----- Original Message -----
From: "Steve Thompson" <ste...@copper.net>
To: <ASSEMBLER-LIST@LISTSERV.UGA.EDU>
Sent: Monday, August 06, 2018 4:21 AM
Subject: Re: EX
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.
Exception, EX.
Of course. In the context, EX can modify everything.
And anyway, why would you want to EX an EX?
That results in S0C3 on MVS or PIC 3 in any other
O/S environment (DOS, TOS, VM, CMS, etc.).
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