The chapter 1 overview on computer hardware has been moved to a new location at www.summercore.com/primer/chap012001.html

this OLD version below will remain just because many of us are "GETTING OLD" as paul simon reminds us ... steve bergen 5/6/2001 .. the day that Charles Coleman turns 60 (see my birthday song to him at www.summercore.com/music ;-)

Written by Steve Bergen and Lynne Schalman
The Original Teaching Company, PO Box E, Lexington MA 02173

16 pages in this chapter ...
16 chapters in this book ...
16 NBA championships for the Boston Celtics ...
Coincidence? We don't think so!

1 BIT COLOR ... 8 BIT COLOR ... 16 BIT COLOR ... 24 BIT COLOR ... ANALOG ... ASCII ... BASE TWO ... BAUD RATE ... BINARY CODE ... BIT ... BYTE ... CD ROM ... COMMERCIAL ... CPU ... CRT ... DAISY CHAIN ... DAISY WHEEL ... DIGITAL... DISK DRIVE ... DOS ... DOT MATRIX ... ECHO ON AND OFF ... FLOPPY DISK ... FRICTION FEED ... GIGABYTE ... GRAY SCALE ... HARD DISK ... HEXADECIMAL ... INKJET PRINTER ... INTERFACE ... K or KILOBYTE ... LASER PRINTER ... LASER DISC ... MEGABYTE ... MIDI ... MODEM ... MONITOR ... MOTHERBOARD ... NETWORK ... NYBBLE ... OCR ... PARALLEL ... PRINTERS ... PIXEL ... PUBLIC DOMAIN ... RAM ... ROM ... SCANNERS ... SCSI ... SERIAL ... SHAREWARE ... STAR NETWORK ... TRACTOR FEED ... VIDEO BOARD ... VIRUSES ... VOICE DIGITIZER ...

The CPU is the CENTRAL PROCESSING UNIT.

It resides on one or more chips inside the computer. Each chip is a grid of silicon wires encased in a plastic container about the size of a fingernail. Although there are other chips involved and this is an oversimplification, it works well to think of the CPU as the CENTRAL component of your system. It functions the way your brain functions, coordinating the activities of the other devices.

Other chips along with the CPU are RAM chips and ROM chips. RAM chips are always blank when the electricity is off.

When you start up a computer and begin to work, the information you type as well as the program that is in process all fit inside of RAM. When people refer to a 64K computer or 128K computer, they are referring to the size of the RAM capacity. RAM stands for RANDOM ACCESS MEMORY. On an Apple II computer, some software packages require 128K and not just 64K in order to work properly, because the program takes up a lot of room. On the GS, some software requires 1000K to 1250K of RAM. On the IBM, it is common for some packages to require 640K to 10 meg of RAM. On the MAC a lot of the current software requires anywhere from 1 to 10 meg of RAM.

ROM chips are crucial to the computer, but not very interesting to you as the consumer.

A ROM chip typically contains frozen information that the manufacturer wants accessible at all times, independent of disks. This might be word processing, BASIC or general screen codes. On the Apple II, BASIC is built into ROM so that if you turn on an Apple II without a disk, the machine understands BASIC . On the old ADAM computer by Coleco, word processing was frozen into ROM so it was automatically capable of word processing when you turned it on. ROM stands for READ ONLY MEMORY.

In order for the CPU and other chips to talk to other devices, an INTERFACE board is used to connect the electronics of one machine to that of the other.

The board usually contains a printed circuit with a few special purpose chips. It is common to have disk drive, modem and printer INTERFACES. The innovation of the MacPlus back in 1987 when it came out was that it had a SCSI INTERFACE built-in, allowing for easy connection to hard disk drives it is the main circuit board that all the RAM CHIPS, ROM CHIPS and other INTERFACE BOARDS plug into.

One special interface board is called the VIDEO BOARD.

Its function is to connenct the computer to the screen and frequently has multi-colored wires emerging from it.

Another important board is called the MOTHERBOARD.


It is the main circuit board that all the RAM CHIPS, ROM CHIPS and other INTERFACE BOARDS plug into.


Both the computer chip and the disk drive store information that is coded electronically.

