The computer is full of chips. Let’s examine them.
If you unscrew the system unit (the box containing the CPU and memory) and peek at the circuitry inside, you’ll see a green plastic board, on which is printed an electrical wiring diagram.
Since the diagram’s printed in copper (instead of ink), the diagram conducts electricity; so it isn’t just a diagram of an electrical circuit; it is an electrical circuit!
The green plastic board — including the circuit printed on it — is called a printed-circuit board (PC board). Each wire that’s stamped onto the PC board is called a trace.
The typical computer contains several PC boards.
Motherboard & babies
In your computer, the largest and most important PC board is called the motherboard (or, more briefly, mobo). It lies flat on the bottom of the system unit.
The other PC boards are smaller. Those little baby boards (about the size of a postcard) are called PC cards.
The typical motherboard has several slots on it. Into each slot, you can put a PC card.
If you buy a modern notebook computer, you’ll see the case’s right-hand wall has a special slot in it. You can shove a card into that slot without opening the notebook’s case.
The kind of card that fits into that special slot is small and thin — the size of a credit card. That kind of card was invented by the Personal-Computer Memory-Card International Assocation (PCMCIA) and therefore called a PCMCIA card. That slot is called a PCMCIA slot.
People have trouble remembering what “PCMCIA” stands for. Cynics say it stands for “People Can’t Memorize Computer Industry Acronyms”. Since “PCMCIA” also stands for “Politically Correct Members of the CIA”, computerists pronounce “PCMCIA” in two breaths: they say “PCM”, then pause, then say “CIA”.
Some PCMCIA cards are very thin. Other PCMCIA cards are thicker, so they can hold extra circuitry. A PCMCIA card and its slot are called Type 1 if their thickness is 3.3 millimeters, Type 2 if 5 millimeters, Type 3 if 10.5 millimeters, Type 4 if 18 millimeters.
On each PC board, you’ll see black rectangles. If you look closely at a black rectangle, you’ll see it has tiny legs, so it looks like a black caterpillar. (Though farmers think it looks like a “black caterpillar”, city folks think it looks more like a “yucky roach”. Kids call it just “a black thingy with legs”.)
The “caterpillars” come in many sizes. In a typical computer, the shortest caterpillars are three-quarters of an inch long and have 7 pairs of legs; the longest are two inches long and have more legs.
Though each black caterpillar has legs, it doesn’t move. It’s permanently mounted on the PC board.
Each leg is made of tin and called a pin.
Sadistic hobbyists play a game where they yank the caterpillars from a PC board and throw the caterpillars across the room. That game’s called “tin-pin bowling”.
Hidden inside the caterpillar is a metal square, called a chip, which is very tiny. The typical chip is just an eighth of an inch long, an eighth of an inch wide, and a hundredth of an inch thick! On that tiny metal chip are etched thousands of microscopic electronic circuits! Since all those circuits are on the chip, the chip’s called an integrated circuit (IC).
Each chip serves a purpose. If the chip’s purpose is to “think”, it’s called a processor chip. If the chip’s purpose is to “remember” information, it’s called a memory chip. If the chip’s purpose is to help devices communicate with each other, it’s called an interface chip. If the chip’s purpose is to act as a slave and helper to other chips, it’s called a support chip.
So a chip is either a processor chip or a memory chip or an interface chip or a support chip — or it’s a combination chip that accomplishes several purposes.
How chips are designed
To design a chip, the manufacturer hires an artist, who draws on paper a big sketch of what circuits are to be put onto the chip. It helps if the artist also has a degree in engineering — and knows how to use another computer to help draw all the lines.
After the big sketch is drawn, it’s photographed.
Have you ever photographed your friend and asked the photography store for an “enlargement”? To produce a chip, the chip’s manufacturer does the opposite: it photographs the sketch but produces a “reduction” to just an eighth of an inch on each side! Whereas a photo of your friend is made on treated paper, the tiny photo of the chip’s circuitry consists of metal and semiconductors on treated silicon so the photo’s an actual working circuit! That photographic process is called photolithography (or photolith).
Many copies of that photo are made on a large silicon wafer. Then a cookie cutter slices the wafer into hundreds of chips. Each chip is put into its own caterpillar.
