1.2 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 directing data to and from memory. In a microcomputer,
the CPU is contained on a single chip. Minicomputers and mainframe
computers have CPUs that occupy several chips.
The speed of a microcomputer can be further accelerated by the
addition of a math-coprocessor chip. This is a chip designed for
the sole purpose of performing mathematical operations. The addition
of a math coprocessor can accelerate math-intensive programs by
a factor of 10 or more. The CPU of a microcomputer cannot do anything
until it has data with which to work. All data that enters the
CPU for processing
FIGURE 1.5 RELATIONSHIP OF CPU TO MEMORY AND 1/0 DEVICES
originally comes from devices located outside the box that
houses the CPU. These devices are called input devices because
their function is to get data into the computer. Input devices
can consist of such things as the keyboard, a floppy or hard disk,
a mouse, another computer, or a laboratory instrument. Figure
1.5 illustrates some common input devices that will be discussed
more fully in Section 1.3.
To know what the CPU is doing, we must be able to view its operation.
To do this, data must be sent out of the CPU to output devices,
which may consist of printers, plotters, and video monitors. Output
can also be stored for later viewing by saving it on floppy or
hard disks. Output devices will be discussed in Section 1.4 and
disk storage will be covered in Section 1.6. Input and output
is collectively known in computer jargon as 1/0.
Data that is entered into the computer must be stored while it
awaits processing by the CPU. It must also have someplace to go
after it has been processed. The area of the computer that holds
this data is the memory. The different types of computer memory
will be discussed in Sections 1.5 and 1.6. All of the electronic
and mechanical components of a microcomputer system are collectively
known as the hardware.
In summary, a microcomputer consists of a central processing unit
that accepts data from an input device, processes the data, and
then sends it to an output device. During the processing, data
is
FIGURE 1.6 TYPICAL MICROCOMPUTER HARDWARE
stored in the computer's memory. A typical microcomputer configuration is shown in Figure 1.6.
1.3 Input Devices
When a microcomputer is first turned on, its memory is empty. Before it can begin processing any data, the data must somehow get into the memory of the microcomputer. The four most common methods of entering data into a computer are shown below.
1. Information is typed in from the keyboard.
2. Data are read in from secondary storage devices like floppy
disks, hard disks, or tape drives.
3. Data are collected and entered into the computer from interface
devices such as analog to digital converters.
4. Information is entered into the computer from drawing devices
such as a digitizer (a type of drawing pad), a mouse, or a light
pen.
Most information that is processed by a computer originally
gets into the computer by being typed in from a keyboard. Therefore,
most of this section will be devoted to describing the standard
IBM PC keyboard, illustrated in Figure 1.7.
FIGURE 1.7 STANDARD IBM PC KEYBOARD
The keyboard is divided into three sections. The middle section,
which is the largest, looks much like a standard typewriter keyboard.
It contains all the letters of the alphabet, standard punctuation
symbols, the numbers 0 through 9, and several special purpose
keys. The arrangement of the keys in this section is referred
to as a QWERTY format, which comes from the order of the first
six keys in the second row. Most data enters a computer from this
middle section of the keyboard.
The two rows of keys on the left are called special function keys
(or just function keys). On some keyboards, the function keys
are located across the top, just above the number keys of the
middle section. Unlike the keys from the middle section, what
happens when you press a function key depends on what software
you are running. Software consists of the programs that control
the operation of the microcomputer. The operation of the function
keys differs from one program to the next.
On the night side of the keyboard is the numeric keypad. The keys
in this section are arranged like those on a calculator and are
designed to speed the entry of numeric data. The NUM LOCK (number
lock) key, when pressed, toggles the numeric keypad between the
number mode and the cursor control arrow mode. The operation of
the special purpose keys (Ins, Del, Ctrl, and so on) will be discussed
later.
If data have previously been saved on a secondary storage device
such as a floppy disk, then this data can serve as input to the
computer. Secondary storage devices are covered more fully in
Section 1.6.
1.4 Output Devices
For the user to view the results of the microcomputer's work, data must be sent from the microcomputer to an output device. Typical output devices are video monitors, printers, plotters, and secondary storage devices that can hold the data for future viewing. The output device found on nearly all microcomputers is the video monitor or just monitor. For the the video monitor to operate, a video display adapter card must be installed in the computer. There are two primary types of monitors available for use with microcomputers.
They are television style cathode ray tube (CRT) monitors and the liquid crystal display (LCD) monitors.
FIGURE 1.8 DOT MATRIX DISPLAY
5 x 7 Matrix
The second most common output device is the printer. Despite their many differences, printers can be placed in one of two categories.
1. Dot matrix printers These printers produce images by printing
small dots in matrix patterns. Figure 1.8 shows what a typical
set of 5 X 7 matrix characters would look like if they were magnified.
