Re: Re: Digest from [email protected]

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Bryan Kattwinkel wrote:
> <div class="moz-text-flowed" style="font-family: -moz-fixed">Phil 
> Roets wrote:
>
>> I am developing a multitask application on a MFC5282 board and 
>> intended to create a semaphore type
> > method to control access to some of the common variables.  Replacing 
> the 'tst.b' with 'tas.b' in the
> > snippet :
>>        move.l   4(%sp),%a0
>>        move.l   #128,%d0
>>        tas.b     (%a0)
>>        jbne     .L4
>>        move.b #-128,(%a0)
>>        clr.b      %d0
>> ..L4:
>>        rts
>> causes a 'invalid instruction for the architecture ...
>
> Because the 5282 does not have that instruction. (tas is ISA-B, 5282 
> is ISA-A+).
>
> Use bset, which sets 1 bit (by number, not mask!) and sets the z flag 
> to the *previous* value of the bit:
>
>     clr.l   D0              /* bit 0 */
>     bset.b  D0,(A0)         /* set one bit atomically */
>     sne     D0              /* previous state */
>     rts
>
> If you need a counting semaphore, you can add/subtract to a long word 
> of memory atomically (in this case the flags represent the state at 
> the end of the operation).
>
> These operations generally have to be written in assembly; typical C 
> compilers will not generate the atomic-memory forms [destination 
> (a0)]. They are atomic with respect to the single 5282 processor; tas 
> would only be needed with multiple CPUs, which I don't think you can 
> do with that chip.
> </div>
Thanks a lot - It helps when one knows the 800 page reference guide a 
bit better!
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