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From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: net.micro.apple
Subject: Re: Apple //e Interrupts
Message-ID: <239@neoucom.UUCP>
Date: Fri, 1-Aug-86 20:17:08 EDT
Article-I.D.: neoucom.239
Posted: Fri Aug  1 20:17:08 1986
Date-Received: Sat, 2-Aug-86 10:42:20 EDT
References: <283@ski.UUCP> <175@uvicctr.UUCP> <2245@brl-smoke.ARPA> <943@rti-sel.UUCP>
Organization: Northeastern Ohio Universities College of Medicine
Lines: 35
Summary: The disk drive motor

True, it would have been very convenient to make the keyboard
strobe generate interrupts as an option-- that is something I never
understood, as it wouldn't have really cost much to implement.

I've always been impressed at how much it is possible to eake out
of an Apple II+.  The Apple is still one of my favorite machines to
work with.

As far as interrupts and DOS 3.x go.  DOS routines always perform a
SEI instruction before entering time-critical sections.  The
problem crops up because some parts of the ROM code use $45 as a
scratch pad, and so does the BRK/IRQ detection code at $FA40, thus
an interrupt arriving at just the wrong moment can cause deviant
behavior.  I got around that by burning a ROM that had a different
IRQ vector at $FFFE.

I wouldn't recommend changing one's ROM unless absolutely
necessary.  ProDOS has fixed the problems of sloppy interrupt
handling, the only problem is the delay-- oh well.

I always wished I could meet Woz and ask him why the Apple II
avoids all use of the interrupts internally--  I suppose that he'd
remark so as to provide 100% flexibility for peripherals to use
them.

By the way, a little addenda to the previous article... I don't
really think that the apple diskette can be spun backwards, if
inserted nomally, in an unmodified Apple drive.  The spindle motor
is driven directly from the 12 volt supply, and has no means for
polarity reversal or speed control-- otherwise, quite correct.

Bill
NEOUCOM
Rootstown, Ohio
wtm@neoucom  (...!allegra!neoucom!wtm)