31 Jan
2022
31 Jan
'22
11:18 p.m.
On Jan 31, 2022, at 4:10 PM, Dave McGuire
<mcguire(a)neurotica.com> wrote:
On 1/31/22 3:00 PM, Paul Koning wrote:
Johnny says that RSX said so, for the fast machines like 11/70 and 11/44. And that makes
sense, with basic instruction times under a microsecond and Unibus accesses rather
expensive due to mapping. The software CRC takes 7 instructions per byte, all register
instructions except for one table reference. The KG11 does a bit at a time, with a 10 MHz
clock if I read it right, so that fits the statement that the operation is complete in a
microsecond so software may not need to test the Done bit depending on what machine you
have. At least two unibus cycles plus a microsecond processing time is not obviously
faster than 7 fast CPU instructions.
The KG11 dates back to the 11/20; as I said earlier, it very obviously makes sense on that
machine where software CRC would probably take around 30 microseconds per byte.
paul
What I mentioned is one or several PDP-11
products, available for RSTS and I believe RSX. There definitely is an "RJ2780
emulator" -- the official name may not be quite that. And I think there was also a
3271 emulator, though I'm not sure if that existed on RSTS.
I have no idea what these things do, since 2780 is something I've never used. Some
sort of remote job entry station? Anyway, it is the only RSTS software that uses a KG11.
DECnet doesn't because on RSTS DECnet only supported devices with hardware CRC: DMC
and friends and later Ethernet.
Yes, 2780 is an RJE protocol.
I did an unsupported software DDCMP, but that
does CRC in software (8 bits at a time, the classic table driven fast software
implementation). That probably outruns a KG11 and in any case I never had access to that
hardware.
The KG11 looks like all TTL logic; are you really sure it'd be faster to do it in
software running on similar-generation hardware?