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. --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