The caterpillar’s purpose is just to hide and protect the chip inside it; the caterpillar’s just a strange-looking package containing the chip. Since the caterpillar’s a package that has two rows of legs, it’s called a dual in-line package (DIP). That DIP’s only purpose is to house the chip.
Computer hobbyists are always talking about chips & DIPs, and at parties serve chips & dips, and are called “dipchips”.
If you ask a computer dealer to sell you a chip, the dealer also gives you the chip’s DIP (the entire caterpillar). Since you’ve asked for a chip but also received a DIP, you might get confused and think that the caterpillar (the DIP) is the chip. But that caterpillar’s not the chip; the chip hides inside the caterpillar.
The typical caterpillar-and-chip costs $3. You might pay somewhat more or somewhat less, depending on how fancy the chip’s circuitry is.
If the circuits in a chip are defective, it’s called a “buffalo chip”. Folks who dislike that tacky term say “potato chip” or “chocolate chip” instead, like this: “Hey, the computer’s not working! It must be made of chocolate chips!”
You can get chips from these famous mail-order chip suppliers:
The following chip suppliers are newer and often charge less:
How chips chat
The chip inside the caterpillar acts as the caterpillar’s brain. The caterpillar also contains a “nervous system”, made of thin wires that run from the brain (the chip) to the legs (the pins). The wires in the caterpillar’s nervous system are very thin: each wire’s diameter is about half of a thousandth of an inch.
If one caterpillar wants to send electrical signals to another caterpillar, the signals go from the first caterpillar’s brain (chip) through the caterpillar’s nervous system to its legs (pins). Each pin is attached to a trace (wire) on the PC board. The signals travel through those traces, which carry the signals across the PC board until the signals reach the second caterpillar’s pins. Then the signals travel through the second caterpillar’s nervous system to that caterpillar’s brain (chip).
To communicate with each other, the caterpillars use a secret code. Each code is a series of 1’s and 0’s. For example, the code for the letter A is 01000001; the code for the letter B is 01000010; the code for the number 5 is 101; the code for the number 6 is 110.
That’s called the binary code, because each digit in the code has just two possibilities: it’s either a 1 or a 0. In the code, each 1 or 0 is called a binary digit.
A binary digit is called a bit. So in the computer, each bit is a 1 or a 0.
When a caterpillar wants to send a message to another caterpillar, it sends the message in binary code. To send a 1, the caterpillar sends a high voltage through the wires; to send a 0, the caterpillar sends little or no voltage through the wires.
So to send the number 5, whose code number is 101, the caterpillar sends a high voltage (1), then a low voltage (0), then a high voltage (1). To send those three bits (1, 0, and then 1), the caterpillar can send them in sequence through the same leg (pin); or for faster transmission, the caterpillar can send them through three pins simultaneously: the first pin sends 1, while the next pin sends 0 and the third pin sends 1.
The speed at which bits are sent is measured in bits per second (bps).
The part of the computer that thinks (“the brain”) is called the processor (or central processing unit or CPU).
In a maxicomputer or minicomputer, the processor consists of several chips, which are processor chips.
In a microcomputer, the processor is so small that it consists of just a single chip, called a microprocessor. It sits on the motherboard. Yes, in a typical microcomputer, the part that does all the thinking is just a tiny square of metal, less than ¼" on each side!
In IBM-compatible PCs, the microprocessor uses a design invented by Intel. Intel has gradually improved that design by putting more circuitry on the chip:
The Intel Pentium could have been called the “Intel 586”, but Intel called it the “Pentium” instead so Intel can trademark the name and prevent companies from copying it. It’s the first computer chip that sounds like a breakfast cereal: “Hey, kids, to put zip into your life, try Penti-yumms. They build strong computer bodies, 5 ways!”
The Intel 8088 was used in the original IBM PC and in a fancier computer called the IBM PC XT. Any IBM-compatible PC containing that chip is called an XT-class computer.
The Intel 286 was used in a computer called the IBM AT. Any IBM-compatible PC containing that chip is called an AT-class computer.
The 8088, 286, 386, and 486 chips are all outdated; they’re no longer actively marketed. All new IBM-compatible PCs contain Pentiums — or imitations of it made by Intel’s competitors.
The most popular imitation of the Pentium chip is the Athlon chip, made by Advanced Micro Devices (AMD). The Athlon chip tends to run faster than the Pentium chip and costs less: it’s a better deal!
