iWoz (40 page)

hertz
A unit of frequency equal to a cycle per second. Named for the German physicist Heinrich Hertz.

hexadecimal
A Base 16 system commonly used by today's digital computers, which work with binary digits (1 and 0) and bytes (eight Is and 0s, or bits) of information at a time. Two hexadecimal digits can represent a byte, as follows:

infinite loop
Often also called an "endless loop," this is a piece of coding that accidentally or by design repeats indefinitely.

instruction
This is a key term in computer technology. It is an order created by a computer program delivered to a computer processor. Each instruction, at its most basic level, is just an order for the computer to do something (like "add" or "subtract") with the Is and Os that make up computer data. See assembler language; register.

interrupt
This refers to a signal that comes from a device attached to a computer—or from a program running on that computer—that causes the CPU system software to stop and pay attention to what should be done next. Almost all computers today use interrupts. That is, they run whatever program they're running instruction by instruction until they are "interrupted" by a device or another program. For instance, if you hit the G key while a program is running, the system will pause, notice that the keyboard has interrupted it, and run the program that will display "G" on-screen.

kilobyte
A unit of computer storage equal to approximately 1,000 bytes of data—more precisely, 2 to the tenth power, or 1,024 bytes.

logic gate
A single computer circuit that has several points of input but only one point of output. It is an elementary building block of a circuit. Most logic gates have two inputs and one output.

At any given moment, every terminal is in one of the two conditions—low (0) or high (1)—defined by the voltage level. The state, 0 or 1, changes often as data is processed. For example, the AND gate is called that because if 0 is false and 1 is true, the gate acts the same way as the standard AND operator in Boolean algebra.

With an OR gate, the output is true (or 1) if either or both of the inputs are true (or 1). If both inputs are false (0), then the output is false (0).

The XOR (exclusive-OR) gate acts in the same way as the logical "either/or." The output is "true" if either, but not both, of the inputs is "true." The output is "false" if both inputs are "false" or if both inputs are "true."

A logical inverter, sometimes called a NOT gate to differentiate it from other types of electronic inverter devices, has only one input. It reverses the logic state.

The NAND gate operates as an AND gate followed by a NOT gate. It acts in the manner of the logical operation "and" followed by negation. The output is "false" if both inputs are "true." Otherwise, the output is "true."

The NOR gate is a combination OR gate followed by an inverter. Its output is "tine" if both inputs are "false." Otherwise, the output is "false."

The XNOR (exclusive-NOR) gate is a combination XOR gate followed by an inverter. Its output is "true" if the inputs are the same, and "false" if the inputs are different.

Using combinations of logic gates, complex operations can be performed. In theory, there is no limit to the number of gates that can be arrayed in a single device. But in practice, there is a limit to the number of gates that can be packed into a given physical space. Arrays of logic gates are found in digital integrated circuits (ICs). As IC technology improves, the physical space it takes up becomes smaller and smaller. That means faster chips in smaller packages—and increasing computer power at decreasing prices. See
Moore's Law
.

machine code
The basic-level language that a computer can understand, this refers to a stream of binary digits—Os and Is—or bits. See bit; byte.

memory
The electronic holding place for instructions and data that your computer needs to reach quickly. Typically referred to as "RAM" (short for random-access memory), memory is typically located on a set of microchips located physically close to the computer processor. When you turn a computer off, all information held in RAM disappears.

Moore's Law
Intel founder Gordon Moore in 1964 made the following now-famous observation: that due to improvements in manufacturing, every eighteen months engineers would be able to double the number of transistors on a chip. Moore's Law has held true to this day.

motherboard
The physical layout inside a computer that contains its basic circuitry and components. The motherboard most typically contains the CPU, the main system memory, the basic input/output system (BIOS), a group of expansion slots, and additional interconnection circuitry. Also sometimes referred to as the "main board" and the "system board."

NOR
gate
See
logic gate.

