This chapter comes from the 33rd edition of the "Secret Guide to Computers & Tricky Living," copyright by Russ Walter. To read the rest of the book, look at


The computer is full of chips. Let’s examine them.


Chip technology

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).

The motherboard lies flat, on the system unit’s bottom, in a laptop or tablet or smartphone or traditional desktop computer.

In an all-in-one computer, the motherboard is vertical, behind the screen.

In a tower computer, the motherboard is vertical, attached to the tower’s right edge.

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.


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.

The “caterpillars” come in many sizes. In a typical computer, the shortest caterpillars are ¾ of an inch long and have 7 pairs of legs; the longest are 2 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.

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).

4 purposes

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” info, it’s called a memory chip.

If the chip helps devices communicate with each other, it’s an interface chip.

If the chip acts as a slave & helper to other chips, it’s a support chip.

So a chip is either a processor chip, a memory chip, 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 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 2 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, serve chips & dips at parties, and are called “dipchips”.

Buying chips

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.

You can get chips from this famous mail-order chip supplier:

JDR Microdevices

1850 S. 10th St., San Jose CA 95112, 800-538-5000 or 408-494-1400

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).

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 B is 01000010; the code for the number 5 is 101; the code for 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 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!

Intel’s designs

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:

Chip’s name                     Year invented   Transistors on chip

Intel 8088                             1979                          29,000 transistors

Intel 286 (also called 80286) 1982                       134,000 transistors

Intel 386 (also called 80386) 1985                       275,000 transistors

Intel 486 (also called 80486) 1989                    1,200,000 transistors

Intel Pentium                        1993                    3,100,000 transistors

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.

Most programs require you to have a Pentium-class chip (Pentium or 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), to honor German physicist Heinrich Hertz.

A “million cycles per second” is called a megahertz (MHz).

1000 megahertz is called a gigahertz (GHz). It’s a billion hertz. Intel has invented fast Pentiums that go at 1, 2, 3, and even 3.9 gigahertz.

Slower than a Pentium

The Pentium’s 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 old chips (the 8088, 286, 386, and 486): the Pentium can perform more tasks simultaneously; it performs more parallel processing.

Old chips seem slower: too often during a clock cycle in earlier chips, part of the chip “does nothing” while waiting for the other chip parts 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 (UF) is ½.

During a clock cycle, a 386 accomplishes a quarter as much useful work as a Pentium, so the 386’s UF is ¼. A 286’s UF is 1/5. An 8088’s UF is 1/20.

Old 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:

Chip        Megahertz                                                                        UF

Intel 8088 4.77, 7.18                                                                              1/20

Intel 286  6, 8, 10, 12                                                                            1/5

Intel 386  16, 20, 25, 33                                                                       ¼

Intel 486  20, 25, 33, 50, 66, 75, 100                                                     ½

Pentium   60, 66, 75, 90, 100, 120, 133, 150, etc., up through 3900      1

For example, suppose you buy an Intel 486 going at 100-megahertz. Since it suffers from a UF 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.

The slowest chip is a 4.77-megahertz 8088. Since it suffers from a UF of 1/20, it acts about as fast as a 0.2385-megahertz Pentium. That’s 16,352 times slower than the fastest Pentium, which goes at 3900 megahertz. Yes, the fastest IBM-compatible computers think over 16,000 times faster than the slowest ones! That’s progress!

The usefulness factor (UF) 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.

Variant chips

Old chips have variants:

The Intel 8088 comes in 2 versions. One version (called simply the “8088”) goes slightly slower than the other version (called the 8086).

The Intel 386 comes in 2 versions. One version (the 386SX) goes slightly slower than the other version (the 386DX).

The Intel 486 comes in 2 versions. One version (the 486SX) goes slower than the other version (the 486DX). Moreover, the 486DX comes in 3 varieties: the original 486DX, the 486DX2, and the 486DX4.


The Pentium comes in many versions. Here are the most popular, listed from slowest to fastest:

Version                  Invented  Comment

Pentium classic         1993           Pentium Pro is a faster variant

Pentium MMX         1995           understands 57 more instructions

Pentium 2                1997           resembles Pentium MMX but 30% faster

Pentium 3                1999           understands 70 more instructions

Pentium 4                2000           Pentium 4M uses less electricity, for latops

Pentium D                2005           D means dual: caterpillar contains 2 chips

Pentium Core Duo    2006           1 chip contains 2 cores, so acts like 2 chips

Pentium Core 2 Duo 2006           1 chip contains 2 cores, so acts like 2 chips

Pentium Core i3       2010           1 chip contains 2 cores, so acts like 2 chips

Pentium Core i5       2010           crude version in 2009, but now 2 or 4 cores

Pentium Core i7       2010           crude version in 2008, but now 4 or 6 cores

To help low-income folks, Intel eventually decided to make cheaper Pentiums, called Celeron. They go slower.

