HDD Cache Vs CPU Cache

[fault]

Hey, here's a newbie question sorry...

I was wondering what's the difference between HDD cache and CPU cache? Is CPU cache what you call "registers"? And is HDD cache just like onboard RAM or something? It's not registers as well is it? And how come CPU cache has two levels?

Thanks.

[jamend]

Modern CPUs actually have 2 or even 3 kinds of cache, excluding the registers. CPU cache stores information that was retrieved from slower RAM based on whether it expects to need it again. This saves it a subsequent trip to the RAM because it can just retrieve the same thing from the much faster cache. HDD cache follows a similar principle in that it stores data from the platters based on what the operating system is doing (and the cache is of course much faster than reading/writing from the platters).

[fault]

Ah ok... thanks for that. So what sort of memory technology is your typical CPU/HDD cache built on?

[Fred Derf Veteran]

  fault said:

Hey, here's a newbie question sorry...

I was wondering what's the difference between HDD cache and CPU cache? Is CPU cache what you call "registers"? And is HDD cache just like onboard RAM or something? It's not registers as well is it? And how come CPU cache has two levels?

Thanks.

The HDD cache buffers hard disk access and the RAM cache buffers memory access.

The HDD cache need not use the same high performance cache memory that the RAM cache uses because disk access is much, much slower than memory access. (Hard disks are measured in miliseconds while RAM is measured in nanoseconds).

CPUs can have up to three levels of cache (at least).

Generally, the first CPU cache is an instruction cache that is designed to help branch prediction in order to prevent the performance loss that comes with the long pipelines of the new CPUs when they encounter a branch. Essentially it stacks the instructions so that the CPU can process them more efficiently. The second cache is a high speed buffer between the CPU and the RAM memory that's on the motherboard.