The system unit is the box containing the CPU and other goodies (such as the speaker, power supply, and memory). If you unscrew that box and pry it open to see the circuitry inside, you’ll see a green plastic board, on which is printed an electrical wiring diagram.

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.

In your computer, the largest and most important PC board is called the motherboard. 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.

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.

Some PCMCIA cards are very thin. Other PCMCIA cards are slightly 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.

Each leg is made of tin and called a pin.

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.

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).

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.

Binary code 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.

The speed at which bits are sent is measured in bits per second (bps).

The old way’s called bipolar.

The new way’s called metal-oxide semiconductor (MOS, which is pronounced "moss"). It’s more popular because it costs less, consumes less electricity, and can hold more circuitry inside the chip.

The most popular kind of MOS is called negative-channel MOS. (It’s also called n-channel MOS or NMOS, which is pronounced "en moss".) The main alternative, called complementary MOS (or CMOS, pronounced "sea moss"), consumes even less electricity but can’t hold as much circuitry inside the chip. CMOS chips are used in simple-minded battery-operated computers (such as digital watches, pocket calculators, pocket computers, and notebook computers) and in some parts of larger computers.

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 the IBM PC and clones, the microprocessor uses a design invented by Intel. I’ll explain Intel’s microprocessors now (and discuss competitors later).

In the original IBM PC (and in the IBM PC XT), the microprocessor was the Intel 8088. IBM computers (and clones) containing that chip are called XT-class computers.

Later, Intel invented an improved version, called the Intel 80286. Since "80286" is too long a number for us humans to remember, most of us just call it the Intel 286. IBM used it in the IBM PC AT computer. That’s why computers containing that chip are called AT-class computers.

After inventing the Intel 286, Intel invented a further improvement (called the Intel 386), then an even further improvement (called the Intel 486).

In 1993, Intel began selling an even further improvement, which ought to be called a 586; but Intel calls 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 bodies, 5 ways!"

While inventing the Pentium, Intel gave it this secret code-name: "P5". Many folks still call that chip the P5.

Most computers built today contain a Pentium. A few computers built today contain a 486, which is slightly slower than a Pentium.

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.

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).

A 60-megahertz Pentium is called a Pentium-60. A 200-megahertz Pentium is called a Pentium-200.

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.

During a clock cycle, a 486 accomplishes half as much useful work as a Pentium. We say the 486’s usefulness factor is ½.

Intel 8088 4.77, 7.18 ¹/₂₀

Intel 286 6, 8, 10, 12 ¹/₅

Intel 386 16, 20, 25, 33 ¹/₄

Intel 486 20, 25, 33, 50, 66, 75, 100 ¹/₂

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.

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 and afterwards. It runs most programs about 15% faster than a Pentium classic; for example, a 200-megahertz Pentium MMX runs programs about 15% faster than a 200-megahertz Pentium classic. That’s because the Pentium MMX is designed slightly better than a Pentium classic and contains twice as much internal level-1 cache memory (an extremely fast form of memory that holds a copy of what’s coming from other memory). It’s called MMX because it also understands 57 extra instructions (called MultiMedia eXtensions), which can theoretically increase the speed of multimedia (video & sound) dramatically; but no important programs have been invented yet to make good use of those 57 extra instructions. Those 57 extra instructions just duplicate some of the intelligence found on fancy video-&-sound cards anyway. Intel’s official name for this chip is "Pentium with MMX Technology", but most folks say just "Pentium MMX".

The Pentium 2 is the newest and fastest kind of Pentium. Invented in May 1997, it runs most programs about 30% faster than a Pentium MMX. Like the Pentium MMX, it understands the 57 multimedia instructions. It’s too expensive for most folks; just rich folks buy it. Intel’s official name for this chips 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. The Pentium Pro was nicknamed the 686 or P6; the Pentium 2 now inherits those same nicknames.

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.

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. Instead of buying a math coprocessor chip made by Intel, you can buy an imitation made by Cyrix or ULSI:

Internal accumulator Each chip contains registers. Each register can hold a binary code number (such as 01000001). The chip’s main register is called the accumulator.

If the accumulator is wide enough to hold 32 bits inside it (such as 10000110111001111110010101010101), the accumulator is called 32-bit; the chip is said to contain a 32-bit accumulator and be 32-bit internally.

If the accumulator is narrower and holds just 16 bits, the accumulator is called 16-bit. In that case, the chip can handle code numbers that are 16 bits long but not code numbers that are 32 bits long. If you try to feed that chip a 32-bit code number, the chip won’t understand it.

