[Unix-jun72] Minor bug in sources e09-07

James A. Markevitch jam at magic.com
Sun May 4 03:35:46 AEST 2008

> >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

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