Re: Digest from [email protected]

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.
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