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Date: Mon, 16 May 2011 07:38:13 -0700
From: "Andy \"Krazy\" Glew" <andy@SPAM.comp-arch.net>
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Subject: Re: What are the various alternate uses for the PC's LSB ?
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On 5/16/2011 5:22 AM, Terje Mathisen wrote:
> Andy "Krazy" Glew wrote:
>> On 5/13/2011 10:53 AM, Terje Mathisen wrote:
>>> Test&Set, in the form XCHG reg,[mem] is the original x86 sync primitive,
>>> the main problem is that it only provides a boolean gate, not something
>>> which can be used as-is.
>>
>> Nit-pocking:
>>
>> XCHG is atomic exchange, not test-and-set.
>>
>> test-and-set is something more primitive.
>
> Nit-picking indeed:
>
> IMHO xchg is the more primitive, since you need to wrap it in a loop to
> aquire an access token or similar:
>
> mov ax,1
> ;; lea bx,[flag_variable]
> spinloop:
> xchg ax,[bx]
> test ax,ax
> jnz spinloop
>
> Terje


Terminology check:

dst_or_flag := bit-oriented-test-and-set(location,bitnum) ::
     <atomic>
         tmp := load( location ) ; old value
	dest_or_flag := (tmp >> bitnum) & 1;  old value of bit
         tmp |= 1<<bitnum;
         store( location, tmp )
     </atomic>

dst_or_flag := int-oriented-test-and-set(location) ::
     <atomic>
         tmp := load( location ) ; old value
	dest_or_flag := tmp;  old value of int, or maybe compare to zero
         tmp = 1;
         store( location, tmp )
     </atomic>

dst := xchg(regnew,location)
     <atomic>
         dest := load( location ) ; old value
         store( location, regnew )
     </atomic>

dst := cmpxchg(cmpval,regnew,location)
     <atomic>
         tmp := load( location ) ; old value
         dest := temp
         if( dest == cmpval )
             tmp = regnew
         else
             tmp = tmp
         store( location, tmp )
     </atomic>

The test-and-sets always need to be wrapped in a spinloop to acquire an 
access token or similar.  Called a test-and-set spinloop.

XCHG can be treated as a test-and-set, and wrapped in a spinloop.
Or you can use a Graunke Thakkar lock, and insert onto a queue, no spinloop.

Ditto CMPXCHG, except the non-spinloop queue insert for CMPXCHG is 
better known.

This is why I call test-and-set more primitive: because it typically 
only sets a single bit, or sets an integer to a fixed value, it cannot 
be used for queue insertion.  But is easier to implement externally.