On Wed, 22 Mar 2006 12:35:47 UTC, glen herrmannsfeldt
<gah@[EMAIL PROTECTED]
> wrote:
> John Small wrote:
> > On Wed, 22 Mar 2006 03:40:58 UTC, bo774@[EMAIL PROTECTED]
(Kelly
> > Bert Manning) wrote:
>
> >>does that end up unpacking a single byte at the specified
displacement?
> >>PARM5D DS 0H
> >> BCTR R14,0 R14 = HEX LTH
> >> EX R14,PARM5EX PACK WAIT TIME
> (snip)
>
> >>PARM5EX PACK DWORD+6(2),0(0,R15) USE 1ST 2 BYTES ONLY
>
> > R14 gets ORed with the second byte of the assembled PACK instruction.
> > As assembled this 2nd byte would be X'10', the first nibble being the
> > "length - 1" of the target and the second nibble the "length - 1" of
> > the source.
>
> > If R14 has a value of 2 then the effective length of the source is 3
> > and the ORed second byte is X'12'. And 3 characters will fit exactly
> > into 2-byte packed field.
>
> But there is BCTR so R14 should be 1 when it gets to the EX.
>
You are right about R14 not being 2.
I was responding to the OP's text "If R14 has a value of 2 wouldn't
the effective length of the target field be 3 bytes..." and not
looking at the code.
The OP's problem seemed to be that he thought that R14, as used in
this code, would determine the run-time target length and possibly
overrunning the DWORD field. And my response was an attempt to say
that R14, as used in this code, is intended to affect the run-time
source length.
Upon further thought, the code snippet in the original post does not
ensure that R14 is positive. If R14 is negative or zero upon entry to
the code snippet then it could affect both of the run-time lengths,
depending on the low-order byte of R14 after the BCTR. So a
non-positive value in R14 could cause overruns in the DWORD field.
For example, a value of 0 in R14 upon entry to the code snippet...
- would be changed to -1 by the BCTR
- the -1 in R14 would cause the EX'd PACK instruction (after the
run-time OR of the 2nd byte of the PACK instruction) to be the
equivalent of:
PACK DWORD+6(16),0(16,R15)
So if the OP was trying to find a bug in this code that was causing
overruns, then the bug may be that this code does not ensure that R14
has a valid length (1-16 on entry, 0-15 after the BCTR) for a source
field.
--
John Small
(remove the z's for email address)
|
