committed by Mark Pizzolato
parent fa5e0ed157
commit 030a47bdb4
1 changed files with 6 additions and 2 deletions
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4
PDP8/pdp8_fpp.c
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@ -25,6 +25,8 @@
fpp FPP8A floating point processor fpp FPP8A floating point processor
05-Jan-22 RHM Fix fencepost error in FP multiply for extended
precision
21-Oct-21 RMS Added device number display 21-Oct-21 RMS Added device number display
03-Jan-10 RMS Initialized variables statically, for VMS compiler 03-Jan-10 RMS Initialized variables statically, for VMS compiler
19-Apr-09 RHM FPICL does not clear all command and status reg bits 19-Apr-09 RHM FPICL does not clear all command and status reg bits
@ -1184,6 +1186,7 @@ for (i = 0; i < cnt; i++) {
wc++; /* do another word */ wc++; /* do another word */
lo--; /* and next mpyr word */ lo--; /* and next mpyr word */
fpp_fr_algn (c, 24, wc + 1); fpp_fr_algn (c, 24, wc + 1);
if (wc < FPN_NFR_MDS) /* don't assume guard word */
c[wc] = 0; c[wc] = 0;
c[0] = c[1] = fill; /* propagate sign */ c[0] = c[1] = fill; /* propagate sign */
} }
@ -1374,6 +1377,7 @@ if (sc >= (cnt * 12)) { /* out of range? */
} }
while (sc >= 12) { while (sc >= 12) {
for (i = cnt - 1; i > 0; i--) for (i = cnt - 1; i > 0; i--)
if (i <= FPN_NFR_MDS) /* Don't overwrite if EP */
a[i] = a[i - 1]; a[i] = a[i - 1];
a[0] = sign; a[0] = sign;
sc = sc - 12; sc = sc - 12;