[Color Computer] [Coco] 6809 Test and Set instruction?

[James Diffendaffer]

>Don't hold me to the specifics of how it was implemented but I 
>believe the
>6809 designers used CPU handshaking on dedicated pins to get atomic
>read-modify-write cycles.

>There are two main differences between the 6809 and the 6809E. One is 
>the
>clock generator built in to the 6809, the other is the definition of 
>two of those pins that aren't connected in the coco.

>As I said I'm a little vague on the specifics but since 
>multiprocessing
>was implemented by having two CPUs share memory and access it during
>opposite phases of the clock all that they needed to do to keep one 
>CPU
>from stepping on a read-modify-write cycle initiated by the other was 
>to
>signal it not to mess with memory during that cycle.
>
>If memory serves there was a Busy/*BusAvailable pin on the master 
>(6809)
>and a corresponding pin on the slave (6809E) used to arbitrate this, 
>with
>read-modify-write instructions locking the bus until they were done
>(typically 2-3 machine cycles).
>
>If you really need to know exactly how it was done look at the
>Motorola 6809 Programmers manual.
>
>
>HTH
>
>Brett K.Heath


Hah!  I knew I saved this link for a reason!
http://www.zetetics.com/bj/papers/6809cpu.htm








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