On 08/05/2018 08:13 AM, Robin Vowels wrote:
From: "Paul Gilmartin"
<00000014e0e4a59b-dmarc-requ...@listserv.uga.edu>
Sent: Friday, August 03, 2018 3:09 AM
A principal use of EX is to be able to use a register mask to
modify the
target. CDC 3800 had a clever alternative to this, a
modify-next-instruction
instruction (I forget what it was called). The target was
always the following
instruction; execution continued after that instruction -- no
need to branch
around. Its principal use was to enable CDC 3800 extended
addressing in old
CDC 3600 short-address instructions. Addressing was not
otherwise modal.
IBM might have done well to provide a modify-next rather than a
long-address,
pipeline breaking, dreadfully expensive, EX.
(They probably had the discussion and had good reasons not to
do it.)
(Can EX modify the CC mask in a target branch instruction? A sure
branch prediction breaker.)
EX can "modify" everything, but it does not modify the subject
instruction.
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Exception, EX. That results in S0C3 on MVS or PIC 3 in any other
O/S environment (DOS, TOS, VM, CMS, etc.).
