--- I letter concerns the cache. My mac has a 256k external cache and no backside(0k) cache. How high can caches go? Which is more important-external or backside? BTW-What do they do? They relate to ram? processor? 8600 400Mhz 608MB ram 9.1 4 hardrives(SCSI, firewire, ATA, and original. Sweating up a storm in S.Texas>>David
Cache is a special type of memory that operates between the CPU and Memory. Everytime the CPU requests a chunk of memory the cache looks to see if the requested data is stored in the cache. If it is available the cache supplies it much sooner than the memory can. If it is not available it is read from memory and a copy is stored in the cache in the event it can be used again. When new data is put in the cache it replaces older data. A larger cache has a higher chance of having any given piece of data in it.
There are different levels of caches. All PPC processors have an L1 cache which operates at the full speed of the CPU and is part of the processor chip. I don't recall the size but it's likely no more than 128K and probably less. This is the most effective cache but also the most expensive.
L2 cache operates at the speed of the computer's bus but still faster usually than the memory. L2 cache usually resides on the motherboard and is often in a DIMM module.
In between the two is the backside cache. It typically operates at 1/3 to 1/2 the speed of the processor. Backside cache was introduced into the PPC line with the G3 and is supported in subsequent processors (G4 and G5s so far). It uses memory external to the actual PPC chip and is typically mounted on the CPU daughter board or ZIF. Backside cache is what gave the G3 it's big performance increase over the prior designs. It is a much faster cache than the L2 (but not as fast as the L1) but has a size comparable to the L2.
Cache usually maxes out at 1Mb but I may have seen 2Mb. L1 cache is usually not even listed unless you look up the CPU. AFAIK only machines that shipped with pre-G3 processors had L2 cache, everything later has Backside cache.
Backside cache is rated both by size and speed. Speed is usually rated as a fraction of the CPU speed. 1/2 speed (or 2:1) is preferable. While some processors boards have the capability of 1:1 cache it's often iffy at best. I've no solid data to back this up but my feeling is that a 2:1, 512K cache is better than a 3:1 1024K cache. 2:1 1024K cache beets both of course.
The computer design must deal with issues involving the cache:
Flushing the cache after startup so the cache(s) data matches the RAM.
Only using cache for RAM and not for IO (which is accessed as memory).
Flushing parts of cache when Virtual Memory swaps RAM to/from Disk in which case the RAM and cache won't match.
I do hope this reply teaches you not to ask silly technical questions. ;) -- Clark Martin Redwood City, CA, USA Macintosh / Internet Consulting
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