The basic unit of information is the BYTE which is equivalent to one character or symbol of text


For example, the word HELLO is a 5 byte word. Because spaces are just as significant as letters, NEW YORK is one 8 BYTE word, which is the way most New Yorkers say it anyway. In fact, it is very useful in teaching or learning word processing to realize that SPACE BAR and RETURN each constitutes 1 BYTE. You might say that the computer alphabet has 29 letters: A-Z, SPACE BAR, TAB and RETURN. Thinking of SPACE BAR, TAB and RETURN as letters in the computer alphabet clarifies the concepts of inserting and deleting spaces.

1000 BYTES = 1 K. How many BYTES on a piece of paper?

Well, a good approach is to consider double-spaced text with wide margins. For example, you might have 50 characters across the page, with a total of 20 lines down. Therefore, we have 20 X 50 bytes or 1000 bytes on the page. Since computer people like to introduce new terms and abbreviations, they translate 1000 bytes into 1 KILOBYTE and then abbreviate it as 1 K. So if your computer is 64 K, it means the chip capacity can theoretically hold up to 64 pages of double spaced text in memory at one time. Of course, since actual use involves loading the word processing program first (which might take up 50K), the memory space of a 64 K computer would be much less than 64 pages.


In the old days of the 1970¹s, a typical CPU with a 12K brain might access a mailing list of 5,000 names and addresses for which each name/address consisted of 200 bytes of data. The software which fit into the 12K of memory could analyze the data which was 5,000 x 200 = 1,000,000 bytes or 1 megabyte of storage space. Specific names would be loaded as needed, without the need to store all 5,000 records in memory at one time.

Just as 1000 bytes equals 1 kilobyte (1K), 1 million bytes (1000K) is 1 MEGABYTE and 1 billion bytes (1000 megabytes) is 1 GIGABYTE.

And so we can say that the old standard 800K floppy disk holds 4/5 of a MEGABYTE (usually pronounced MEG). Hard disks might range from 20,000K to 400,000K which we would call 20 meg to 400 meg. And CDs hold typically up to 650,000K, which we call 650 meg or over 1/2 a GIGABYTE.

HOW IS EACH BYTE CODED? Although this is not particularly useful, many people find it interesting. Via magnetism, it is possible to magnetize a spot on the plastic floppy disk or a piece of wire to be either ON or OFF. The inside of a chip looks like a huge grid of horizontal and vertical wires.

Consider a 1 inch chip that has 100 horizontal wires and 100 vertical wires.

It looks like a gigantic tic-tac-toe board with 10,000 different junction spots or intersection points. Imagine that each one can be TURNED ON or OFF with magnetism, behaving like a light bulb.

Let us call each junction of wires a BIT and let us think of grouping the thousands of BITS into groups of 8.



It turns out that we can design a code of ONs and OFFs so that each group of 8 BITS holds one letter or symbol of the English alphabet. How? Well, first let's figure out how many different patterns of ON and OFF we would find with 8 BITS lined up. If we had just a 2 bit computer, we might have 4 different patterns for the 2 spots: If we had just a 3 bit computer, we might have 8 different patterns for the 3 spots: Using the symbols 1 for ON and 0 for off simplifies the explanation:
A 2-bit computer has 4 patterns: 00 01 10 11
A 3-bit computer has 8 patterns: 000 001 010 011 100 101 110 111
A 4-bit computer has 16 patterns:
0000 0001 0010 0011 0100 0101 0110 0111
0001 1001 1010 1011 1100 1101 1110 1111

This system of 1's and O's is really the BASE TWO or BINARY number system in math; the word BIT is an acronym of the phrase BInary digiT.

Here are the numbers from 1 to 14 written in BASE TWO:
	0001 = 1	        1000 = 8

	0010 = 2	        1001 = 9

	0011 = 3	        1010 = 10

	0100 = 4	        1011 = 11

	0101 = 5	        1100 = 12

	0110 = 6        	1101 = 13

	0111 = 7	        1110 = 14

Here is Larry Bird's uniform number (33) written as a BASE TWO number:
100001

Each position stands for a power of TWO instead of a power of TEN:


Now let's figure out how many patterns in an 8 bit computer? Since a 2 bit computer has 4 patterns and a 4 bit computer has 8 patterns, can you see that the number of patterns doubles as we add a bit. Can you figure out a good rationale for why it is so?

Take the 8 patterns for the three bit computer. Put a ZERO in front of each pattern or put a ONE in front of each pattern. These are the 16 patterns that correspond to the four bit computer.

