This book follows how switches were etched together onto a chip, from the integrated circuit to the memory chip. It is a simplified picture, because a real computer has more parts. A computer is built from a few main parts, including a CPU and memory. A program is a list of instructions for a computer, and the processor in a computer follows the ones that a programmer writes. A computer's processor has two parts: the arithmetic-logic unit, a simple calculator, and the control unit that follows the program's instructions. The CPU of a computer can also add two numbers, and the parts of a computer are connected by a set of wires called the bus. The computer does not do anything automatically: some program code always tells it how.
A bit is the smallest piece of computer storage, and it holds a 0 or a 1. Binary is a number system that uses only two digits, 0 and 1. A byte is a group of 8 bits, and one byte can store one letter or other character, such as A or x. Each extra bit doubles the number of different patterns that bits can make, and a byte, which is 8 bits, can make 256 different patterns. Inside a computer, things such as pictures are stored with only 0s and 1s. A picture on a computer screen is made of many tiny squares called pixels, and in the RGB scheme, each of red, green and blue light is a number from 0 (off) to 255 (brightest). Everything that is seen on a computer screen, even text, is an image that the device has been programmed to display. All of these ideas are ways to keep numbers and pictures as ones and zeros that a machine can handle.
A machine code instruction is extremely simple, such as adding two numbers or testing whether a number is zero. Programming languages work by translation to machine code, and no single machine code instruction prints. Only machine code can actually run on the CPU, so source code written in a programming language must be translated first. A compiler can translate a program's source code into machine code ahead of time. A CPU's machine code is hardwired into its design, and each family of CPUs has its own machine code. Loops and if-statements run because some of them jump back to an earlier point or skip the next instruction. In the fetch/execute cycle, the processor fetches an instruction from memory, decodes it to learn what to do, executes it, and repeats. Instructions can be stored as numbers too, an idea that led von Neumann and his collaborators to a general purpose, reprogrammable computer.
RAM is fast memory that holds the programs and files a computer is using right now, and it loses what it holds when the power is turned off. When a computer first powers up, the hardware runs a tiny boot loader program that lets an operating system start. The operating system is a supervisory program that starts first when the computer boots up. To launch a program, the operating system gives it an area of RAM, loads its first machine code there, and tells the CPU to start running it. The operating system gives each program its own area of memory, so that each program only reaches its own resources. One CPU can seem to run many programs at once by running a few instructions from each in turn, called round robin.
All digital computers rely on binary ones and zeros and on rules of logic set out by the English mathematician George Boole in the 1850s. Boole argued that just three operations, AND, OR and NOT, can perform all logic functions, but his logic had no practical use for decades until Claude Shannon applied it to telephone switch design in the 1930s. Computers are digital, but must convert continuously varying analog signals such as sound, light and heat into ones and zeros to process them. On computer networks, bits are communicated by light, voltages or sound. In 1947, John Bardeen and Walter Brattain built the first semiconductor amplifier. It was named the transistor. A transistor is a solid state switch with no moving parts, and it is turned on and off by an electric signal. Transistors can work as switches in computers.
The first transistor was a sliver of germanium with two closely spaced gold contacts, and they were held in place by a plastic wedge. A small change in the current through one contact caused a larger change in the current at the other contact, so it amplified signals. It amplified its input signal up to 100 times. The 1956 Nobel Prize in Physics went to Shockley, Bardeen and Brattain for their research on semiconductors and for their discovery of the transistor effect. A semiconductor is a material between a conductor and an insulator, and most semiconductor components are made of silicon. By 1960, most transistors were silicon, which could work at temperatures up to 150 degrees Celsius. Microscopic transistors are etched onto a fingernail-sized piece of silicon to make a chip. So by then there was a small, fast switch made of silicon, and the next step was to put many of them on one chip.
Jack Kilby built the first integrated circuit in 1958 by etching transistor, capacitor and resistor elements on one germanium slice. Integrated circuits made transistors, wires and other parts on one chip, using a process similar to printing. Computer technology took off with the integrated circuit around 1960, and it took off again with the microprocessor in the 1970s, when a chip could hold a complete micro computer. Moore's law was a prediction, not a law of nature, that the number of components on a chip would double every eighteen months. A 12-core microprocessor chip shown at the Computer History Museum holds 1.8 billion transistors. So the integrated circuit joined many switches into one chip, and that one idea, used again and again, is how a chip came to hold so many of them.
Faster chips made slow magnetic core memory a bottleneck, and by the early 1980s semiconductors were the main type of memory. RAM can fetch any byte in a few nanoseconds, and a nanosecond is one billionth of a second. A hard drive can be far slower than RAM: one college course says at least a million times slower. Dynamic RAM packs more bits on a chip by using fewer transistors to reach each storage capacitor, and by 2000, DRAM chips holding over 1,000 million bits were available. Much of the chip area of modern microprocessors and graphics engines holds memory, not logic. So the story of the chip is a story of switches that grew smaller and more numerous, and of memory that had to keep up with the fast chips they made.
Words to know
- integrated: joined together
- microprocessor: complete micro computer
- germanium: a semiconductor
- capacitor: storage element
