Re: cas latch

  • From: "Stig Hornuff" <hornuff@xxxxxxxxxxx>
  • To: norman.dunbar@xxxxxxxxxxxxxxxxxxxxxxxxx, Mark.Bobak@xxxxxxxxxxxxxxx, jonathan@xxxxxxxxxxxxxxxxxx
  • Date: Tue, 31 Oct 2006 10:11:19 +0000

Hi Norman,

HPUX (PA-RISC 2.0), does not have test-and-set or compare-and-swap normally needed by semaphore operations. BUT it does have a compare-and-clear instruction by which the same thing can be achieved: A true atomic semaphore action.


----Original Message Follows----
From: "Norman Dunbar" <norman.dunbar@xxxxxxxxxxxxxxxxxxxxxxxxx>
Reply-To: norman.dunbar@xxxxxxxxxxxxxxxxxxxxxxxxx
To: <Mark.Bobak@xxxxxxxxxxxxxxx>, <jonathan@xxxxxxxxxxxxxxxxxx>
CC: <oracle-l@xxxxxxxxxxxxx>
Subject: Re: cas latch
Date: Tue, 31 Oct 2006 08:34:37 +0000

Morning Jonathan,

>> Correct,  HP doesn't do CAS.
So does that mean that HPUX doesn't have an atomic 'test and set'
machine code instruction then ?

If so, how on Earth can Oracle emulate one for the CAS latch. I'm of
the belief (but I'm easily led) that a latch operation has to be atomic
so that it cannot be interrupted half way through and hijacked by
another process.

The fact that HPUX doesn't has such an atomic instruction has caused my
brain to leak out through my ears trying to figure out a way of
implementing a latch in software with a process that cannot be
interrupted. All I can come up with is a disable interrupts at the start
and enable at the end.

Any clues gratefully received. (Purely for my own curiosity.)


Regards, Norman.

Norman Dunbar. Contract Oracle DBA. Rivers House, Leeds.

Internal : 7 28 2051
External : 0113 231 2051

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