Elements of a Microcomputer

About the elements of a microcomputer

The primary function of a digital computer is the manipulation of data. The three elements which are essential to the fulfillment of this task are the central processing unit, the memory and the input-output interface. These elements are collectively known as the computer hardware, and each element exists physically as one or more integrated circuit chips mounted on a printed circuit board. Where the central processing unit (CPU) consists of a single microprocessor, it is usual to regard the system as a microcomputer. The distinction between microcomputer, minicomputer and mainframe computer is a very arbitrary division made according to relative computer power. However, this classification has become somewhat meaningless, with present-day 'microcomputers' being more powerful than the mainframe computers of only a few years ago.

 

The central processing unit (CPU) part of a computer can be regarded as the brain of the system. The CPU determines what computational operations are carried out and the sequence in which they are executed. During such operations, the CPU makes use of one or more special storage locations within itself known as registers. Another part of the CPU is the arithmetic and logic unit (ALU) which is where all arithmetic operations are evaluated. The CPU operates according to a sequential list of required operations defined by a computer program, known as the computer software. This program is held in the second of the three system components known as the computer memory.


The computer memory also serves several other functions besides this role of holding the computer program. One of these is to provide temporary storage locations which the CPU uses to store variables during execution of the computer program. A further common use of memory is to store data tables which are used for scaling and variable conversion purposes during program execution.


Memory can be visualized as a consecutive sequence of boxes in which various items are stored for a typical memory size of 65 536 storage units. If this storage mechanism is to be useful, then it is essential that a means is provided for giving a unique label to each storage box. This is achieved by labeling the first box as 0, the next one as 1 and so on for the rest of the storage locations. These numbers are known as the memory addresses. Whilst they can be labeled by decimal numbers, it is more usual to use hexadecimal notation.


Two main types of computer memory exist and there are important differences between them. The two kinds are random-access memory (RAM) and read-only memory (ROM). The CPU can both read from and write to the former, but it can only read from the latter. The importance of ROM becomes apparent if the behavior of each kind of memory is considered when the power supply is turned off. At power-off time, RAM loses its contents but ROM maintains them, and this is the value of ROM. Intelligent instruments normally use ROM for storage of the program and data tables and just have a small amount of RAM which is used by the CPU for temporary variable storage during program execution.


The third essential element of a computer system is the input-output (I/O) interface, which allows the computer to communicate with the outside world by reading in data values and outputting results after the appropriate computation has been executed. In the case of a microcomputer performing a signal processing function within an intelligent instrument, this means reading in the values obtained from one or more transducers and outputting a processed value for presentation at the instrument output. All such external peripherals are identified by a unique number as for memory addresses.


Communication between these three computer elements is provided by three electronic highways known as the data bus, the address bus and the control bus. At each data transfer operation executed by the CPU, two items of information must be conveyed along the electronic highway: the item of data being transferred and the address where it is being sent. Whilst both of these items of information could be conveyed along a single bus, it is more usual to use two buses which are called the data bus and the address bus. The timing of data transfer operations is important, particularly when transfers take place to peripherals such as disk drives and keyboards where the CPU often has to wait until the peripheral is free before it can initialize a data transfer. This timing information is carried by a third highway known as the control bus.


The latest trend made possible by advances in very large-scale integration (VLSI) technology is to incorporate all three functions of central processor unit, memory and I/O within a single chip (known as the computer on a chip). These chips are already well established in domestic appliances and vehicle fuel control systems, and their exploitation within intelligent instruments is likely to grow rapidly.


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