From: jam@magic.com (James A. Markevitch)
Subject: [Unix-jun72] Minor bug in sources e09-07
Date: Sat, 3 May 2008 10:35:46 -0700 (PDT) [thread overview]
Message-ID: <200805031735.KAA01301@mist.magic.com> (raw)
> >There is a bug in the svn sources on page e09-07, near the bottom.
> >The call to sleep should read:
> >
> > jsr r0,sleep; 0:..
> >
> >Note that there should be a colon, not a semi-colon after the 0.
> >Presumably, this code was never executed, else it would have
> >resulted in a halt.
>
> thanks!
>
> but what *does* that syntax do? 0:.. ?
The two-instruction sequence is:
movb tty+3(r1),0f / put clist id in sleep argument
jsr r0,sleep; 0:..
The "0:" on the second line is a label and it is referenced by the "0f"
in the first line. The first line is putting a value into the argument
being passed to the sleep subroutine. Self-modifying code.
The ".." assembles to a 0.
With the incorrect code, "0" assembled to a 0 and ".." assembled to
a 0, so there was one extra word of zeroes, and the return from the
sleep would have executed it (halt) instead of the "br 1b" on the
next line.
James Markevitch
next reply other threads:[~2008-05-03 17:35 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-05-03 17:35 James A. Markevitch [this message]
-- strict thread matches above, loose matches on Subject: below --
2008-05-03 16:21 James A. Markevitch
2008-05-03 17:26 ` Brad Parker
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