>
>IBM used to have a processor that was used internally in control 
>units that allowed context switching between tasks by setting a 
>single byte. The switch bought a dedicated address space, registers 
>and all. No save restore of anything needed. Really neat for some 
>special apps, but not for general use.

Zilog's Z-80 had the alternate register set.
In embedded systems, we dedicated those to the ISRs, to minimize latency.

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