Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!world!jeffb
From: jeffb@world.std.com (Jeffrey T Berntsen)
Newsgroups: comp.sys.apple2
Subject: Re: Multitasking on a II
Message-ID: <1990Dec20.143649.11598@world.std.com>
Date: 20 Dec 90 14:36:49 GMT
References: <289@generic.UUCP>
Organization: The World @ Software Tool & Die
Lines: 21

ericmcg@pnet91.cts.com (Eric Mcgillicuddy) writes:

>The 65C02 and 65816 have two uninteruptible instruction, TSB and TRB. These
>are Test and Set (or Reset) Bit. Perhaps the *RESET line would prevent
>interuption, but nothing else will stop them from completing their function
>(usually marking a semaphore). Any subsequent accesses to the semaphore will
>show it as being in use. I hope Prodos uses this for the "PRODOS_BUSY" flag
>and for locking Handles. (?).

I know that ProDOS couldn't use them because ProDOS runs on a ][+ or unenhanced
//e both of which have plain 6502's.  The original 6502 doesn't have those
instructions.  ProDOS uses shift instructions for its busy flag; that was 
documented in a technote someplace because there was a bug in it at some point
(fixed now).  I don't know how ProDOS locks file handles or even if it does
at all.  I'd be interested in finding out myself but don't feel like
disassembling ProDOS right now.

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Jeffrey T. Berntsen                   |  Looking for a good .sig
jeffb@world.std.com                   |
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