[Gllug] Atomic increment/decrement

[Jonathan Dye]

> -----Original Message-----
> From: t.clarke [mailto:tim at seacon.co.uk]
> Sent: 07 August 2002 14:40
> To: gllug at linux.co.uk
> Subject: [Gllug] Atomic increment/decrement
> 
> 
> I have no ideas about mutexes - never fortunately had the need.
> 
> But I do remember many years ago having to do something 
> vaguely similar on
> a TI990 computer.
> 
> As I recall, the fundamental issue was one of getting the CPU 
> to do an atomic
> operation (of any sort) by first using a atomic 'test and 
> set' cpu opcode.
> The routine was in assembler and did something along the lines of
> test and set a flag word
> if the set succeeded you have control so do the atomic operation
> and reset the flag word
> if no success spin for a bit and try again
> 
> The whole thing depended on an atomic cpu test-and-set 
> instruction and was
> written in assembler.
> 
> I assume that unless the C language supports such a 
> test-and-set cpu instruction
> directly, you will have do write something in assembler to 
> gain access to the
> Intel X86 opcodes directly.

It's not supported by the c lanugage but it is somtimes a system call that
is implemented in the kernel so that you can have atomicity (is that a
word?) across threads/processes/processors.  It would probably be done using
atomic cpu instructions (or with kernel locks).

JD

> Tim

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