The typical program uses just 16-bit instructions. (Instead of using a 32-bit instruction, it uses a pair of 16-bit instructions.)

But a few fancy programs use 32-bit instructions. To run those 32-bit programs, you must buy a chip that’s 32-bit internally. The chart shows that to run the fanciest programs (32-bit), you must buy at least a 386SX.

External data path The column marked "external data path" tells how many of the chip’s pins transmit data.

That technique of using just a few pins to transmit many bits is called multiplexing. Computerists say the 386SX is "a 32-bit chip multiplexed onto 16 pins"; they say the 386SX is a multiplexed 386DX.

Notice that the 386SX is just as smart as the 386DX — it understands the same 32-bit codes — but it transmits them more slowly (as 2 bursts of 16, instead of 1 burst of 32). So the 386SX is smart but a slow communicator — like Einstein with his mouth full and trying to talk through a narrow drinking straw.

The 8088 is a multiplexed 8086. Like the 8086, the 8088 thinks about 16 bits; but the 8088 must send them out in two 8-bit bursts.

Address The computer’s main memory (which consists of RAM chips and ROM chips) is like a city: each location in it has an address. If the main memory is large enough to hold lots of info, it has lots of addresses.

External megahertz If a chip is simple (an 8088, 8086, 286, 386SX, 386DX, 486SX, or 486DX), it communicates at the same speed as it thinks. For example, a chip that thinks at 20 megahertz communicates at 20 megahertz.

A 486DX2 chip communicates half as fast as it thinks. For example, a 50-megahertz 486DX2 chip communicates at 25-megahertz. That chip is said to be 50 megahertz internally, 25 megahertz externally. Since that chip thinks twice as fast as it communicates, it’s called a clock-doubled chip.

A 486DX4 chip communicates a third as fast as it thinks. For example, a 75-megahertz 486DX4 chip communicates at 25-megahertz. Since that chip thinks three times as fast as it communicates, it’s called a clock-tripled chip. Since the chip is clock-tripled, it ought to be called a "486DX3"; but Intel calls it a "486DX4" instead because Intel wants to pretend that it’s better than "486DX3" chips manufactured by IBM.

8088 The most popular imitation of the 8088 is the V20 chip. It’s made by Nippon Electric Company (whose abbreviation is NEC, which is pronounced "neck"). At 10 megahertz, it’s faster than Intel’s original!

8086 The most popular imitation of the 8086 is NEC’s 10-megahertz V30 chip.

286 Imitations of the 286 are made by Harris and come in 16-megahertz and 20-megahertz versions.

386 Imitations of the 386SX and 386DX are made by Advanced Micro Devices (AMD) and come in 40-megahertz versions.

486 AMD’s imitations of the 486 are excellent and come in 66-megahertz, 80-megahertz, 100-megahertz, and 120-megahertz versions.

Cyrix and IBM make awful 486 imitations that go much slower than Intel’s originals and ought to be called "386½" instead of "486". Cyrix’s imitation of the 486SX is called the 486SLC; Cyrix’s imitation of the 486DX is called the 486DLC. IBM’s imitation of the 486DX is called the Blue Lightning (BL).

Pentium classic Imitations of the Pentium classic are made by AMD and Cyrix.

Pentium Pro Cyrix makes a Pentium Pro imitation. Cyrix calls it the 686, but cynics call it the "586½".

Pentium 2 Imitations of the Pentium 2 are made by AMD and Cyrix.

AMD’s imitation is called the K6 MMX. The K6 MMX is slightly slower than a Pentium 2 but much faster than a Pentium MMX. Its price is ridiculously low: its quantity-1000 price is just $109 for 166MHz, $189 for 200MHz, $289 for $233MHZ. The K6 MMX 166MHz is an amazing bargain: its price is nearly as low as a Pentium classic 166MHz, but it performs better than a Pentium MMX 200MHz. Its ridiculously low price makes the K6 MMX the best value of chips made today: it gives you the most power per dollar. All reviewers love it and recommend it.

Intel’s biggest competitor is Motorola. It manufactures the 6809E microprocessor, the 68000 (which is faster and understands advanced commands), several souped-up versions of the 68000, and the Power PC:

What’s the Power PC? Motorola’s fastest microprocessor, the Power PC, was invented by a team of researchers from three companies (Motorola, Apple, and IBM), all working together. That’s why it’s called the love-triangle chip. It was invented to prevent Intel from monopolizing the microcomputer marketplace.

The first version of the Power PC, called the Power PC 601, was manufactured just by IBM. Later versions (the Power PC 603, 604, and 604e) are manufactured by both Motorola and IBM.

