Microcomputer Hardware

1.1 Introduction to Microcomputers

Even though electronic digital computers have existed since the 1940's, microcomputers have been around only since the late 1970's. The appearance of microcomputers coincided with the development of large-scale integrated circuits (LSI chips). An integrated circuit is a tiny chip of silicon on which is layered multiple electronic circuits. Figure 1.1 illustrates the size of a common silicon chip.

A chip like the ones shown in Figure 1.1 can hold thousands of electronic circuits containing millions of individual components. In order to connect wires to the chip so that signals can be sent to and from it, the chip must be mounted in a plastic package containing wire leads. This plastic case is called a dual in-line package, or DIP for short. The name is quite descriptive since a DIP consists of two rows of leads on each side of the plastic case. A DIP is shown in Figure 1.2.

Figure 1.2 Chip in a DIP

A typical microcomputer will contain dozens of chips that perform such functions as storing data, routing data, displaying data, and performing arithmetic and logic operations on data. These chips are usually plugged into special sockets like the one shown in Figure 1.3. These

Figure 1.3 Chip and Socket

sockets are then attached to a large printed circuit board called the motherboard. The motherboard of a microcomputer is shown in Figure 1.4.

Figure 1.4 Microcomputer Motherboard

In addition to a variety of chips, the motherboard of a microcomputer also contains a series of expansion slots. These are the long sockets shown in the upper-left corner of Figure 1.4. These sockets are designed to hold additional printed circuit boards for such things as video display adapters, serial interfaces, and instrument interfaces.

Although microcomputers from a variety of manufacturers appeared in the 1970’s, it was the introduction of the Personal Computer (PC) from IBM in 1981 that initiated the widespread use of microcomputers in the sciences. The IBM-PC was the first microcomputer from a major vendor to combine the important features of large memory capacity (up to 640,000 characters), rapid processing speed (a 4.77 MHz clock), high resolution graphics, and convenient expansion capabilities. Within months of the introduction of the IBM-PC, other computer manufacturers such as Hewlett-Packard, AT&T, and Zenith introduced copies of the IBM-PC that performed exactly like the model from IBM. These copies were sometimes referred to as clones or compatibles. The acceptance of the IBM-PC architecture by so many different computer companies helped to establish it as an industry standard. This standard, at least in part, is still with us today. We still use the term PC to refer to any microcomputer whose heritage traces back to the original IBM-PC.

1.2 Computers are Digital Devices

All data transfer, storage, and processing done by a microcomputer is performed digitally using binary (base two) codes. This binary system translates every character entered in the computer into a set of 1's and 0's. For example, the PC represents the capital letter "C" as 1000011. The advantage of binary coding over other methods is that a sequence of only two possible states is required to represent a character in the electronic circuits of the computer. The binary digit 1 could be represented by a signal level of +5 volts and a binary 0 could be represented by a signal level of 0 volts. Therefore, the smallest piece of information that needs to be stored in memory is a single binary digit. A single binary digit is called a bit (abbreviated with a lowercase b). Different groupings of bits are used to represent different characters or instructions.

A collection of eight bits is called a byte (abbreviated with an uppercase B). One byte can represent any one of 256 characters (2⁸ = 256). The word "hello" would require a total of five bytes of memory – one byte for each character in the word. Since we are primarily concerned with how many characters the memory banks of a computer can hold, memory size is referred to in units of bytes. The memory size of modern computers is typically measured in megabytes (MB). In binary arithmetic, the power of two which is closest to 1,000,000 is 2²⁰ (2²⁰ = 1,048,576). Therefore, in computer jargon, the prefix "mega" stands for 1,048,576. A similar situation exists for the prefix "kilo". Normally, the prefix "kilo" means 1,000 such as in kilometer (km). When speaking of computer memory, however, the prefix "kilo" means the power of two that is closest to 1,000. This is 2¹⁰ which is 1,024. Therefore, a computer having 256 kilobytes of memory has the ability to store as many as 262,144 (256 X 1,024 = 262,144). Frequently, the word kilobyte is abbreviated with a capital K. Fortunately, the distinction between the normal use of the terms "kilo" and "mega" and there use in reference to computers is of little importance.

1.3 Microcomputer Parts

All microcomputers, regardless of the brand, share a common overall design. This design is illustrated in Figure 1.5. The CPU, shown in the middle of the diagram, is the brain of the computer. CPU stands for central processing unit and it is the chip that contains all the circuitry for performing arithmetic and logic operations and for directing data to and from memory. In a microcomputer, the CPU is contained on a single chip. Minicomputers and mainframe computers have CPU's that occupy several chips.

Figure 1.5 Relationship of CPU to Memory and I/O Devices