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From: ewhac@well.UUCP (Leo 'Bols Ewhac' Schwab)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Official Warnings...
Message-ID: <11394@well.UUCP>
Date: 24 Apr 89 10:51:16 GMT
References: <3082@haven.umd.edu>
Reply-To: ewhac@well.UUCP (Leo 'Bols Ewhac' Schwab)
Organization: Lt. Col. Oliver North, Overseas Shipping and Receiving.
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Quote: "Men, take Binkley's head."  -- Coach Bloom

In article <3082@haven.umd.edu> brett@sylvester-tr.UMD.EDU (Brett S Bourbin) writes:
>	In the Jan/Feb 1989 issue of Amiga-Mail, under the "Official Warnings
>to ROM-Jumpers, Structure-Hackers, and Others", it states that you should
>NOT use the TAS assembly instruction.  What is the reason for this?
>
	A zillion other people have offered explanations.  Now it's my turn.

	TAS uses an indivisible memory cycle to do its work.  It's supposed
to be used in multi-processor systems, where many CPU's share the same
memory.  TAS causes the CPU to grab the memory location and hold onto it
until it's finished modifying it, thereby preventing other CPU's from
possibly inspecting the datum before the modification is complete.  It's a
form of hardware semaphore facility.

	However, the Amiga doesn't have multiple CPU's (the Copper doesn't
count in this instance).  So TAS is not needed, and as has already been
explained, the behavior of TAS on the Amiga is guaranteed to be
indeterminate.

	Use the BSET instruction instead.  It's functionally similar.  It
even runs a little quicker.

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Leo L. Schwab -- The Guy in The Cape	INET: well!ewhac@ucbvax.Berkeley.EDU
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