Cache controllers, can Xenix use them?
was-John McMillan
jcm at mtunb.ATT.COM
Tue Mar 21 10:01:25 AEST 1989
Probably the best advice on this topic was to read a recent [PC-mag., Byte?]
article on this particular cache. I did, and I DIDN'T re-submit
clarifications. SBC's note suggests some are needed. (I'll also
point out that I believe I stated up front that I was NOT addressing
THIS cache directly as I hadn't seen the spec's on it.)
In article <422 at brian386.UUCP> news at brian386.UUCP (Wm. Brian McCane) writes:
>In article <1425 at mtunb.ATT.COM> jcm at mtunb.UUCP (was-John McMillan) writes:
>> Caches usually require kernel software for:
>> 1) Boot-time checkout (validation);
Nothing in the article indicated any validation is performed.
>> 2) Defective cache shutdown/workaround;
Nothing in the article indicated any shutdown mechanism is available.
>> 3) Context-switch flushing;
This cache is placed between Mem Manager and Phys Mem: it looks at
Phys Memory only. (Placing an cache where it reads Virtual Memory
addresses is probably a pretty stupid thought on my part.)
>> 4) Memory-mapped hardware cache-BLOCKING;
Nothing in the article indicated any mechanism is provided to permit
memory-mapped hardware addresses. This would be gross. More information
is needed.
>> 5) DMA-overlapped page flushing/blocking;
One of this caches features is that it monitors the Physical Mem Address
lines and disqualifies any entries it contains for DMA-addressed memory.
>> 6) Text-loading cache-flushing (split text & data caching);
(Ref: (3) above. This point is irrelevant if physical memory is cached.)
...
>Cache checking should be done in the power up routines just like the
>keyboard. And if it is defective, the system probably won't boot,
>since I believe cache off on most systems means it doesn't cache, instead
>it just passes the data blindly.
Some systems can run with their cache disabled. AT&T has even
distributed benchmarks of their machines where they'd forgotten
to enable the cache: ouch! (Generates some embarrassingly low
throughput numbers.)
jc mcmillan -- att!mtunb!jcm
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