hide random home http://www.zdnet.com/pcmag/issues/1507/pcmg0097.htm (PC Press Internet CD, 03/1996)

PC Magazine -- April 9, 1996

Cache Confusion

While researching hard disks, I found drives available with and without caches. I don't understand: Isn't the external L2 cache sufficient? Does it make sense to buy a hard disk with a cache if you already have an L2 cache? And how do things work if you have two caches? Are the two caches seen as one memory area regardless of combined cache size?

    J. L. Sundstrom, via the Internet

PC MAGAZINE: Your confusion is understandable, but the explanation is pretty simple. The cache on a hard disk has nothing to do with the L2 cache. A hard disk cache consists of memory built into the hard disk's electronics. It usually results in better performance: Access to the hard disk data is quicker, because some of the hard disk's data is held in memory right on the drive. This makes it possible to spool the data to the hard disk controller at the same time that the drive heads are moving to the next location on the drive where data is needed.

The L2 cache, on the other hand, exists as a buffer between the system memory and the CPU. It holds recently accessed data, so the next piece of information the CPU requires will likely be there when needed. The CPU can access the faster SRAM (static RAM) used for the L2 cache memory in less time than it can access the slower DRAM (dynamic RAM) used for the system memory.

The cache on the hard disk works between the hard disk and its controller, while the L2 cache acts between the CPU and the system memory. Thus, they perform similar but separate tasks.

--Alfred Poor

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