31 Jan
2022
31 Jan
'22
11:57 p.m.
On 1/31/22 5:18 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.
Ok, executing mostly out of cache I can see now how that might be
faster, especially on something like an 11/70 with a screaming fast
processor but a slow Unibus interface. Interesting!
-Dave
--
Dave McGuire, AK4HZ
New Kensington, PA
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?