Don't trust TAS

[Walter Bays]

>	john at frog.UUCP (John Woods, Software) writes:
>	I would be interested to know how one does multi-processor locking on a
>	SPARC, however (or other RISC processors).
>
In article <2150 at ubc-cs.UUCP> pajari at grads.cs.ubc.ca (George Pajari) replies:
>
>I think that the M68000 provides hardware signals which *may* lock
>the bus *if* the hardware designer has done the bus interface properly,
> [Discussion of system in which TAS did not lock the bus.
> He fixed it in software.]
>As mentioned above one can implement mutual exclusion without
>hardware support (i.e. entirely in software)...a good survey of 
>the techniques is in Alan C. Shaw's book 'The Logical Design of 
>Operating Systems'.

Good point.  Not always necessary for RISC, though.  The Clipper has
a test-and-set instruction.  I would bet the HP Precision series has
a test-and-set, since they have standard multi-processor models.  I'd
think even the purest of RISC architects would implement it.  (Even though
it takes more than one clock cycle :-)  The main reason there are few
RISC multi-processors is probably that few people want to pay for them.
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