4 MB Level 2 cache

[beachycove]

While looking for something else, I just turned up in this document, note #2, that pci Macs in the 7300-8600 range support a theoretical maximum of 4 MB of Level 2 cache. (9500 and 9600 machines are exceptions because L2 cache is soldered to the logic board and use of the dimm slot for extra cache seemingly does nothing.)

Given that some tests suggest that 1 MB of L2 cache can nearly double the performance of a 604e machine, I wonder what 4MB would do for one?

Compatible 4 MB cache dimms were never manufactured, were they?

 

[Unknown_K]

The affects of cache is limited, over a certain amount it doesn't add much if any speed increases because of other factors. If the bigger cache chips are the same form factor as the regular ones and you are handy with surface mount chips then swap some out and benchmark it. If the benchmark program fits completely in the cache then you will see exactly how fast the CPU is.

If you wanted to see major speed inprovments in computers then make the RAM speed faster then the FSB of the CPU so you don't even need cache at all. Somebody years ago shipped a PC (386 maybe) with SRAM for main ram because for a brief instant it was cheaper then the normal DRAM used. SRAM was what was used for cache back then and much faster then DRAM, so the CPU ran with no wait states and was a screamer.

 

[OtakuMegane]

4MB would probably give some improvement but not likely enough to be worth it. There were a couple 2MB cache DIMMs made way back but it never went from there because the performance gain wasn't big and they cost way too much.

One of the problems was the cache didn't run anywhere close to CPU speed like the onboard ones today. It was better than having just the aging DRAM memory bus of the day, but still limited.

 

[trag]

Also, large cache can actually diminish performance. The larger the cache, the longer is spent examining the TAG something-or-other, which is the lookup table for the cache. Larger cache means either a larger table, or larger cache segments, which means that swapping data into and out of the cache takes longer.

Either way, there's a point at which the cache lookup time becomes longer than a simple access time.

Performance vs. cache size is a steep curve which plateaus rather steeply--or whatever the opposite of steep would be in this case. Perhaps, plateaus sharply.

 

[beachycove]

That explains a good deal.

Now all I need is a source for an affordable 1MB cache dimm for my 2x604e/200MHz 8600.

Were there really, as O-M suggests, 2MB dimms available?

 

[trag]

Were there really, as O-M suggests, 2MB dimms available?

If he says so, I see no reason to disbelieve him. I never saw any that I remember, but that doesn't mean they didn't exist. There was a lot stuff supported for those PowerSurge machines which was either never built or built just long enough to discover there was no demand.

For example, according to the Apple Hardware Design Notes, the 7200 can use 256 MB DIMMs. As far as I know no one ever made any 256 MB DIMMs for the 7200 but that would have been kind of cool. It would have taken the RAM capacity to 1 GB, which is very believable, because the 7200 uses the same ROM as the PowerSurge family, and the memory map seems to be largely a function of the ROM.