The first Celeron, invented in 1998, was a cheaper, slower version of the Pentium 2. The newest Celeron is a cheaper, slower version of the
Pentium Core 2 Duo.

For very low-income folks, Intel makes a version that’s even cheaper & slower, called the Atom. It’s used in netbook computers.

What’s available

Intel has stopped marketing the oldest chips (8086, 8088, 286, 386, 486 and oldest Pentiums). Modern computers use these new Pentiums: the Core i3, Core i5, and Core i7.

Here are prices of various Pentium chips:

Intel Pentium chip   Cores  Cache memory    Gigahertz    Price

Celeron G1620               2             ½ megabyte           2.6GHz              $69

Pentium G3260              2             3 megabytes          3.3GHz              $78

Core i3-4170                  2             3 megabytes          3.7GHz            $155

Core i5-3330                  4             6 megabytes          3GHz               $219

Core i5-3470                  4             6 megabytes          3.2GHz            $239

Core i5-3570                  4             6 megabytes          3.4GHz            $249

Core i7-3770                  4             8 megabytes          3.4GHz            $336

Core i7-4820                  4           10 megabytes          3.9GHz            $369

Core i7-990x                  6           12 megabytes          3.4GHz          $1054

The chart shows the price charged by a discount dealer
(JDR Microdevices) for a single chip when this book went to press in September 2016. 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 less. That chart also shows how much cache memory
(fast-access internal memory) is included inside the Pentium chip.


Intel’s competitors have imitated Intel’s chips. Some imitations go faster than Intel’s originals!

Intel’s chip                     Imitations

8088 (4.77 or 7.18 MHz)    NEC’s V20 chip goes faster: 10 MHz.

8086 (8 or 10 MHz)           NEC’s V30 chip goes fast: 10 MHz.

286 (6-12 MHz)                Harris’s 286 goes faster: 16 & 20 MHz versions.

386 (16-33 MHz)               AMD’s 386 goes faster: 40 MHz.

486 SX (20-33 MHz)         Cyrix’s 486SLC goes slower (UF is just 1/3).

486 DX (25-100 MHz)       AMD’s 486 goes faster: 66-120 MHz versions.

Pentium classic (60-200)    AMD’s 586 & Cyrix’s 586 go slower (UF just 2/3).

Pentium 2 (233-450)          AMD’s K6 & K6-2 go slightly slower (UF just 7/8).

Pentium Celeron (266-2800) AMD’s Duron & Sempron go the same speed.

Pentium 3, etc. (450-3600) AMD’s Athlon II & Phenom II & A go the same.

AMD’s A chip is popular, because it’s a CPU chip that includes graphics processing on the same chip. Here are the prices charged by a discount dealer (JDR Microdevices):

AMD A chip          Cores  Cache memory    Gigahertz    Price

A4-3300                  2           1 megabyte              2.5 GHz            $52

A4-3400                  2           1 megabyte              2.7 GHz            $54

A4-5300                  2           1 megabyte              3.4 GHz            $69

A6-5400K               2           1 megabyte              3.6 GHz            $88

A6-6400K               2           1 megabyte              3.9 GHz            $90

A6-3500                  4           4 megabytes             2.1 GHz            $91

A6-3670K               4           4 megabytes             2.7 GHz          $105

A8-3850                  4           4 megabytes             2.9 GHz          $130

A8-5600K               4           4 megabytes             3.6 GHz          $140

A10-5800K             4           4 megabytes             3.8 GHz          $150

Half-assed systems

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 let 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’s 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.

Total cost

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 many hundreds or 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.

Math coprocessor

Each Pentium chip includes math coprocessor circuitry, which handles advanced math fast. That circuitry can multiply & divide long numbers & decimals and 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!

The slowness will annoy you 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.

Here’s the only difference between a 486DX chip and a 486SX chip:

The 486DX chip (and 486DX2 and 486DX4) includes math-coprocessor circuitry; the 486SX does not. Intel invented the 486DX, then later invented the 486SX by using this manufacturing technique: Intel took each 486DX whose math coprocessor was faulty and called it a 486SX. So a 486SX was just a defective 486DX.

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 math coprocessor chip. Put it next to the CPU chip on the motherboard. It contains the math-coprocessor circuitry that the CPU lacks.

