Path: utzoo!attcan!uunet!cs.utexas.edu!swrinde!gem.mps.ohio-state.edu!mips!cprice
From: cprice@mips.COM (Charlie Price)
Newsgroups: comp.sys.mips
Subject: Re: ls -A and the MIPS
Message-ID: <31898@orac.mips.COM>
Date: 21 Nov 89 22:25:31 GMT
References: <14057@boulder.Colorado.EDU> <14101@boulder.Colorado.EDU> <14134@boulder.Colorado.EDU>
Reply-To: cprice@mips.COM (Charlie Price)
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 41

In article <14134@boulder.Colorado.EDU> grunwald@foobar.colorado.edu writes:
>
>How do you force a cache flush? Will a simple O/S call do it as part
>of the O/S overhead?

RISC/os, the MIPS OS, has calls to let any user process
invalidate (clean, flush) portions of the process' virtual space.
The call is:
  cacheflush(start_addr, nbytes, which_caches)

Say "man cacheflush" to receive greater enlightenment.

The only cache flushing that you are likely to need from
the user level is to get rid of "stale" entries in the instruction cache.
The i-cache can contain out-of-date entries when the user
has done either a store or a read(2) into locations used
as instructions.  The i-cache isn't updated automatically as
a result of CPU store instructions or DMA.
When you "flush" the address in the i-cache, it is marked
invalid and the next i-fetch takes a cache miss and gets
the (new) instruction from main memory.


Caution.
MIPS cpu's have a per-page bit in the memory mapping hardware
that allows accesses mapped through the translation buffer to
bypass the cache and always go to main memory.
RISC/os offers a user-level service to make pages of the process'
virtual space "uncached" by setting the no-cache bit in the
page table entry.  Utter
   "man cachectl"
to find out about this service.
You might be tempted to avoid the hassles of invalidating the
i-cache in the right places by making the entire region you
use uncached.
If you do any loops at all executing from uncached locations
is going to be VERY SLOW.  The overhead of invalidating the
cache entries is probably less.
-- 
Charlie Price    cprice@mips.mips.com        (408) 720-1700
MIPS Computer Systems / 928 Arques Ave. / Sunnyvale, CA   94086