Many new programs require you to have a Pentium-class chip (Pentium, Athlon, or similar imitation). Those programs won’t run if your computer is so old that it contains an 8088, 286, 386, or 486.
In an army, when soldiers march, they’re kept in step by a drill sergeant who yells out, rhythmically, “Hup, two, three, four! Hup, two, three, four! Hup, two, three, four!”
Like a soldier, the microprocessor takes the next step in obeying your program just when instructed by the computer’s “drill sergeant”, which is called the computer clock. The clock rhythmically sends out a pulse of electricity; each time the clock sends out a pulse, the microprocessor does one more step in obeying your program.
The clock sends out millions of pulses every second, so the microprocessor accomplishes millions of steps in your program every second!
Each pulse is called a clock cycle. The clock’s speed is measured in cycles per seconds.
A “cycle per second” is called a hertz (Hz), in honor of the German physicist Heinrich Hertz. A “million cycles per second” is called a megahertz (MHz).
When Intel invented the Pentium chip in 1993, the Pentium’s clock did 60 million cycles per second. That’s 60 megahertz! Intel also invented a faster Pentium, at 66 megahertz, then even faster Pentiums at 75, 90, 100, 120, 133, 150, 166, 200, 233, 266, 300, 333, 350, 400, 450, 500, 550, 600, 650, 667, 700, 733, 750, 800, 850, 866, and 933 megahertz. For example, a 200-megahertz Pentium thinks twice as fast as a 100-megahertz Pentium.
1000 megahertz is called a gigahertz (GHz). It’s a billion hertz. Recently, Intel has invented faster Pentiums that go at 1, 1.3, 1.4, 1.5, and 1.7 gigahertz. For example, a 1-gigahertz Pentium thinks twice as fast as a 500-megahertz Pentium.
Slower than a Pentium
The Pentium is an amazing chip: while it thinks about one part of your program, it simultaneously starts getting the next part of your program ready for processing. That chip’s ability to do several things simultaneously is called parallel processing.
The Pentium is smarter than earlier chips (the 8088, 286, 386, and 486): the Pentium can perform more tasks simultaneously; it performs more parallel processing.
Earlier chips seem slower: too often during a clock cycle in earlier chips, part of the chip “does nothing” while waiting for the other part of the chip to catch up. Those earlier chips therefore accomplish less useful work during a clock cycle than a Pentium.
You’ve seen that those early chips accomplish less useful work during a clock cycle than a Pentium. Moreover, they accomplish fewer clock cycles per second than a Pentium; they have fewer megahertz:
For example, suppose you buy an Intel 486 going at 100-megahertz. Since it suffers from a usefulness factor of ½, it accomplishes just ½ as much useful work per cycle as a 100-megahertz Pentium, so it acts about as fast as a 50-megahertz Pentium. A 20-megahertz 386, which suffers from a usefulness factor of ¼, acts about as fast as a 5-megahertz Pentium. A 10-megahertz 286, which suffers from a usefulness factor of 1/5, acts about as fast as a 2-megahertz Pentium.
The slowest chip is a 4.77-megahertz 8088. Since it suffers from a usefulness factor of 1/20, it acts about as fast as a 0.2385-megahertz Pentium. That’s 14,256 times slower than the fastest Pentium, which goes at 3400 megahertz. Yes, the fastest IBM-compatible computers think over 10,000 times faster than the slowest ones! That’s progress!
The “usefulness factor” is just an approximate average. During a cycle, for example, a 486 accomplishes about ½ as much useful work as a Pentium, on the average; but on certain tasks a 486 accomplishes more than “½ as much”, and on other tasks it accomplishes less.
The Intel 8088 comes in two versions. One version (called simply the “8088”) goes slightly slower than the other version (called the 8086).
The Intel 386 comes in two versions. One version (called the 386SX) goes slightly slower than the other version (called the 386DX).
The Intel 486 comes in two versions. One version (called the 486SX) goes slower than the other version (called the 486DX). Moreover, the 486DX comes in three varieties: the original 486DX, the 486DX2, and the 486DX4.
7 Pentiums Intel’s invented 7 versions of the Pentium.
The Pentium classic is the oldest and slowest kind of Pentium. Invented in 1993, it’s the kind of Pentium found in most computers built from 1993 through 1996.
