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Role of Cache in the pc

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Role of Cache in the PC

In early PCs, the various components had one thing in common: they were all really slow :^). The processor was running at 8 MHz or less, and taking many clock cycles to get anything done. It wasn't very often that the processor would be held up waiting for the system memory, because even though the memory was slow, the processor wasn't a speed demon either. In fact, on some machines the memory was faster than the processor.

In the 15 or so years since the invention of the PC, every component has increased in speed a great deal. However, some have increased far faster than others. Memory, and memory subsystems, are now much faster than they were, by a factor of 10 or more. However a current top of the line processor has performance over 1,000 times that of the original IBM PC!

This disparity in speed growth has left us with processors that run much faster than everything else in the computer. This means that one of the key goals in modern system design is to ensure that to whatever extent possible, the processor is not slowed down by the storage devices it works with. Slowdowns mean wasted processor cycles, where the CPU can't do anything because it is sitting and waiting for information it needs. We want it so that when the processor needs something from memory, it gets it as soon as possible.

The best way to keep the processor from having to wait is to make everything that it uses as fast as it is. Wouldn't it be best just to have memory, system buses, hard disks and CD-ROM drives that just went as fast as the processor? Of course it would, but there's this little problem called "technology" that gets in the way. :^)

Actually, it's technology and cost; a modern 2 GB hard disk costs less than $200 and has a latency (access time) of about 10 milliseconds. You could implement a 2 GB hard disk in such a way that it would access information many times faster; but it would cost thousands, if not tens of thousands of dollars. Similarly, the highest speed SRAM available is much closer to the speed of the processor than the DRAM we use for system memory, but it is cost prohibitive in most cases to put 32 or 64 MB of it in a PC.

There is a good compromise to this however. Instead of trying to make the whole 64 MB out of this faster, expensive memory, you make a smaller piece, say 256 KB. Then you find a smart algorithm (process) that allows you to use this 256 KB in such a way that you get almost as much benefit from it as you would if the whole 64 MB was made from the faster memory. How do you do this? The short answer is by using this small cache of 256 KB to hold the information most recently used by the processor. Computer science shows that in general, a processor is much more likely to need again information it has recently used, compared to a random piece of information in memory. This is the principle behind caching.
"Layers" of Cache

There are in fact many layers of cache in a modern PC. This does not even include looking at caches included on some peripherals, such as hard disks. Each layer is closer to the processor and faster than the layer below it. Each layer also caches the layers below it, due to its increased speed relative to the lower levels:


Devices Cached

Level 1 Cache

Level 2 Cache, System RAM, Hard Disk / CD-ROM

Level 2 Cache

System RAM, Hard Disk / CD-ROM

System RAM

Hard Disk / CD-ROM

Hard Disk / CD-ROM


What happens in general terms is this. The processor requests a piece of information. The first place it looks is in the level 1 cache, since it is the fastest. If it finds it there (called a hit on the cache), great; it uses it with no performance delay. If not, it's a miss and the level 2 cache is searched. If it finds it there (level 2 "hit"), it is able to carry on with relatively little delay. Otherwise, it must issue a request to read it from the system RAM. The system RAM may in turn either have the information available or have to get it from the still slower hard disk or CD-ROM.

It is important to realize just how slow some of these devices are compared to the processor. Even the fastest hard disks have an access time measuring around 10 milliseconds. If it has to wait 10 milliseconds, a 200 MHz processor will waste 2 million clock cycles! And CD-ROMs are generally at least 10 times slower. This is why using caches to avoid accesses to these slow devices is so crucial.

Caching actually goes even beyond the level of the hardware. For example, your web browser uses caching itself, in fact, two levels of caching! Since loading a web page over the Internet is very slow for most people, the browser will hold recently-accessed pages to save it having to re-access them. It checks first in its memory cache and then in its disk cache to see if it already has a copy of the page you want. Only if it does not find the page will it actually go to the Internet to retrieve it.

Level 1 (Primary) Cache

Level 1 or primary cache is the fastest memory on the PC. It is in fact, built directly into the processor itself. This cache is very small, generally from 8 KB to 64 KB, but it is extremely fast; it runs at the same speed as the processor. If the processor requests information and can find it in the level 1 cache, that is the best case, because the information is there immediately and the system does not have to wait.

Note: Level 1 cache is also sometimes called "internal" cache since it resides within the processor.
Level 2 (Secondary) Cache

The level 2 cache is a secondary cache to the level 1 cache, and is larger and slightly slower. It is used to catch recent accesses that are not caught by the level 1 cache, and is usually 64 KB to 2 MB in size. Level 2 cache is usually found either on the motherboard or a daughterboard that inserts into the motherboard. Pentium Pro processors actually have the level 2 cache in the same package as the processor itself (though it isn't in the same circuit where the processor and level 1 cache are) which means it runs much faster than level 2 cache that is separate and resides on the motherboard. Pentium II processors are in the middle; their cache runs at half the speed of the CPU.

Note: Level 2 cache is also sometimes called "external" cache since it resides outside the processor. (Even on Pentium Pros... it is on a separate chip in the same package as the processor.)

Disk Cache

A disk cache is a portion of system memory used to cache reads and writes to the hard disk. In some ways this is the most important type of cache on the PC, because the greatest differential in speed between the layers mentioned here is between the system RAM and the hard disk. While the system RAM is slightly slower than the level 1 or level 2 cache, the hard disk is much slower than the system RAM.

Unlike the level 1 and level 2 cache memory, which are entirely devoted to caching, system RAM is used partially for caching but of course for other purposes as well. Disk caches are usually implemented using software (like DOS's SmartDrive).

Peripheral Cache

Much like the hard disk, other devices can be cached using the system RAM as well. CD-ROMs are the most common device cached other than hard disks, particularly due to their very slow initial access time, measured in the tens to hundreds of milliseconds (which is an eternity to a computer). In fact, in some cases CD-ROM drives are cached to the hard disk, since the hard disk, despite its slow speed, is still much faster than a CD-ROM drive is.

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