Accounting Principles V11

Clive W. Humphris

COMPUTER SYSTEMS: Computer Hardware Configuration.  


All computers comprise the same basic hardware structure, built around the central processing unit 'CPU'. Their main differences are in the size or width of the data/addressing busses, registers and the processor clock speed.

Microcomputers originally had an eight bit data bus. Nowadays microprocessors, PC's frequently extend this to a 16 or 32-bit width. Mini and mainframe computers go even further with 64 or greater data bus widths. The speed of a computer is determined roughly by its clock rate. This equates to the time it takes for instructions to be executed. Most instructions however, require more than one clock cycle.

There are four main groups of computers (known as platforms). Desktop and Laptop machines are categorised as 'microcomputers' and may be connected together to provide much greater processing power through distributing their workload across many machines. 'Minicomputers' are the mid range machines and might be found supporting the local computer operations in say a supermarket or office. Providing the central hub for hundreds of individual work stations distributed around a building.

'Mainframe' computers provide the processing power for organisations such as banks, insurance companies and airline operations. They can cost hundreds of thousands of pounds, require air conditioning and usually lots of support staff. The largest of all are the 'Supercomputers' that can cost millions and are reserved for huge number crunching operations, the stock exchange, weather forecasting, or scientific calculations etc.

External Memory (Hard Disk)

The only real difference between the hard disk on a desktop PC and a mini or mainframe computer is its physical size and therefore amount of data it can hold combined with the speed of access and data transfer.

Data is arranged in thousands of individual tracks on platters, stacked to provide multiple disk surfaces. See the disk drive topics for an explanation of how the data is laid out and distributed across the disk surfaces. Hard disk memory is called 'secondary storage' and is permanently stored until overwritten. This type of storage medium is known as 'non volatile' which means the data remains even when the computer power is turned off.

Central Processing Unit (CPU)

The CPU is the heart of the computer and carries out the functions of moving data between the various registers and peripheral units under the control of computer software.

Before the CPU can perform any useful function or run operations such as word-processing or spreadsheets it has to have a piece of software loaded called the 'operating system'. 

This software ensures the computer is configured correctly and all input and output devices can communicate with one another. On your PC this operating system is called 'Windows '95, '98, 2000, NT, XP or Windows 7. Below that in the operational hierarchy is a another level called DOS (disk operating system) which handles all the file operations.

Input Devices

A computer relies on input data and there are a whole range different methods of collecting it. 

P1str = P1str & "Data doesn't only consist of customers names and addresses, it could also come from continuously monitoring a sensor in a Power Station for example, to ensure the generator voltage output is adjusted in response to consumer demand. Another might be product scanning in a supermarket or pick up from engine sensors in a motor vehicle.

No matter where the data comes from it's likely that it will be required to convert it from one type to another. 
Electrical, Gas or Temperature sensors will most probably output analogue voltage levels, which need to be converted into digital binary signals that the computer understands. Customers in a shop for example pay in some form of currency and this needs to be converted into a binary representation within the electronic till and output using a series of light emitting diodes as a visual display.

Output Devices


Output devices comprise those units which enable information or data to be presented in a form that we humans can understand. Generally these include printers, plotters and visual display units (VDU).

However, computer output signals can be used for much more than displaying data. Consider a robot control system, input sensors will pickup signals regarding the position of riveting tool on a car body for example, calculate that it needs to be moved by say 20mm. 
The computer output data is now fed to a stepper motor to move the riveting tool. Much the same as the paper in a printer is moved for a Line-Feed instruction. Output devices are connected to the serial and parallel ports including the latest data port called a Universal Serial Bus (USB) which allows very fast data transfer.

Cache Memory


Cache Memory is made up of static RAM having very fast access times and is often part of the CPU chip (primary cache). Cache operations use special data bussing or pipelining (allowing more than one instruction to be processed at a time) to ensure it has a high priority over other data transfers.

As an intermediate store for data being worked on, the cache will contain data related to that currently being processed, once completed it is then transferred back to the dynamic RAM. If the cache memory is contained in a separate IC then it is called 'secondary cache memory'. There is also a further type of cache memory called 'disk cache' where it provides a faster memory interface between the dynamic RAM and the considerably slower disk drive.

Main Memory (RAM)


Dynamic RAM (Random Access Memory) is a series of registers within an integrated circuit which can be accessed in any order. This is where each binary word consisting of eight or more bits (word) can be individually addressed and read or written to. The 'data word' is normally the smallest group of binary bits that can be manipulated in RAM. For most other memory activities i.e. disk or tape, then data is operated on in much larger groups called clusters.

Dynamic RAM uses transistors with tiny capacitors which store each individual binary digit either a 0 or a 1. One disadvantage of this type of memory is that it has to be continually refreshed to ensure the electrical charge does not leak away. 

It is also volatile in that it will loose its memory contents when the power is removed. RAM is known as a primary storage. In computer specifications DRAM is also stated as EDO (Extended Data Out Random Access Memory) this type has faster access times and less expensive than conventional DRAM. Standard DRAM can only access one block of data at a time, EDO RAM can start fetching the next block of memory at the same time that it sends the previous block to the CPU. DDRS (Double Data Rate Synchronous) is a further type of DRAM, that allows data transfers on both edges of each clock cycle, effectively doubling the memory chip's data throughput, also consuming less power, a major requirement in portable computers.

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