The Pentium MMX is slightly faster. Invented in January 1997, it’s the kind of Pentium found in most computers built in 1997.
The Pentium 2 is even faster. Invented in May 1997, it became popular when Intel dropped the price in 1998. It runs most programs about 30% faster than a Pentium MMX. Like the Pentium MMX, it understands the 57 multimedia instructions. Intel’s official name for this chip is “Pentium II”; but to avoid Roman numerals I’ll write “Pentium 2”.
The Pentium 2 replaces an old 1995 expensive version, called the Pentium Pro, which ran some programs fast but ran other programs slowly (even slower than a Pentium classic!) and lacked MMX.
To help folks who can’t afford a real Pentium 2, Intel began selling a cheaper version, called the Pentium Celeron, in 1998. It’s slower.
In February 1999, Intel invented a speeded-up Pentium 2, called the Pentium 3. Using a technique called Single-Instruction Multiple-Data (SIMD), it understands 70 extra instructions, called Streaming SIMD Extensions (SSE), which few programs use yet.
The newest Pentium is the Pentium 4. It’s controversial: it runs some programs faster than the Pentium 3 but runs other programs slower!
Megahertz Here’s how many megahertz are available:
Prices Here are some prices:
That chart shows the price charged by discount dealers (such as Spartan Technologies, The Chip Merchant, and JDR Microdevices) for a single chip when this book went to press in June 2004. By the time you read this, prices might be lower, since Intel drops prices frequently (about every 2 months). If you buy 1000 chips at a time directly from Intel, you pay even less.
Intel’s competitors have imitated Intel’s chips. Some of the imitations go faster than Intel’s originals!
Here are the prices charged by discount dealers (such as Spartan Technologies, The Chip Merchant, and JDR Microdevices):
While a chip waits for your commands, the chip accomplishes nothing useful during the wait: it just mumbles to itself.
To make full use of a fast Pentium, make sure you know what commands to give the computer. To help the chip reach its full potential, buy lots of RAM, big disk drives, and a quick printer. Otherwise, the Pentium will act as idiotic as if it’s in the army: it will just “hurry up and then wait” for other parts of the system to catch up and tell it what to do next.
A mind is a terrible thing to waste! To avoid wasting the computer’s mind (the CPU), make sure the other computer parts are good enough to match the CPU and keep it from waiting.
If you get suckered into buying a computer that has a fast Pentium chip but insufficient RAM, insufficient disk drives, and a slow printer, you’ve bought a computer that’s just half-fast; it’s half-assed.
When you buy a microcomputer, its advertised price includes a microprocessor, motherboard, and other goodies. Pay for the microprocessor separately just if you’re inventing your own computer, buying parts for a broken computer, or upgrading your computer by switching to a faster microprocessor & motherboard.
Though the microprocessor is cheap, the computer containing it can cost thousands of dollars. That’s because the microprocessor is just a tiny part of the computer. In addition to the microprocessor, you want memory chips, interface chips, support chips, PC boards (to put the chips on), I/O devices (a keyboard, screen, printer, speaker, and mouse), disks, and software.
Used-computer stores and garage sales get you IBM clones for these prices:
Those prices include nearly everything you need (such as the CPU, memory chips, disks, keyboard, and a screen displaying many colors) but do not include a printer or software. Those prices are approximate; the exact price you pay depends on the CPU’s speed (how many megahertz) and on the other components’ speed, quality, and size.
Notice that a 286 computer costs $30, which is $10 more than an 8086 computer. That’s because a 286 computer includes a better CPU chip and also comes with a better keyboard, better screen, better memory chips, and better disks.
Each Pentium chip includes math coprocessor circuitry, which handles advanced math fast. That circuitry can multiply & divide long numbers & decimals; it can also compute square roots, logarithms, and trigonometry.
Primitive chips — the 8088, 8086, 286, 386SX, 386DX, and 486SX — do not include such circuitry.
To make a primitive chip do advanced math, you must feed the chip a program that teaches the chip how to break the advanced problem down into a series of simpler problems. That program runs slowly — nearly 100 times slower than if a math coprocessor were present!
You’ll be very annoyed at the slowness if you’re a scientist trying to do advanced math — or an artist trying to rotate a picture, since the computer computes the rotated image’s new coordinates by using trigonometry. For example, if you draw a 3-D picture of a house and then ask the computer to show how the house looks from a different angle, you need a math coprocessor to avoid a long delay.