The greater the potential number of dots In the matrix, the better
the printed copy will look. Therefore, printed copy from a 5 X
7 dot matrix printer will not took as good as that from a 7 X
9 dot matrix printer. Dot matrix printers will also produce high
resolution graphics. Dot matrix printers differ in the technology
used to produce the dots. Printers that are traditionally called
dot matrix produce their dots by fining small pins against the
ribbon and paper. These printers are capable of printing at over
3.00 characters per second (cps). Inkjet printers produce their
dots by spraying ink on the paper. Different colored inks can
be used to produce color images. Thermal printers produce their
dots by either burning holes in specially sensitized paper or
by melting the ribbon onto the paper. These printers are capable
of speeds up to 80 cps. Even laser printers are dot matrix printers.
They use a laser to create the dot pattern on the paper which
then picks up toner to produce the final image. Laser printers
are capable of printing up to 15 pages per minute.
2. Fully formed character printers in this category produce their
images by striking wheels, balls, or thimbles which contain the
complete character against the ribbon and paper. (For this reason
they are sometimes called impact printers.) This is the same way
a standard typewriter produces its images. A common example of
a fully formed character printer is the daisywheel printer. In
this type of printer, spokes containing characters are struck
by a hammer forcing the spoke against the ribbon and paper. This
type of printer is also referred to as a letter quality printer
because the output looks just like it was produced by a standard
typewriter.
FIGURE 1.9 FLATBED PLOTTER
The drawbacks to these printers are they are slow (less than 60 cps), noisy, and are usually incapable of producing graphics.
Output from a microcomputer can also be sent to a plotter, which creates its image by drawing on paper with a felt tip pen. Some plotters will automatically change pens to produce multicolored drawings. Plotters are primarily used for creating graphs, charts, and diagrams. They are far too slow for producing a full page of text. A plotter is illustrated in Figure 1.9. Another type of computer output is sound.
Primary Memory
A microcomputer would be incapable of performing even the simplest
task if it did not contain some type of memory. Consider an example
in which you want the microcomputer to add the numbers 2 and 2.
When you type the first 2 in from the keyboard the CPU does not
yet know what you intend to do with it so it has to store the
number. When you enter the plus sign it now knows you intend to
do some arithmetic but it still needs another number. Finally,
you enter the second 2 and the CPU performs the calculation and
stores the result in memory. A microcomputer uses memory to store
the programs that control its operation, to store data waiting
for processing, and to store the results of operations performed
by the CPU.
Primary memory, or storage, is electronic memory that is directly
addressable by the CPU. This memory is contained in integrated
circuits called memory chips. Each memory location is assigned
a number called an address. The CPU uses these addresses to keep
track of information stored in memory. Since primary memory is
completely electronic, transfer of data to and from it is extremely
fast.
A microcomputer contains several types of primary memory. RAM
(Random Access Memory) is used for storing information that changes.
This is the memory in a computer that is accessible to the user.
RAM is used to store user programs that control what the CPU does.
It stores the data used by these programs and the results of operations
performed by these programs. How much RAM a computer has determines
the size and sophistication of the tasks a microcomputer can perform.
RAM is an example of volatile memory. This means that everything
stored in RAM is lost when the power is turned off, even for an
instant.
Another type of memory found in all microcomputers is ROM (Read
Only Memory). ROM can be read by the user but cannot be altered.
ROM is nonvolatile, retaining the information stored in it even
when the power is turned off. ROM is used primarily to store the
instructions a microcomputer needs to get itself started after
you turn on the power. This start up process is called booting
or bootstrapping and figuratively means that the computer pulls
itself up by its own bootstraps. The boot instructions are placed
in ROM by the computer manufacturer and cannot be altered by the
user. ROM can be used to store other programs supplied by the
manufacturer. Programs stored in ROM are sometimes referred to
as firmware.
Examples of other kinds of memory chips include PROM, EPROM, and
EEPROM. PROM (Programmable Read Only Memory) is a type of ROM
that can be programmed by the user. However, once it is programmed,
the contents cannot be changed. EPROM (Erasable Programmable Read
Only Memory) is a type of PROM chip that can be erased and reprogrammed.
EPROMs are erased by shining ultraviolet light on them. An example
of an EPROM chip is shown in Figure 1. 10. EEPROM (Electronically
Erasable Programmable Read Only Memory) is much like EPROM except
that EEPROM chips can be erased by an electrical signal instead
of ultraviolet light.
All data transfer, storage, and processing done by a microcomputer
is performed digitally using binary (base two) codes. This
FIGURE 1.10 EPROM CHIP
binary system translates every character entered in the computer
into a set of I's and O's. For example, the PC represents the
capital letter "C" as 10000 11. 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 I could be represented
by a signal level of + 5 volts and a binary 0 could be represented
by a signal 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. Different groupings of bits
are used to represent different characters.