Intel CPU        Which Intel math coprocessor to buy

8088 or 8086     8087

286                   287

386SX              387SX

386DX              387DX

486SX               487SX

Better yet, give up and buy a new computer, containing a Pentium!



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.

The RAM  chips remember info temporarily.

The ROM chips remember info permanently.

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 the computer doesn’t have enough RAM chips to hold an 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 short problem 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. Examples:

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 the phrase LOVE 2 KISS U consists of 13 bytes. If you store that phrase in the RAM, 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. 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. The abbreviation for terabyte is T.

In honor of the words “kilobyte”, “megabyte”, “gigabyte”, and “terabyte”, programmers name their dogs Killer Byte, Make A 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 a total of 64 kilobytes (64K). That row of chips is called a 64K chip set. Each chip in that set is called a “64K chip”, but you need a whole row of those 64K chips to produce a 64K RAM.

If your computer is slightly fancier (such as the Apple 2c), it has two rows of 64K chips. 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 4 rows of 64K chips. Since each row is a 64K RAM, the 4 rows together total 256K.

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.

Parity chip

The original IBM PC contained an extra chip in each row, so each row contained 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.

If the strip is classic,

it contains a single row of chips, pops into one of the motherboard’s slots,

and is called a Single In-line Memory Module (SIMM).

If the strip is modern,

it contains two rows of chips (one row on each side of the strip)

and is called a Dual In-line Memory Module (DIMM).

If the strip is old-fashioned and weird,

it pops into a series of pinholes instead of a slot

and is called a Single In-line Pin Package (SIPP).

Staples charges $15 for a 2-gigabyte DIMM, $27 for a 4-gigabyte DIMM, $46 for an 8-gigabyte DIMM.

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.

In old-fashioned computers,

each SIMM fits into a motherboard slot by using 30 big pins.

In computers that are more modern, each SIMM uses 72 big pins instead.

The typical DIMM uses 168, 184, or 240 big pins.

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”).

Faster circuitry

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:

In 1987 came the first improvement, called Fast Page Mode (FPM).

In 1995 came Extended Data Output (EDO), which went even faster.

In 1996 came Synchronous DRAM (SDRAM), which went even faster.

In 1999 came Rambus DRAM (RDRAM), which went even faster.

In 2000 came Double Data Rate SDRAM (DDR SDRAM),

which had 184 pins and went about as fast as RDRAM but cost less.

In 2003 came DDR2 SDRAM,

which has 240 pins and transfers data twice as fast as DDR SDRAM.

Early versions of DDR2 SDRAM didn’t work well;

but at the end of 2004, DDR2 SDRAM improved enough to be practical.

In 2007 came DDR3 SDRAM,

which has 240 pins and transfers data twice as fast as DDR2 SDRAM.


If you want to buy an extra SIMM or DIMM to put in your computer, make sure you buy the same kind as what’s already in your computer. Make sure the extra SIMM or DIMM has the same number of pins (30, 72, 168, 184, or 240?), 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, DDR, DDR2, or DDR3).


The original IBM PC came with just 16K of RAM, but you could add extra RAM to it.

To run modern Windows software, you need at least 4G of main RAM; but many people still use older software that can run on 1G, 512M, 256M, or even less RAM.



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:

RAM chips remember, temporarily, info supplied by you.

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

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 IBM-compatible PC, the motherboard contains a ROM BIOS chip. That chip contains the ROM BIOS and also the ROM bootstrap. If your computer’s made by IBM, that chip is typically designed by IBM; if your computer’s made 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 designed 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).

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:

An ultraviolet-erasable PROM (UV-EPROM) gets erased by shining an intense ultraviolet light at it for 5 minutes (or leaving the chip in sunlight for 2 weeks). That technique erases the entire chip.

An electrically erasable PROM (EEPROM) gets erased by sending it a 25-volt shock for a tenth of a second. That technique erases just one byte in the chip: to erase many bytes, you must perform that technique many times.

Flash memory gets erased by sending it a 3-volt shock for 1 second.
That technique erases a whole 64-kilobyte block at once, “in a flash”.
It’s the most popular type of erasable PROM. It’s used in digital cameras (to store pictures), cell phones, and reprogrammable BIOS chips. If the flash memory pretends to be an extra hard disk & drive, it’s called a
solid-state drive (SSD) and runs faster than a traditional hard disk & drive. A solid-state drive that plugs into the system unit’s USB port is called a
USB flash drive (and is about the size of your thumb); it costs $5 for 16 gigabytes, $8 for 32 gigabytes, $12 for 64 gigabytes, $20 for 128 gigabytes, at Best Buy.

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 less to copy.