But if you use the computer just as a souped-up typewriter (to record and edit your writing) or as an electronic filing cabinet (to record names and addresses on a mailing list), you’ll never notice the lack of a math coprocessor, since you’re not doing advanced math.
Each 486DX chip (and 486DX2 and 486DX4) includes math-coprocessor circuitry; the 486SX does not. So here’s the only difference between a 486DX and a 486SX: the 486SX lacks math-coprocessor circuitry.
If your CPU lacks math-coprocessor circuitry (because your CPU is an 8088, 8086, 286, 386, or 486SX), here’s how to do math quickly: buy a supplementary chip, called a math coprocessor chip. Put it next to the CPU chip on the motherboard. It contains the math-coprocessor circuitry that the CPU lacks.
Although the CPU (the computer’s brain) can think, it can’t remember anything. It can’t even remember what problem it was working on!
Besides buying a CPU, you must also buy memory chips, which remember what problem the CPU was working on. To find out what the problem was, the CPU looks at the memory chips frequently — millions of times every second!
You need two kinds of memory chips: RAM and ROM.
Let’s begin by looking at RAM chips. If a chip remembers info just temporarily, it’s called a random-access memory chip (RAM chip).
When you buy RAM chips, they contain no info yet; you tell the CPU what info to put into them. Later, you can make the CPU erase that info and insert new info instead. The RAM chips hold info just temporarily: when you turn the computer’s power off, the RAM chips are automatically erased.
Whenever the CPU tries to solve a problem, the CPU stores the problem in the RAM chips, temporarily. There it also stores all instructions on how to solve the problem; the instructions are called the program.
If you buy more RAM chips, the CPU can handle longer problems and programs. If the computer doesn’t have enough RAM chips to hold the entire problem or program, you (or a programmer) must split the problem or program into several shorter ones instead and tell the CPU to work on each of the short ones temporarily.
How RAM is measured
A character is any symbol you can type on the keyboard, such as a letter or digit or punctuation mark or blank space. For example, the word HAT consists of 3 characters; the phrase Mr. Poe consists of 7 characters (M, R, the period, the space, P, O, and E). The phrase LOVE 2 KISS U consists of 13 characters.
Instead of saying “character”, hungry programmers say byte. So LOVE 2 KISS U consists of 13 bytes. If, in the RAM, you store LOVE 2 KISS U, that phrase occupies 13 bytes of the RAM.
RAM chips are manufactured by a process that involves doubling. The most popular unit of RAM is “2 bytes times 2 times 2 times 2 times 2 times 2 times 2 times 2 times 2 times 2”, which is 1024 bytes, which is called a kilobyte. So the definition of a kilobyte is “1024 bytes”. It’s about a quarter as many characters as you get on a typewritten page (assuming the page is single-spaced with one-inch margins and elite type).
The abbreviation for kilobyte is K. For example, if a salesperson says an old computer has a “512K RAM”, the salesperson means the main circuitry includes enough RAM chips to hold 512 kilobytes of information, which is slightly over 512,000 bytes.
A megabyte is 1024 kilobytes. Since a kilobyte is 1024 bytes, a megabyte is “1024 times 1024” bytes, which is 1,048,576 bytes altogether, which is slightly more than a million bytes. It’s about how much you can fit in a 250-page book (assuming the book has single-spaced typewritten pages). The abbreviation for megabyte is meg or M.
A gigabyte (pronounced “gig a bite”) is 1024 megabytes. It’s slightly more than a billion bytes. The abbreviation for gigabyte is gig or G.
A terabyte is 1024 gigabytes. It’s slightly more than a trillion bytes.
In honor of the words “kilobyte”, “megabyte”, “gigabyte”, and “terabyte”, many programmers name their puppies Killer Byte, Maker Byte, Giggle Byte, and Terror Byte.
Rows of RAM chips
In a primitive microcomputer (such as the Commodore 64), the RAM is a row of eight chips on the motherboard. That row of chips holds 64K altogether. So it holds 64 kilobytes, which is slightly more than 64 thousand bytes (since a kilobyte is slightly more than a thousand bytes).