OR gate
See
logic gate.

oscilloscope
A laboratory instrument commonly used to display and analyze the waveform of electronic signals. On a screen, the device draws a graph of voltage over time.

processor
The logic circuitry in a computer that responds to a computer instruction. Generally people use the term "processor" to refer to a computer's central processing unit (CPU). Another common term for the CPU is "microprocessor." See
CPU
.

PROM
Short for programmable read-only memory, this is a type of computer chip with data that can only be changed with a special machine. Such a machine, often called a "PROM programmer," actually blows a fuse on the chip—hence the term "burning a PROM." See
EEPROM; EPROM.

RAM
Short for random-access memory, this is the type of memory chip a computer uses for short-term storage and calculation. This kind of "memory" is not to be confused with the permanent storage a hard disk or CD-ROM drive provides. RAM chips lose their contents whenever you power down the computer. See
memory.

register
In a computer processor, a register is a holding place for any kind of data, including a storage address, individual characters, or a computer instruction. For example, a computer instruction might command that the contents of two registers be added together. A register is typically large enough to hold a 32-bit instruction, though there are smaller registers, such as half-registers, in some computer designs.

resistance
Denoted by the representation R, this is the opposition a given substance offers to the flow of current. Measured in ohms.

resistor
An electronic component that controls the flow of current in a circuit by resisting, or turning away, electricity. Typically, resistors are mounted on a printed circuit board or built into a chip.

ROM
Built into every computer, ROM, short for read-only memory, is computer memory that contains data that can only be read. It is designed to store data permanently, and not be erased or modified by a user. A ROM chip contains the program that allows a computer to be restarted and still remember its basic settings every time. Unlike the computer's RAM (random-access memory), the data on this chip stays intact even when you turn the power to the machine off. The ROM is typically powered by a small long-life battery. See
EEPROM; EPROM; PROM; RAM.

signal
Most simply, this is an electric current or field used to carry data from one place to another. A direct current (DC) signal that can be switched on and off is a simple form of carrying information—it's how the early telegraph signals worked. A more complicated signal consists of an alternating current (AC) to carry more than one stream of data at a time.

sine wave
The most familiar alternating current waveform, varying with time. A waveform is a pictorial representation of how alternating current (AC) varies over time.

slot
See
expansion slot.

storage
Refers to the place in a computer where data is held in electromagnetic or optical form for access by a computer processor. The term "primary storage" generally refers to the place in memory where data is held; the term "secondary storage" generally refers to permanent data holding on hard disks, tapes, and other storage media.

transistor
A tiny device for regulating electronic signals. Invented by three scientists at Bell Laboratories in 1947, this was a key invention that enabled computers and computerized devices. Before transistors, vacuum tubes were in use—but they quickly became obsolete after the transistor was widely available for the purpose of regulating current (voltage). Transistors act as incredibly tiny and effective switches for electronic signals. See vacuum tube.

transistor circuit
See
transistor.

vacuum tube
Also known as an "electron tube," a vacuum tube was once commonly used to amplify electronic signals. It is now mostly obsolete, having been replaced in electronics by the transistor. See
transistor.

Acknowledgments

First, I must thank my parents for helping me find my own values and for assisting my education.

This book could not have happened without believers in the publishing industry. John Brockman did a splendid job in finding our publisher, W. W. Norton. We lucked out to get the legendary nonfiction editor Angela von der Lippe on this book. Her true interest was part of what I needed to get this project done. Countless others had important roles as well.

More thanks than I can ever give should go to Gina Smith, who had the drive to create this book and who met with me on countless occasions to put it all together. Just having a schedule and a purpose and prodding made all the difference. We would get together and speak stories into recorders, and go over and over the paragraphs to get the right sound. Thanks also to Michele Earl for a lucky and unusual encounter that led to my meeting Gina.

I have to thank those responsible for my successes in life. To Miss Skrak for seeing so much in me. To Mr. McCollum for finding so much valuable education beyond the school he worked for. To Steve Jobs for wanting to do Great and Big things. To Randy Wig- gington, Chris Espinoza, Dan Sokol, Bill Fernandez, and the Homebrew Computer Club members for the whole appreciation of a