On 8/6/2018 18:12, Dan Greiner wrote:
I was once asked why the execute exception existed. That is, why not just let
the hardware — or, in this odd case, the firmware — cascade down a chain of
multiple EX instructions, ORing the bits of the R1 field with the subsequent
target instruction, whatever instruction that might be. Aside from there being
absolutely no practical reason for wasting circuits on such folly, the answer
is obvious ... the EX instruction could target itself, and the CPU would get
its knickers tied into a knot without an exception.
Obvious, maybe, but maybe not. B * isn't prevented from spinning the
CPU. It has no knickers, and cares not if you carelessly make it work
furiously at nothing.
sas