That row of chips is called a 64K chip set. Each chip in that set is called a “64K chip”, but remember that you need a whole row of those 64K chips to produce a 64K RAM.
The most popular style of 64K chip is the TI 4164. Although that style was invented by Texas Instruments, other manufacturers have copied it.
If your computer is slightly fancier (such as the Apple 2c), it has two rows of 64K chips. Since each row is a 64K RAM, the two rows together total 128K.
If your computer is even fancier, it has many rows of 64K chips. For example, your computer might have four rows of 64K chips. Since each row is a 64K RAM, the four rows together total 256K.
64K chips first became popular in 1982. If your computer is so ancient that it was built before 1982, it probably contains inferior chips: instead of containing a row of 64K chips, it contains a row of 16K chips or 4K chips.
During the 1980’s, computer engineers invented 256K and 1M chips.
If your computer has very little RAM, you can try to enlarge the RAM, by adding extra rows of RAM chips to the motherboard. But if the motherboard’s already full, you must buy an extra PC card to put the extra chips on. That extra PC card is called a RAM memory card.
The original IBM PC contains an extra chip in each row, so each row contains 9 chips instead of 8.
The row’s ninth chip is called the parity chip. It double-checks the work done by the other 8 chips, to make sure they’re all working correctly!
So for an original IBM PC (or imitations of it), you must buy 9 chips to fill a row.
Strips of RAM chips
If your computer is modern and you want to insert an extra row of RAM chips, you do not have to insert 8 or 9 separate chips into the motherboard. Instead, you can buy a strip (tiny memory card) that contains all 8 or 9 chips and just pop the whole strip into the computer’s motherboard, in one blow.
Here’s what SIMMs and DIMMs cost:
You can get those prices from discount dealers, such as:
Some computers use SIMMs containing a set of just 2, 3, or 4 chips. That set of special chips imitates 8 or 9 normal chips.
A nanosecond is a billionth of a second. The typical SIMM contains chips that are fast: they retrieve info in 60 nanoseconds. Some SIMMs and DIMMs contain chips that are even faster: 10 nanoseconds.
Dynamic versus static
A RAM chip is either dynamic or static.
If it’s dynamic, it stores data for just 2 milliseconds. After the 2 milliseconds, the electrical charges that represent the data dissipate and become too weak to detect.
When you buy a PC board containing dynamic RAM chips, the PC board also includes a refresh circuit. The refresh circuit automatically reads the data from the dynamic RAM chips, then rewrites the data onto the chips before 2 milliseconds go by. Every 2 milliseconds, the refresh circuit reads the data from the chips and rewrites the data, so that the data stays refreshed.
If a chip is static instead of dynamic, the electrical charge never dissipates, so you don’t need a refresh circuit. (But you must still keep the power turned on.)
In the past, computer designers used just static RAM because they feared dynamic RAM’s refresh circuit wouldn’t work. But now refresh circuits are reliable, and the most popular kind of RAM is dynamic.
Dynamic RAM is called DRAM (pronounced “dee ram”). Static RAM is called SRAM (pronounced “ess ram”).
The circuitry on SIMM and DIMM cards has improved, to let a stream of data get from the memory card to the CPU chip faster. Such improvements have fancy names:
If you want to buy an extra SIMM or DIMM to put in your computer, make sure you buy the same kind as the others that are already in your computer. Make sure the extra SIMM or DIMM has the same number of pins (30, 72, or 168?), the same number of chips on it (2, 3, 4, 8, 9, or more?), operates at the same number of nanoseconds (10 or 80?), and uses the same technology (FPM, EDO, SDRAM, RDRAM, or DDR).
Let your memory grow
Here’s how much RAM you typically get altogether:
The original IBM PC came with just 16K of RAM, but you could add extra RAM to it. Here’s how much RAM the typical IBM-compatible PC contains now:
To run modern IBM PC software, you need at least 128M of main RAM; but many people still use old IBM PC software that can run on 64M, 32M, or even 16M of RAM.
How RAM is divvied
For IBM-compatible PCs having a lot of RAM, here’s how it’s divvied up.
The first 640K of main RAM is called the base memory (or conventional memory). It’s the part of the RAM that the computer can handle easily and fast.
The next 384K is called upper memory. It’s relatively unimportant, since most programs don’t know how to use it.