The Power PC is used in Apple’s fastest computer, the Power Mac.

Intel emulation Suppose your computer’s microprocessor is made by Motorola, but somebody gives you software written for Intel microprocessors instead. You can run that software on your computer if you feed your computer an Intel emulator (software that makes Motorola microprocessors imitate Intel’s). But Intel emulator software runs slowly. To accomplish tasks faster, buy software that runs directly on Motorola microprocessors without needing an Intel emulator.

Math coprocessor Want a Motorola math coprocessor? For the 6809E CPU, no math coprocessor is available. For the 68000 or 68020, buy the 68881 math coprocessor ($49). For the 68030, buy the 68882 math coprocessor ($69). The 68040 comes in two versions: the standard version (called the 68RC040) includes math-coprocessor circuitry; the stripped-down version (called the 68LC040) does not. The Power PC includes math-coprocessor circuitry.

In Motorola & Mac jargon, a math coprocessors is called a floating-point unit (FPU).

Accelerator cards To make an old Mac go faster, insert a CPU accelerator card, which contains a faster CPU chip. Here are your choices:

You can get those cards from Sonnet Technologies (800-786-6260, 714-261-2800) and Source Technologies (800-900-4599, 954-725-9777) Before buying one of those cards, ask the salesperson whether it’s compatible with your Mac.

Primitive old microcomputers contain microprocessors invented by Zilog and MOS Technology. They’re not Intel clones.

Zilog, which was owned by Exxon, made the Z-80A microprocessor, which was super-cheap: it cost just $2! It’s in many obsolete computers, such as the Radio Shack TRS-80 models 1 & 2 & 3 & 4 & 12, the Kaypro 2 & 4 & 10, the Epson QX-10 & Geneva, the Timex-Sinclair 1000 & 1500, and the Coleco Adam.

The 6502 microprocessor was invented by MOS Technology, which became part of Commodore. It was also manufactured by other chip makers, and you could get souped-up versions that understood extra commands and went faster.

If a chip is even fancier (such as the 68-pin Intel 286 or the 132-pin Intel 386DX), it requires too many pins to fit in a DIP. Instead of coming in a DIP, the chip usually comes in a pin grid array (PGA), which is a square having many pins underneath it, as if it were a square porcupine lying on its back.

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 — about a million times every second!

The part of the computer’s main circuitry that contains the memory chips is called the main memory.

If you buy extra memory chips (so that your computer can remember extra information), and the extra memory chips don’t all fit on the motherboard, you must buy an extra PC card to mount them on; that extra card is called a memory card. If the memory card comes in a cute little cartridge that you can pop into and out of the computer easily, it’s called a memory cartridge.

You need two kinds of memory chips: RAM and ROM. The RAM chips remember information temporarily; the ROM chips remember information permanently. Let’s begin by looking at RAM chips.

If a chip remembers information just temporarily, it’s called a random-access memory chip (RAM chip).

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.

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".

The abbreviation for kilobyte is K. For example, if a salesperson says the computer has a "64K RAM", the salesperson means the main circuitry includes enough RAM chips to hold 64 kilobytes of information, which is slightly over 64,000 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.

A terabyte is 1024 gigabytes. It’s slightly more than a trillion bytes.

Rows of RAM chips In a cheap microcomputer (such as the Commodore 64), the RAM is a row of eight NMOS chips. 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.

During the 1980’s, computer engineers invented 256K and 1M chips. The most popular style of 256K chip is called the 41256, which you can get from discount dealers for $2. A 1M chip costs $4.

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.

Parity chip The IBM PC and some clones contain an extra chip in each row, so that 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!

SIMMs and SIPPs If your computer is ultra-modern and you want to insert an extra row of RAM chips, you do not have to insert 8 or 9 separate chips. Instead, you can buy a strip that contains all 8 or 9 chips and just pop the whole strip into the computer’s motherboard, in one blow.

The typical strip of chips is called a Single In-line Memory Module (SIMM) and pops into one of the motherboard’s slots. If the strip pops into a series of pinholes instead, the strip is called a Single In-line Pin Package (SIPP).

You get those prices from discount dealers (such as Memory Express at 800-877-8188 or 818-333-6389). SIPPs cost $5 more than SIMMs.

The typical SIMM contains chips that are fast: they retrieve information in 70 nanoseconds. (A nanosecond is a billionth of a second.) Old-fashioned SIMMs contain slower chips, requiring 80 nanoseconds; the fanciest SIMMs contains extra-fast chips, requiring just 60 nanoseconds. The newest SIMMs include circuitry called Extended Data Out (EDO), which transfers data from the SIMM to the CPU faster.

