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From: brianr@tekig5.PEN.TEK.COM (Brian E Rhodefer)
Newsgroups: comp.sys.amiga.tech
Subject: Interrupt Servers
Keywords: A6 scratch
Message-ID: <6493@tekig5.PEN.TEK.COM>
Date: 29 Jun 90 00:32:59 GMT
Distribution: na
Organization: Tektronix Inc., Beaverton, Or.
Lines: 25

My program uses a Vertical Blanking Interrupt server whose only
function is to signal a task every N frames.  To call Exec's
Signal() function, of course, the server loads A6 with &SysBase,
but I have a few misgivings about this.

My copy of the Exec volume of the RKM, in the chapter on Interrupts
(Ch 7, I believe), says that Interrupt Server routines can't count
on the contents of A6, and that only regs D0-D1/A1/A5 are scratch.

I disassembled the Vertical Blank interrupt (for my system), and found
that D0-D1/A1/A5-A6 are all saved & restored by the Kernel.  With the
explicit warning against depending on A6's contents, it seems to me
that either

 a) if everyone indeed preserves A6, as the RKMs appear to mandate, then
    they should be able to depend on the Kernel's loading of A6 with SysBase
 or

 b) if nobody depends on A6, as the RKMs suggest, and the kernel
    saves&restores it, then A6 can be regarded as scratch

Is the RKM inaccurate/incomplete here?  I don't have to push&pop A6
in my Server routine, do I?

Brian Rhodefer