Those two parts (the conventional memory and the upper memory) consume a total of 640K+384K, which is 1024K, which is one megabyte.
The rest of the main RAM (beyond that first megabyte) is typically called the extended memory.
If a chip remembers information permanently, it’s called a read-only memory chip (ROM chip), because you can read the information but can’t change it. The ROM chip contains permanent, eternal truths and facts put there by the manufacturer, and it remembers that info forever, even if you turn off the power.
Here’s the difference between RAM and ROM:
The typical computer includes many RAM chips (arranged in rows) but just a few ROM chips.
What kind of info is in ROM?
In your computer, one of the ROM chips contains instructions that tell the CPU what to do first when you turn the power on. Those instructions are called the ROM bootstrap, because they help the computer system start itself going and “pull itself up by its own bootstraps”.
In the typical microcomputer, that ROM chip also contains instructions that help the CPU transfer information from the keyboard to the screen and printer. Those instructions are called the ROM operating system or the ROM basic input-output system (ROM BIOS).
In the typical microcomputer, one of the ROM chips tells the computer how to make each character on the screen out of dots. That chip is called the character generator.
In famous old microcomputers, several ROM chips contain definitions of fundamental English words, which are called BASIC words. For example, those ROM chips contain the definitions of BASIC words such as PRINT, INPUT, IF, and THEN. Those BASIC definitions in the ROM are called the ROM BASIC interpreter.
For example, let’s look inside a primitive computer: the Commodore 64. It contains just 4 ROM chips:
In the typical IBM-compatible PC, the motherboard contains a ROM BIOS chip.
That chip contains the ROM BIOS and also the ROM bootstrap. If your computer is manufactured by IBM, that chip is typically designed by IBM; if your computer is manufactured by a company imitating IBM, that chip is an imitation designed by a company such as Phoenix. Such a chip designed by Phoenix is called a Phoenix ROM BIOS chip. Other companies that design ROM BIOS chips for clones are Quadtel (which was recently bought by Phoenix), Award (which was recently bought by Phoenix), and American Megatrends Incorporated (AMI) (which remains independent).
On a special PC card (called a video display card), you’ll find a ROM chip containing the character generator.
If your computer is old and built by IBM, some chips on the motherboard contain the ROM BASIC interpreter. If your computer is new or an imitation, all of BASIC comes on a disk instead of in ROM chips.
Altogether, the original IBM PC contained six ROM chips: the ROM BIOS chip, the character generator, and four ROM BASIC interpreter chips. Each of those six chips contained 8K, so that the computer’s ROM totaled 48K. On newer computers from IBM and competitors, the total is slightly different.
If your computer attaches to a TV and is old-fashioned (such as a Commodore Vic, Commodore 64, Commodore 128, Atari 800, Atari 800XL, or Radio Shack Color Computer), you can pop ROM cartridges into the computer. A ROM cartridge is a cartridge containing a PC card full of ROM chips. Etched into those ROM chips is a program.
The typical ROM cartridge contains a program that plays a video game, such as Space Invaders or Pac Man or computer chess. You can also buy ROM cartridges that contain programs for word processing, music, art, or tutoring you. Each ROM cartridge costs about $30.
How ROM chips are made
The info in a ROM chip is said to be burned into the chip. To burn in the info, the manufacturer can use two methods.
One method is to burn the info into the ROM chip while the chip’s being made. A ROM chip produced by that method is called a custom ROM chip.
An alternate method is to make a ROM chip that contains no info but can be fed info later. Such a ROM chip is called a programmable ROM chip (PROM). To feed it info later, you attach it to a device called a PROM burner, which copies info from a RAM to the PROM.
Info burned into the PROM can’t be erased, unless the PROM’s a special kind: an erasable PROM (EPROM). You can buy 3 types of erasable PROMs:
Those numbers (for erasure time, voltage, and block size) are typical; but for your chip the numbers might be different, depending on how the chip was manufactured. After you erase an erasable PROM, you can feed it new info.
If you’re a manufacturer designing a new computer, begin by using an erasable PROM, so you can make changes easily. When you decide not to make any more changes, switch to a non-erasable PROM, which costs less to manufacture. If your computer becomes so popular that you need to manufacture over 10,000 copies of the ROM, switch to a custom ROM chip, which costs more to design and “tool up for” but costs less to make copies of.