A collection of eight bits is called a byte. One byte can represent
any of 256 characters (2 =256). The word "bit" would
require a total of three bytes of memory one byte for each character
in the word. Since we are primarily concerned with how many characters
the memory of a computer can hold, memory size is referred to
in units of bytes or kilobytes (kilo = 1000). In binary arithmetic,
the power of 2 that is closest to 1000 is 2" (2" = 1024).
Therefore, in computer jargon, the prefix kilo stands for 1024.
Frequently, the word kilobyte is abbreviated K. A computer having
256K of RAM has the ability to store as many has 262,144 characters
in random access memory locations (256 X 1024 = 262,144).
The maximum amount of memory the CPU of a computer can directly
access is a function of the number of bits the CPU can handle
at one time. The number of bits a CPU can process at one time
is called its word length. The word length of a CPU also affects
how fast the CPU can process data. The Intel 8088 CPU in the
FIGURE 1.11 DISK READ WRITE HEAD
1.6 Secondary Memory
Secondary memory (or storage) refers to nonvolatile storage
devices that are usually mechanical in nature and therefore. are
much slower at transferring data to the CPU." The most common
secondary storage device for use with microcomputers is the floppy
disk (usually just called disk). Disks come in a variety of sizes
including 8 inch, 5.25 inch, and 3.5 inch. The disk is made of
a flexible plastic that is coated with a magnetizable substance
like oxides of iron or chromium. Signals can be recorded onto
the disk in much the same way that signals are recorded on magnetic
tape. A diagram of a disk read write head from a disk drive is
shown in Figure 1. 11. A current corresponding to the signal sent
by the CPU of the computer passes through a wire coil which surrounds
an iron core. This produces a magnetic field which is concentrated
in the gap of the iron core. The disk, which rotates beneath the
read write head, is magnetized by this field. A series of magnetic
pulses corresponding to the original binary data sent out by the
CPU is recorded on the disk. Reversing this process allows data
already stored on a disk to be read tack into the computer.
FIGURE 1.14 HARD DISK DRIVE
Another popular secondary storage device for use with microcomputers
is the hard disk. The basic principles of operation for a hard
disk are similar to those of the floppy disk. In a hard disk,
however, the magnetic medium is coated on a rigid metal platter.
This platter rotates ten times faster than that of a floppy disk.
Therefore, a hard disk is much faster at transferring data to
the CPU of the microcomputer. Another important difference between
a hard disk drive and a floppy drive is that the read write heads
of the hard disk drive do not touch the surface of the disk the
way they do in a floppy drive.
Hard disks, like the one shown in Figure 1. 14, hold much more
data than a floppy disk. An IBM PC floppy disk can hold approximately
1.44MB of data. Hard disks hold twenty megabytes or more of data.
Hard disks are high precision, delicate instruments. They must
be handled with care to avoid head crashes, which occur when the
read write head of a hard drive comes in contact with the rigid
platter. If this happens, all of the data on the disk will probably
be lost. In most cases, the platters in a hard drive are not removable.
Therefore, the data on a hard disk cannot usually be carried from
one computer to another. Most hard disks are designed using what
is known as Winchester technology For this reason, hard disks
are sometimes called Winchesters.
A recent addition to the storage devices of a computer is the CD-ROM (compact disc-read only memory). A CD-ROM is a type of hard disk that uses laser beams rather than magnets to read and write bits of information on the surface of the disk. CD-ROMs, unlike most hard drive platters, are removable.
SUMMARY
Microcomputers are made possible by advances in electronics that led to the development of large scale integrated circuits. These advances made it possible to place the entire CPU of a computer on a single chip.
All computers share the same basic architecture. The main components are: (1) the central processing unit (CPU), (2) input devices, (3) output devices, and (4) memory.
The CPU of a microcomputer performs all the arithmetic, logic, and data handling functions of the microcomputer. The speed at which the CPU can process data is determined by the system clock speed and the word length of the CPU.
The most common input device used with microcomputers is the
keyboard. The PC keyboard is a QWERTY keyboard with the addition
of a numeric keypad and special function keys Other input devices
typically used with microcomputers are disk dives and analog to
digital interfaces.
Output can be directed to a video monitor, printer, or disk drive.
Composite monitors are capable of displaying text and low resolution graphics. TTL and RGB monitors can display high resolution graphics.
Dot matrix printers produce hardcopy at high speed with varying degrees of resolution, whereas fully formed character printers produce letter quality copy.
The basic types of computer memory are RAM and ROM. RAM memory is the workspace used by the CPU for storing data and programs. It is volatile. ROM memory holds instructions supplied by the manufacturer. ROM is nonvolatile. Memory is allocated in eight bit quantities called bytes. One character occupies one byte of memory.
Data that must be saved after the computer is turned off are usually stored on a floppy or hard disk, or secondary storage devices magnetically record binary data onto a rotating disk.