Author: Vincent Lejeune
Date: 03:07:57 06/25/02
Go up one level in this thread
On June 24, 2002 at 22:29:38, Robert Hyatt wrote: >On June 24, 2002 at 13:31:20, Bo Persson wrote: > >>On June 24, 2002 at 10:30:26, Robert Henry Durrett wrote: >> >>>This addresses half the problem. What if the microprocessor wishes to WRITE >>>something. Why not write it directly to a huge cache and bypass RAM entirely? >> >>It does, sort of. >> >>The processor writes to the cache, which will update the RAM *eventually*. The >>processor doesn't have to wait for the update, so if you're lucky you will not >>see the delay. >> >>Current processors even have write-buffers queueing data going to the cache... > >Most use "write back" or "copy back" which means that memory is not updated >until the modified cache line gets replaced by something else. At that point, >the modified (dirty) line is first flushed back to memory before it is replaced >by something new. With luck, this turns a bunch of memory write operations into >a bunch of cache writes with one memory write later on... > >And then there is "victim cache" to hold stuff that was "displaced" for a bit >(from cache) in case it is needed again soon. > > > >> >>>If you had extremely large caches, couldn't RAM be dispensed with entirely? >> >>Or, if you had fast enough RAM, caches could be dispensed of. :-) > > >Micron used to do this. They used to have the fastest 386/486 boxes running. >Their entire memory was SRAM rather than DRAM with an SRAM cache. Made them >very fast. And much more expensive. Of course, 1 gig of SRAM would be very >big compared to 1 gig of DRAM. The improvement of "all Sdram" souldn't be very big, if I remember correctly , at this very old time (386/486) the figure was : 80% of the access memory were in the cache with 16 KB and 95% for 256 KB cache ! The typical amount of ram was between 4 MB and 16 MB > > > > >>That was actually the case for micros until about 15 years ago, when caches were >>introduced alongside the "extremely" fast 486. >> >>A couple of years before that, 120 ns RAM matched an 8 MHz 286 processor pretty >>well. >> >> >> >>> >>>Bob D.
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