I've got a question regarding the use of the L2 cache card in a Mac 7600 that has an accelerator.
The cache card is a 256k one with an access speed of 40mhz. The accelerator's backside cache is 1MB with an 80mhz. access speed.
Seems as if the card-cache would be slowing down the performance. True or no?
If a higher level cache (lower "L" number, think of backside cache as L1.5) has the data then it doesn't matter what the lower level cache does. If it doesn't have the data and the next lower level cache does then it can get it from that cache which is still faster than even lower level devices.
As a general rule the closer to the processor the cache is the smaller it is and the faster it is. In theory you could do away with cache and just use memory that operates at the processor speed. But the memory would be horribly expensive and the logic board would be a problem having to operate that much memory at that high a speed.
When the processor makes a memory access each cache and the main memory each attempt to get that data. The first one that gets the data is used. The L1 cache will always get data faster but only if it has it. Since it's the smallest cache it's the least likely to have it.
Your backside cache is 1Mb so a 256K L2 cache won't help, the backside cache will always have more cached data than the L2. There is some anecdotal evidence that the L2 cache can cause problems but it isn't common.
And, isn't system RAM a deterrent to speed in a Mac? If it has an acess speed of 100mhz., and the CPU's running at 240mhz. wouldn't this be a limiting factor as well?
Yes this is why a cache is used so repeatedly accessed memory locations can be accessed at a faster speed. BTW the memory in a 7600 is accessed at around 16MHz (60nS). But there is nothing you can do about this. The memory is accessed at a fixed rate regardless of it's rated speed.
If you processor is indeed running at 240MHz then your bus is running at 40MHz and so is your L2 cache.
And of course the bus speed of the Mac, which in this case is 45mhz.
The maximum bus speed on these models is 50MHz but varies from 40-50Mhz depending on the processor. The bus speed is set to an integer, or integer plus 0.5 dividend of your processor clock. The L2 cache always runs at the bus speed.
The backside cache of a G3 or greater usually runs at 1/2 or 1/3 the processor speed.
The L1 cache (built into the processor chip) runs at the processor clock speed.
Each type of cache has a maximum rated speed but it is operated at the speed dictated by the hardware. If the rated speed isn't as high as the hardware is running it, it still operates at the hardware speed but will likely malfunction.
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