The end result is that an 8 bit computer has 256 patterns. Each one is equal to one letter of the alphabet, a symbol on the keyboard or perhaps even a graphics character. Some of the 256 patterns may even be left unused. This provides another way to understand a BYTE. A byte is made up of 8 bits and is a coded way of representing one letter of the alphabet.

An even more esoteric word is a NYBBLE which is made of 4 bits.

And so 8 bits make a byte, 4 bits make a NYBBLE, 2 nybbles make a byte.

For example, when you type the 3 byte word YES on your computer, each letter is translated into its 8 bit code. The Y is code 89, the E is code 69, the S is code 83. Each code is 1 byte made up of 8 bits:
Y Code 89 = 01011001 = OFF ON OFF ON ON OFF OFF ON
E Code 69 = 01000101 = OFF ON OFF OFF OFF ON OFF ON
S Code 83 = 01010011 = OFF ON OFF ON OFF OFF ON ON
The computer is therefore storing 3 bytes of information to remember the word YES. Since each byte is made of 8 bits, the computer is storing the 3 bytes as a series of 24 bits. If we are looking at a disk, we would be referring to 24 spots of magnetism on one of the circular tracks of the record. If we are looking inside a chip, we would be referring to 24 intersections of wires on the tic-tac-toe board.

By the way, the most common coding system for relating binary numbers to English symbols is called the ASCII system.

If you look in the back of many computer books or printer manuals, you will find a table of codes that tells you what letter matches what code. By agreement, the ASCII coding system is commonly used. It means that A is code 65, B is code 66 and so on ... through Z which is code 90. ASCII stands for American Standard Coding Information Interchange and it proves that the computer industry is not completely idiosyncratic and eccentric!

Another computer system for coding numbers is called HEXADECIMAL or base 16.

In this system we use one symbol for each number from 1 to 15. We use A for 10, B for 11, C for 12, D for 13, E for 14 and F for 15. In the HEXADECIMAL system, after the one's place we have the 16's place, so the number A3 means 3 in the ONEs place and A (or 10) in the 16's PLACE, i.e. A3 = 10x16 + 3 = 163. For another example, the number 2B means B (or 11) in the ONEs place and 2 in the 16's place, so the number 2B = 2x16 + 11 = 43.

Let's compare the 3 number systems:

decimal...called base 10...10 symbols used: 0 to 9...100,10 and 1's place
binary...called base 2... 2 symbols used: 0 and 1...4,2 and 1's place
hexadecimal..called base 16...16 symbols used 0-9 and A,B,C,D,E,F...256,16 and 1's place

Below are the ASCII codes for the common 26 uppercase letters of the alphabet, A to Z, together with their BINARY and HEXADECIMAL equivalents. On some printers, if you turn on the machine incorrectly, you may find that it prints a series of 2-character codes. These are really the hexadecimal equivalents for the characters that you intended to print. For example, if the computer prints out the codes
4C 41 52 52 59 20 42 49 52 44 20 47 4F 20 44 41 4C 4C 41 53 0D
you can decipher using this chart what was really typed! Beware that 20 means SPACE BAR and 0D means carriage return.
Thanks to Theresa Overall from Lamplighter School (Dallas, TX) for fixing this coded message

	A	65	01000001	41		N	78	01001110	4E

	B	66	01000010	42		O	79	01001111	4F

	C	67	01000011	43		P	80	01010000	50

	D	68	01000100	44		Q	81	01010001	51

	E	69	01000101	45		R	82	01010010	52

	F	70	01000110	46		S	83	01010011	53

	G	71	01000111	47		T	84	01010100	54

	H	72	01001000	48		U	85	01010101	55

	I	73	01001001	49		V	86	01010110	56

	J	74	01001010	4A		W	87	01010111	57

	K	75	01001011	4B		X	88	01011000	58

	L	76	01001100	4C		Y	89	01011001	59

	M	77	01001101	4D		Z	90	01011010	5A

DISK DRIVE: This storage device holds the floppy disks.

Each FLOPPY DISK is actually a circular record encased in a square protective envelope. The record is divided into concentric circles called tracks. Each track is divided into chunks of information called sectors.

