 The magazine of the Melbourne PC User Group Cache, what is it, and what's it for? Tim Hardman |
Cache - it's a strange word, but quite an important one in the computer
industry these days. Let's look at what it is, what it does and why it's needed.
Basically, cache is memory, whether it be a portion of RAM set aside for that purpose or separate memory
chips. The easiest way to explain cache's purpose is with an example. I'm going to use SmartDrive, a small
TSR program that comes with DOS and Windows.
Once you've installed DOS or Windows, your AUTOEXEC.BAT file will usually execute SmartDrive upon startup. It
will be loaded into memory every time you boot your PC normally. (Note: SmartDrive will usually be taken out
of the startup files if Win 95 is loaded because it's designed to take care of these arrangements
itself.)
Okay, SmartDrive loads itself into memory, but what does it do?
Basically it sets aside an amount of RAM (usually 0.5 to 1 MB of XMS or EMS memory). Then it keeps an eye on
which data or files are being accessed from the hard drive(s). What it's doing is taking an educated guess
about what data will be required next, then it loads that data into the SmartDrive-reserved portion of
memory. When the CPU looks for more data, SmartDrive steps in, asks the CPU if it wants any of the data it
has put into its part of RAM. Usually SmartDrive is smart and the CPU says, "Thanks for the data,
you've just saved me heaps of time," (or it would if the CPU talked and had the sort of manners that require
please and thank you).
Getting the data from RAM is much, much faster than getting it fresh from the hard drive. So there's one
reason cache exists.
Other types of cache
Of course SmartDrive isn't the only cache that exists. There is hard-drive cache and cache on the interface
cards that connect them to the bus and in turn to the CPU.
One of the more important instances of cache these days is the internal cache, built into the CPU itself and
the external cache on the motherboard.
Let's talk numbers first
Most motherboards come with at least 256 KB of external cache with the option of upgrading that to 512 KB of
cache. CPUs generally have 128 KB of internal cache. Intel Pentium Pro and Pentium IIs are exceptions to this
general rule.
Wonderfully smart Intel scientists have decided that, instead of trying to make CPUs tick over so fast that
they melt, they'd build more cache into the CPU. This, believe it or not, greatly improves the speed of the
processor. Thus Pentium Pros come with 256 KB of internal cache, Pentium IIs with 512 KB.
Okay, I hear you asking. We have internal and external cache. But what does the stuff actually do?
Remember SmartDrive? Cache pretty much works as SmartDrive does for a hard drive, only it's doing the
guesswork for RAM, rather than the hard drive(s). One reason cache is so important today is the speed of
modern processors. These days CPUs can accept instructions and data so quickly that we cache, otherwise the
processor would be sitting around waiting for data, files or instructions.
To see just what kind of effect cache has on your PC's performance, you could go into your PC's CMOS setup
and disable internal and external cache and see how much slower your PC runs.
Note: Do not do this unless you know what you are doing or have the guidance of an experienced person at
hand. Never, never fiddle with CMOS without a recent, proven backup from which you can restore your
system.
About the author
Tim Hardman is a part-time Computer Technician, currently studying Electronics Engineering at the Royal
Melbourne Institute of Technology. You can contact him via email at
sylvester@crafti.com.au.
Reprinted from the November 1997 issue of PC Update, the
magazine of Melbourne PC User Group, Australia
