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From: brianw@microsoft.UUCP (Brian WILLOUGHBY)
Newsgroups: comp.sys.apple2
Subject: Re: Multitasking on a II
Message-ID: <60167@microsoft.UUCP>
Date: 31 Dec 90 21:25:00 GMT
References: <11297.apple.net@pro-angmar>
Reply-To: brianw@microsoft.UUCP (Brian WILLOUGHBY)
Organization: Microsoft Corp., Redmond WA
Lines: 39

m.tiernan@pro-angmar.UUCP (Michael Tiernan) writes:
>In-Reply-To: message from floyd@pawl.rpi.edu
>
>One thought, the 8088 and subsequent processors of that bastardized family
>have all had one little thing that none of the others (6502, Z80, etc) have
>had and that's some kind of instruction that CANNOT under ANY circumstances be
>preempted.  Now, I am not dead sure of the 8088 but the children of it have
>had this instruction.  Now you can't still say that this legitimizes the idea
>of multiprocessing (or any other word prefixed by "multi") but it did allow
>you to do more than you can without it.
>
>But as has been shown many times you ain't close without hardware level memory
>management.
>
><< MCT >>

What sort of "preemption" are you expecting on an Apple ][ system,
anyway?  If there were multiple processors, or even multiple bus
masters, then perhaps there might be some trouble.  But the truth of
the matter is that only interrupts can stop the flow of a program
running on a 65x02, and all instructions of the 6502 are
non-preemptable by interrupts.  If you need to protect multiple
instructions, then you will need to use the interrupt mask.

I'm not saying that this 80x86 instruction is not useful, but I am
saying that the design of the Apple ][ system would not be benefitted
in any way by the existence of such an instruction.

There are a few peripherals which use DMA for data transfer on the
Apple ][, but there is no harm in preemption here, since those
peripherals do not directly affect the program flow.  They merely
move data to/from memory - so as long as you don't try to overwrite
your system semaphores, there will be no problems due to preemption.

Brian Willoughby
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