Computers: Portable computers

Portable computers

At the moment, the largest hardware growth area within personal computing is the personal computer. As in other areas, PC-compatibles dominate the portable scene, with offerings from most manufacturers.

Portable computers fall into four categories:

• Mains-only portables, which in effect are small desktop PCs which can be easily carried around but which can't be used 'on the move'. Typically, these boast most of the facilities of desktop PCs - including hard disks - but weigh about 15 lbs.

• 'Laptop' computers which also offer most of the facilities of desktop PCs but which include rechargeable batteries and so can be used on the move. These may weigh as much as mains-only portables, and the batteries may only run for about 3 hours between charges.

• 'Notebook' computers which are much lighter - they weigh 6 lb or less - but which may lack some of the facilities of desktop PCs, such as hard disks. Some notebook computers, such as the Poquet and the range from Psion, are not PC-compatible, and they may have keyboards which are rather small and awkward to use, but they have features which nevertheless make them tempting buys for people who want to write letters etc. on the move, such as a low price, a long battery life, and a low weight.

• Hand-held ('palm-top') computers, such as the Psion Organiser and the Micro writer Agenda (see Figure 2.7). These are more like electronic Filofaxes than computers, enabling you to maintain diaries, address lists, etc. in a box which weighs about 1lb and which is about the size of a Filofax. They may include simple word processing and record keeping, and the Agenda includes a cable which enables you to transfer your work to a PC. They are limited by their tiny screen displays, which typically show 20 characters across by four lines deep, and by the tiny keyboards. However, the Agenda includes special 'Micro writer' buttons which, once you learn the various combinations, enable you to achieve a reasonable typing speed.

Leaving palm-top computers aside (as they are too tiny and awkward for normal use), the advantages of portables over ordinary desktop machines are:

• They occupy much less space on the desk. I am writing this book on a portable computer at home, where I would have problems trying to fit a desktop computer on my desk.

• They can be easily carried around for use in other locations, or, in the case of notebook computers, for use on the move.

The first portables lacked many of the features that were standard on desktops, such as large hard disks and good displays, and they were very expensive and fairly heavy. The latest models have come down in price, they offer excellent displays including colour on some models, they are quite light, and they have all the power of their larger brethren. It is reckoned that in few years portable PCs will become the most popular type of computer.

Figure 2. 7 The Micro writer Agenda

Computer buses

The 'bus' in a computer is the wiring along which all the bits that make up each item of the information travel in parallel. An important feature of the IBM PC's bus is its expansion capability- it provides a series of slots into which expansion cards can be added (see page 47). The original PC bus allowed only 8 bits to travel in parallel; when it introduced the AT in 1984, IBM doubled the number of wires in the bus so that it could handle 16 bits, and it also doubled its speed to 8 MHz. The AT bus has proved very successful, there are a large number of expansion cards available for it, and it is still the standard in the PC world. It is now referred to as the Industry Standard Architecture (ISA) bus.

One of the main features of the IBM PS/2 range of computers which were introduced in 1987 was the radically new Micro Channel Architecture (MCA) bus. This offered a number of advantages over the AT bus:

• It doubled the number of wires to handle 32 bits.

• It could run much faster.

• It offered improved multitasking capabilities.

• It allowed expansion cards to use the bus without imposing additional work on the CPU.

• It allowed PS/2 computers to be directly linked to IBM mainframe and minicomputers and so act as terminals.

However, it was not compatible with the AT bus, which meant that PS/2 computers lost an essential element of PC compatibility, for they could not use any existing expansion cards. ·

To compete with the superior technology of MCA, other PC manufacturers led by Compaq announced (in 1988) that a new Extended Industry Standard Architecture (EISA) bus would be developed. Like MCA, this would be a 32-bit bus and would run at a high speed, but it would be compatible with the ISA bus and so take existing expansion cards. The EISA bus is now available, and is provided as standard on fast 80486 machines.

Being 32 bits, MCA and EISA buses are significantly more expensive than the 16-bit ISA bus. At the present time they have little to offer most users, as few expansion cards are able to take advantage of the high speeds that they allow.