From: "Dan Greiner" <[email protected]>
Sent: Tuesday, August 07, 2018 8:12 AM
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.
It was important to block EX used that way, whether
accidentally or purposely.
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