The standard Apple 5.25 inch disk drive uses 35 tracks with 16 sectors per track for a total of 560 sectors. Each sector holds 250 bytes for a total of 560 X 250 = 140,000 bytes. The IBM disk drive uses 40 tracks on each side of the disk, 9 sectors per track for a total of 720 sectors. Each sector holds 500 bytes for a total of 720 X 500 = 360,000 bytes. The Apple GS and Mac computers use 80 tracks per side of the 3.5 inch disk with 10 sectors per track for a total of 800 sectors. Each sector holds 500 bytes for a total of 800 X 500 = 400,000 bytes. The 2 sides of the disk therefore add up to 800,000 bytes.


The most common size disk for microcomputers has been the 5.25 inch floppy. On the Apple II computers, the disk drive uses just 1 side of the plastic. By notching the flip side with a hole puncher or disk notcher, you can use one disk as if it were really made of two disks. The same size plastic is used both front and back by the IBM and that is why we don't ever double notch IBM disks. Because of different specifications, Apple 5.25 drives simply require single-sided, single density floppy disks (abbreviated SS SD), while IBM PC drives require double-sided, double density disks (abbreviated DS DD). Almost all recent IBMs such as the AT computers, AT clones and PS models require double-sided high density disks (abbreviated DS HD) that hold 1200K instead of 360K.

Increasingly, the 3.5 inch floppy has become popular, at first on the Macintosh computer and now on the IBM and Apple II computers. The disk case is made of hard plastic and is small enough to fit into a shirt pocket. The standard disk for the Mac and Apple II holds 800,000 bytes. Even though the casing is hard plastic, these 3.5 inch FLOPPY DISKS are not called hard disks. The inside plastic circular record really does FLOP! The newest Macs and IBMs now use high density 3.5 inch disks that hold 1400 K of data or 1.4 megabytes.


Because different manufacturers use different disk formats, software is generally non-compatible among different machines. The medium is analogous to VHS or BETA videotapes. Unfortunately, you have more than two types! Except for a few special cases--such as IBM and its compatibles--you must use software for your specific computer.

HARD DISKS: A HARD DISK drive is approximately the size of a shoe box, although frequently it is built into the main computer case.

It is best to think of a HARD DISK drive as a circular piece of plastic that has been sealed in a dust free environment. You do not remove a HARD DISK. You do not handle it. You can save or load onto the HARD DISK and move programs and files from your floppy drive to and from the HARD DISK.

Because a hard disk is sealed in a special environment, it can spin faster and store more information. Typically, a hard disk might hold the equivalent of 25-200 floppy disks. And so you might say that a hard disk drive is to a floppy disk drive as a jukebox is to a record player. The advantage of hard disks are obvious: more information available without shuffling floppies, fast access and convenience. Furthermore, we are seeing an increasing number of software packages that either require or recommend the use of a hard disk drive. The down side of hard disks in addition to cost is that the device is more fragile and requires more know-how. It is incumbent that hard disk drive owners learn about subdirectories/file folders, general system information and backup schemes. Pricewise, floppy disk drives cost about $100-$300 while hard disk drives cost about $300-$1000.

DOS: This wonderfully alienating acronym stands for DISK OPERATING SYSTEM and it refers to the storage and coding system for the particular disk.

IBM uses its own DOS called IBM DOS or MS DOS. Apple II disks can either be DOS 3.3 or PRODOS. Radio Shack computers traditionally used TRSDOS and more recently use a version of IBMDOS. And the old Apple III computer used its own DOS that was named the Sophisticated Operating System; owners of these machines could then refer to loading up the Apple SOS (sauce). Varying DOS is one of several reasons for non-compatibility. Fortunately, there are software programs and hardware options that allow a Macintosh drive, for example, to read PRODOS files from an Apple IIe or that allow an IBM drive to read MACDOS files from a Mac computer.

To try to understand what DOS is, think of it as analogous to the fact that records play at 33, 45 or 78 RPM. DOS represents the grooving system that the manufacturer uses to put magnetic tracks onto a disk in order to store information onto that disk. For most computers, when you turn on the machine regardless of floppy or hard drive start-up, the very first thing that happens is that DOS is loaded from the start-up disk into a portion of the RAM chip. A standard IBM knows how to grab info only from a disk that was grooved with IBM DOS. That's why it gets nowhere with a Mac disk.

