On Fri, 15 Apr 2022 at 03:06, Ian Worthington <00000c9b78d54aea-dmarc-requ...@listserv.uga.edu> wrote: > > Thanks Seymour, it had never occurred to me they were totally identical apart > from CC and sign extension. I'd always had a suspicion that there might be > something that happened at the boundaries that I hadn't considered! > Good to know.
This is also true of some related things in the architecure. I spent an embarassingly long time some years back wondering why there were only Rotate Left versions of instructions like RLL/RLLG and the set of Rotate Then x Selected Bits. Of course it's obvious that if you want to rotate right, say 5 bits, you can instead rotate left 59 bits (or 37 for a 32-bit register). What I wasn't seeing right away is that you can specify -5 as the rotate left amount on the instruction (immediate field for RSxBG or in a register and/or displacement for RLL[G]), and the truncation of the resulting twos complement representation of -5 is, duh, 59. Or 37, if you like. Tony H.
