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Author Topic: How long does a Cache Miss take?  (Read 2289 times)

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How long does a Cache Miss take?
« on: June 19, 2013, 08:13:55 AM »

I am trying to figure out the impact of software that does a series of random reads from memory.  Reads which never make use of the cache, never reading from the same place twice, cache thrashing software.

On a typical 50Mhz 060 accellerator card or 50Mhz 030 accellerator card for Amiga, how long does a cache miss take when reading from fastram?

I know it has to read in a whole cache line of 16 bytes as a series of 4 burst moves of 4 bytes each.

For some reason I have the numbers 4-2-2-2 stuck in my head but I think that would have been from my 25Mhz 030 A3000 with Static Column Zip RAM.  So I am guessing the numbers would be like 8-4-4-4 on 060 and 030 cards running at 50Mhz?
(meaning a cache miss would take 20 cycles on those cards)

Does anybody know what the real numbers are?


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Re: How long does a Cache Miss take?
« Reply #1 on: June 19, 2013, 10:14:01 AM »

Fast Page and Static Colum RAM both work with the same timing in an amiga system.(AGA might be the exception)

An 68K upto the 68030 is asynchrone, and it takes 4 clock cycles to read or write anything.

At that time Memory access timing was defined not in clocks but in nano seconds.

The Amiga's chipmem which is compatible with Motorola's MCM54400 chips. (A500, A600 & A3000)
tRC of min 130ns on an amiga it depends if you Have NTSC or PAL 139.5ns or 141ns.

MCM54400A-70 timings(incomplete)
CAC - -/20n
tRCD - 20n/50n
tRP - 50n/-
tRAS - 70n/10

CAC - -/20n
tRCD - 20n/50n
tRP - 50n/-
tRAS - 70n/10

To be honest this tells you very little.
just as a rule of thumb RAS times 2 is the minimum cycle time of RAM.