CRT: The CRT is the screen, sometimes called the MONITOR

It is that simple. The letters stand for Cathode Ray Tube which is what old televisions used to be called. CRTs can be color, green or amber and have varying resolutions usually described as a rectangle of PIXELS, perhaps 200 by 320 for IBM CGA display or 384 by 512 for the 9 inch Mac display. Each PIXEL is one dot on the screen. With Apple II computers before the GS, a color screen did not give adequate clarity for tasks like word processing while green screens were less advantageous for educational software. Accordingly, it was common practice to purchase either green or color depending on use.

Keep in mind that the resolution and/or color attributes are not just a function of the CRT, but of the CPU as well. IBM users buy a specific VIDEO BOARD that is plugged into their unit enabling VGA or CGA or EGA graphics. The appearance of their screen is a function of which CRT they own and which VIDEO BOARD. The Apple II computer always sends out a basic composite color video signal. If you have the computer connected to a green or amber screen, you see only shades. The GS sends out the same video signal but also sends out an RGB display which is a better quality color signal. That is why the GS screen looks richer than a standard Apple screen, but you could easily buy a GS computer and attach it to a composite color monitor. From 1984-90, most Macs were black and white. The newest Macs are all color. The LC was the first color Mac and stood for Low Cost color. In the IBM world, the clones now come standard with VGA color.

PRINTERS in 1980 used to be impressive if they could print italics.

DOT-MATRIX printers such as the IMAGEWRITER use a mechanism that print tiny little dots on your paper, shaped into letters and numbers. Typical associations with DOT-MATRIX printers are that they are fast, reliable, less expensive but moderate quality. The DAISY WHEEL mechanism gave you fully formed characters with no dots. Typically, DAISY WHEEL printers provided good quality in the 1980s but were slow, less reliable, noisy, more expensive and did not print graphics. Today these printers are virtually extinct!

Over the last few years, LASER PRINTERS have become less expensive and have made DAISY WHEELS into trivia questions.

The LASER PRINTER resembles a photostat machine and produces superb quality text and graphics. The laser printer is a computer itself which directs a laser beam to charge a photostatically sensitive drum which then attracts the toner. LASER PRINTERS such as the Hewlett-Packard Laserjet and Apple LaserWriter can now be bought for $500-$3,000. INKJET PRINTERS such as the Apple StyleWriter and Hewlett-Packard Deskwriter are similar in function and quality to laser printers and now exist in the $300-$800 price range. The inkjet printer sprays ink from a matrix of tiny jets. HP now has popularized the C inkjet printers. Deskwriter C and Deskjet C which sell for $400 to $800 and provided low-cost color and laser-like printing.

An important printer feature involves the kind of paper to be used.

Traditionally, dot-matrix printers use TRACTOR FEED paper -- the typical computer paper with perforations and holes. The tractor feed mechanism sits on top of the printer and feeds in the paper continuously. In contrast, daisy wheel, laser and inkjet printers typically use FRICTION FEED paper -- single sheets of paper that slide in as on a paper tray or like an electric typewriter.

One innovation of the mid-1980s was that dot-matrix printers came with a removable TRACTOR to allow for single sheets of paper. Or you can purchase a TRACTOR to fit onto your daisy wheel to allow for continuous feed paper. Another device more typically made for the daisy wheel printer is the cut sheet feeder which allows single sheets of paper to be automatically fed into the machine. A laser printer has a paper tray that holds 100 to 500 sheets.

Printers come in either PARALLEL or SERIAL version, which refers to the way the data is sent from the computer to the printer.

If a byte is made of 8 bits, then a PARALLEL device sends the 8 bits marching on 8 separate wires, like a crowd of people marching 8-across down the street. In contrast, a SERIAL device sends the 8 bits in single file, with a code after each group of 8, e.g. each group of 8 children in single file with the last one in each group wearing a special hat. The only important consideration, however, as far as the consumer is concerned is to match your printer to the interface: PARALLEL printer for parallel interface or SERIAL printer for serial interface. If your computer already comes with a built-in interface -- such as the serial one on the Apple IIc -- then it is important to purchase the corresponding type of printer.

You are now in the middle of chapter 1, approximately the middle of page 9 in the green Summercore book.

Click HERE to go to pages 9-16 beginning with Analog vs Digital.

Click HERE to go to page 1 beginning with the table of contents and list of vocabulary terms