From: mycroft@gnu.ai.mit.edu
To: dj@stealth.ctron.com
Cc: gas2@cygnus.com, hjl@nynexst.com, masaki@eie.minolta.co.jp,
raeburn@cygnus.com
Subject: Re: [masaki@eie.monolta.co.jp: GAS bug and etc]
Date: Fri, 11 Nov 1994 13:42:00 -0000 [thread overview]
Message-ID: <9411112139.AA00683@goldman.gnu.ai.mit.edu> (raw)
The problem with using the 16-bit jcxz is that, even if the offset is
small, if the target isn't in the first 64K of memory, the program is
toast. The reason is that %eip is masked to 0x0000ffff AFTER adding
the offset.
The comment you're referring to:
rel16/32 indicates that these instructions map to two; one with a 16-bit
relative displacement, the other with a 32-bit relative displacement,
depending on the operand-size attribute of the instruction.
only applies to the `rel16/32' versions of the conditional jump
instructions. The JCXZ and JECXZ instructions do not allow 16- or
32-bit displacements.
The `Operation' section for that instruction should be more clear.
BTW, why are you using JCXZ or JECXZ at all? The equivalent sequence:
testl %ecx,%ecx
jz ...
is the same speed on the 386, but faster on the Pentium (and probably
the 486, but I don't have those timing specs).
next reply other threads:[~1994-11-11 13:42 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
1994-11-11 13:42 mycroft [this message]
-- strict thread matches above, loose matches on Subject: below --
1994-11-11 6:11 DJ Delorie
1994-11-11 6:28 ` H.J. Lu
1994-11-11 6:37 ` DJ Delorie
1994-11-11 6:45 ` H.J. Lu
1994-11-11 6:49 ` DJ Delorie
1994-11-11 7:06 ` H.J. Lu
1994-11-11 10:31 ` Richard Stallman
1994-11-11 10:46 ` DJ Delorie
1994-11-11 14:27 ` Richard Stallman
1994-11-11 15:01 ` Arthur Kreitman
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