Caveat - this page gives information on the "PC" view of the world. There are other suitable machines available, for example Apple's iMAC. This page is based on actual experience and is not necessarily an endorsement of one technology over another.

It's time to de-mystify computer adverts. Here are some points to consider when buying a computer.

Take a typical advertisement for a PC:

The phrases the "Intel Pentium II processor 350MHz " and "Intel 440BX AGP set " cover the "Central Processor Unit" (CPU) description. Sometimes known as the "brains" of the computer this part runs the instructions given by the software. The CPU is housed on a "motherboard" (a term that also appears in some ads) which also houses RAM and control chips.

The 350MHz of the example above refers to a clock speed which serves as an indicator of how fast the processor can carry out instructions. The power of microprocessor chips is measured by their ability to quickly process instructions; the measure of this is indicated by the clock speed (in MHz). Generally speaking, faster is better but also more expensive. If a choice has to be made, the determining factor is the speed required by the software running on the machine. Each software package will contain a specification describing its requirements. 

The "Maths co-processor", referred to above as " Intel 440BX AGP set", increases the speed at which mathematical calculations can be performed. All Pentium chips (and equivalents from other manufacturers) will contain a maths co-processor. The "440BX" indicates a high bus speed of 100Mhz. Bus speed determines how quickly messages can be interchanged by the processor and the other components on the motherboard. Some busses work at 66MHz, a Pentium II from 350MHz upward will have the BX chipset and work at 100MHz.

This type of memory is required because the CPU doesn't access the hard drives directly. Any currently active programs (e.g. a word processor document that is being typed) stores information into RAM and the CPU changes it as instructed by the user. The information is saved to the hard drive permanently when the file is saved using the command in the program. Once the computer is switched off, the RAM contents are lost.

This block of memory stores frequently accessed instructions to increase the speed at which these instructions can be carried out.

A reasonable sized cache has a significant impact on the performance of any CPU. "Pipeline burst" cache allows several operations to be carried out simultaneously. "L1" refers to cache which is most immediately available to the CPU as it is housed on the CPU chip. Typically, processors would have about 16K of this type of memory. "L2" originally referred to cache which was housed on a separate chip (on the Pentium). The Pentium Pro and the Pentium II, however, have both L1 and L2 on the CPU chip and so have faster access to the L2 cache. The "512KB Internal Cache" refers to L2 cache in this case.

The hard drive contains all the permanent information held in the computer. Again size matters - 4Gbyte drives can hold approximately 2 million pages of text. This may seem more than you'll ever need, but multimedia files (i.e. those with video clips, pictures or sound included) will easily take up several Mbytes. The operating systems and software packages will also take up a considerable amount of disk space. Microsoft's Windows '95, for instance, takes up in the order of 100MBytes in operation. Home Essentials uses about 90MB. Office '97 uses 192MB.

The speed of a drive may also be quoted, in either rpm (large, say several thousand revs per minute), access time (small, say 10ms) or transfer rate (in Mbytes/sec, 33 is good).

 Different types of hard drives exist. The most relevant are IDE (an early, obsolete type), EIDE (a faster version) and SCSI (pronounced "scuzzy") an expensive fast drive. EIDE is equivalent to ATA-2. The ATA-33 (as advertised) may also be referred to as Ultra DMA or Ultra ATA. This is a faster (33MB/s) drive than ATA-2 which also performs error checking (this improves the accuracy of data transfer).

The important measures for the video card are memory size, resolution and refresh rate. The latter two also apply to the monitor. The larger the memory on the video card, the more natural the movement in a video clip will appear.

The resolution determines the sharpness of the picture and the refresh rate the amount of flicker. Common resolutions are 640x480, 800x600, 1024x768, and 1152x870. A picture looks better if it can provide these resolutions at high refresh rates (low and unacceptable 45Hz, reasonable 70Hz). Both the video card and the monitor have to be capable of these rates.

A larger screen (monitor) is good for Computer Aided Design packages. Screens come in a range from about 14" to 21". The term SVGA implies a monitor that can handle high resolutions, so long as the video card has 2MB or more RAM.

These are used to slot in various adapters. The sound, video and modem cards are slotted into these, for instance. The quoted slots should be in addition to these. PCI are the newer type, most adapters are built to fit these. Older cards fitted into ISA slots. The type of things these might come in useful for include scanner driver cards, network cards (for setting up LANs), ISDN adapters (for fast access to the Internet), SCSI adapters.. ....etc.

 These are now commonly used to load software packages. Each CD can hold about 600MB. The speed of the drive is measured against the speed of a typical audio CD player, hence 2X, 32X etc. Faster is generally better, the minimum specification sold currently is 24X.

The next progression is to DVD-ROM. One of these devices (similar in shape and size to CDs) can hold, currently, 4.7GB. This is expected to go to 17GB in the near future. If a packaged PC is offered with this drive, check that it will read CDs also.

A modem is required in order to link to the Internet or send faxes from a PC. Speeds from 14.4kbits/sec (almost defunct) to 56.6kbits/s are available. Higher is better, although in reality there may not be much difference between 33.3 and 56.6k. Check that a 56k modem meets the standard "V.90".

Three types are common, desktop, tower and mini-tower. The choice is a matter of space and personal taste. Desktops sit on the desk, usually but not necessarily under the monitor. Towers sit upright on the floor or beside the monitor. It can be easy to kick tower systems if they are housed under a desk! A tower is big enough to fit extra hard drives; a mini-tower is the more usual offering.

Current MS Windows offerings for operating systems are Windows '95, Windows '98 and Windows NT. The last of these designed for server systems. For a bundled PC Windows '98 is most likely to be offered.

This is a standard piece of equipment; although ergonomically designed ones may be useful if much repetitive work has to be done.

The three major types of printers are Dot Matrix, Ink jet and Laser. Dot Matrix is very crude; Ink Jet provides good quality at a reasonable price. Use a Laser Jet if you need professional output. Remember to count the cost of maintaining printers, as toner cartridges and colour refills can be expensive. It is a necessary item, however. The paperless office is still a pipedream. Ensure that cables to connect your PC to the printer are included in any offer.

These devices are chiefly used for transferring information from one PC to another. Some software is sold on floppy disk, but most is now provided on CD-ROM. Present day floppy drives take 3.5" disks that can hold 1.4Mbytes of data. These are HD (high density) disks, using older DD (double density) disks gives a maximum of 720k.

A recent development is the 120MByte disk which can also read 1.4MB ones. This may be useful, particularly as a backup device.

 Once your hard disk is beginning to fill up with files of your own making, it's a good idea to back up data on a regular basis. This can be done using floppy disk drives using 3.5" disks. With compression techniques, it is possible to fit a large amount of data onto these disks, but a full back up of a 6.4G disk is going to take most of somebody's day!

A quicker method is to buy a plug-in Zip drive that puts a large amount of data onto a small tape in minutes.

Other useful links -

PC basics - a site hosted by Intel which "does what it says on the tin". Aimed primarily at parents buying PCs for sons / daughters, it would also provide a useful reference for teachers who want to get up to speed.

Apple iMAC - a site hosted by Apple for a non PC (that's non Personal Computer, not non Politically Correct!) view of the world. The iMAC concept is particularly aimed at internet access and might also be asuitable solution for a school's ICT requirements.

On line dictionary of computer terms - a site hosted by Imperial College London, which provides a glossary and search facility for computer terminology. A bit technical for people coming to grips with the technology, but a useful reference for the odd term that baffles you.