Let your memory grow In a typical computer, the RAM contains several rows of chips, so that the total RAM contains several megabytes.

Mac The original Mac (nicknamed the Slim Mac) included 128K of RAM. Then came a version nicknamed the Fat Mac, which included 512K. Next came an improvement called the Mac Plus, which included 1M.

Names of classic computers The Commodore 64 computer got its name because it contained 64K of RAM. Then Commodore invented an improved version, the Commodore 128, which contained 128K of RAM.

The Laser 128 imitates the Apple 2c. Each comes with 128K of RAM.

IBM 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 PC or clone contains now:

The first 640K of main RAM is called the base memory (or conventional memory). That’s the part of the RAM that the computer can handle easily and quickly.

The next 384K is called upper memory. It’s relatively unimportant, since most programs don’t know how to use it.

The rest of the main RAM (beyond that first megabyte) can be either expanded or extended. Here’s the difference between "expanded" and "extended":

If a chip acts as "expanded" RAM, the CPU gets the chip’s information by copying that information to the upper memory area. Then the CPU examines what’s in the upper memory area. That process is slow, since you must wait for the CPU to copy the chip’s information to the upper memory area. That process was invented because it’s the only way an 8088 or 8086 chip can handle RAM beyond a megabyte. Extended RAM is faster and simpler but requires a 286, 386, 486, or Pentium — and is understood just by programs that are modern.

For an 8088 or 8086 CPU, the expanded RAM comes on an expanded RAM card. That card contains the RAM chips and the hardware necessary to control them. That card is expensive.

For a 286 CPU, you can buy an expanded RAM card, an extended RAM card (which is cheaper), or a combination card that you can switch between the two.

A trio of companies (Lotus, Intel, and Microsoft) agreed on the technical details of how expanded memory should be handled. Their agreement is called the Lotus-Intel-Microsoft Expanded Memory Specification (LIM EMS). Expanded memory fitting their specification is called EMS memory. To manage that expanded memory, you need a special program, called the expanded memory manager (EMM).

The same trio of companies, working together with a fourth company (AST), developed an agreement on extended memory. Their agreement is called the Lotus-Intel-Microsoft-AST eXtended Memory Specification (or LIMA XMS). Extended memory fitting their specification is called XMS memory. To manage that extended memory, you need a program called the extended memory manager. The most popular extended memory manager is called "HIMEM.SYS".

The first 64K of extended memory is called the high memory area (HMA), because it’s just slightly higher than the base memory and upper memory. (The rest of the extended memory should be called "even higher memory", but nobody does.)

NMOS RAM versus CMOS Most RAM chips are NMOS. The prices I quoted you were for NMOS.

Dynamic versus static A RAM chip is either dynamic or static.

If it’s dynamic, it stores data for only 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 and 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.

Dynamic RAM is called DRAM (pronounced "dee ram"). Static RAM is called SRAM (pronounced "ess ram").

Bipolar cache In a maxicomputer, minicomputer, or fancy microcomputer, the RAM is divided into two sections. One section is huge, contains many rows of NMOS chips, and is called the main RAM. The other section is tiny, contains just a few bipolar chips, and is called the cache (which is pronounced "cash").

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.

RAM chips remember, temporarily, info supplied by you.

ROM chips remember, forever, info supplied by the manufacturer.

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, NEW, RUN, LIST, GO, TO, END, STOP, INPUT, IF, and THEN. Those BASIC definitions in the ROM are called the ROM BASIC interpreter.

Commodore 64 For example, let’s look inside a primitive computer: the Commodore 64. It contains just four ROM chips. The first chip contains 8K, for the ROM bootstrap and ROM BIOS. The second contains Commodore’s 8K ROM BASIC. The third contains Commodore’s 4K character generator. The fourth contains ¼K that tells the computer how to make the screen produce pretty colors.

IBM In the typical IBM PC or clone, 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 designed by IBM; if your computer is a clone, 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 American Megatrends Incorporated (AMI), Award (a smaller company), and Quadtel (which is now owned by Phoenix.)

On a special PC card (called a video display card), you’ll find a ROM chip containing the character generator.

Extra ROM chips Some microcomputers include extra ROM chips that tell the computer how to handle specific applications, such as word processing and accounting.

ROM cartridges 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.

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).

To erase a typical EPROM, shine an intense ultraviolet light at it for 20 minutes. That’s called an ultraviolet-erasable PROM (UV-EPROM).