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From: dirks@canoe.cis.ohio-state.edu (william r dirks)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Official Warnings...
Message-ID: <43941@tut.cis.ohio-state.edu>
Date: 18 Apr 89 13:01:11 GMT
References: <3082@haven.umd.edu> <5881@ardent.UUCP> <43928@tut.cis.ohio-state.edu>
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In article <43928@tut.cis.ohio-state.edu> Ethan R Dicks <erd@cis.ohio-state.edu> writes:
>The warning is intended to warn programmers that the TAS is NOT atomic
>in the Amiga because of the possible actions of the custon chips.  Structures
>which are sensitive to updates from multiple processes are NOT adequately
>protected by the TAS mechanism.

 The TAS instruction _is_ indivisible with respect to the CPU itself.
 Though it may be possible for DMA to _suspend_ the instruction and 
 write to the memory location in quesiton, it is not possible to have
 an interrupt in the middle of an instruction.  This is because
 the 68000 does not support instruction continuation.  Once an 
 instruction is begun, it will run to completion before any interrupt
 (including a context switch) is handled.  Guaranteed.

 Now the 68010 and up DO support instruction continuation, but I
 believe the TAS instruction (and CAS, CAS2 on the '020) are handled
 differently (restarted from the beginning) to guarantee an atomic
 operation.  Although I admit I can't find where it says that.

 ______________________________________________________________________
| Name:      Bill Dirks                                                |
| Address:   dirks@baloo.eng.ohio-state.edu                            |
| Quote:     "Survival and preservation must cancel out programming."  |
|_________________________Star_Trek,_"What_Are_Little_Girls_Made_Of?"__|
-=-
 ______________________________________________________________________
| Name:      Bill Dirks                                                |
| Address:   dirks@baloo.eng.ohio-state.edu                            |
| Quote:     "Survival and preservation must cancel out programming."  |