CS100, Spring 1996: Answers to Quiz 4

This page contains questions and answers for the fourth quiz in CS100: Principles of Computer Science, a course based on the book The Most Comples Machine.

The answers given here are sample answers only. There is always some variation in the answers that would receive full credit.

Question 1: Here is a list of names and terms from the history of computing. Pick any four of these and briefly explain the role of each in that history: Ada Augusta (Countess of Lovelace); the Analytical Engine; "human computers"; relays; Eckert and Mauchly; the ENIAC; John von Neumann; Alan Turing; integrated circuits.

Answer: Here are possible answers for all nine of the options listed in the question:

• Ada Augusta. The Countess of Lovelace was a supporter of Charles Babbage and wrote about his Analytical Engine. Her writings included programs for the Engine, and hence she is often referred to as the first computer programmer.
• The Analytical Engine. In the mid-1800s, the English mathematician Charles Babbage designed this computing machine, which was never built. It would have had many similarities with modern computers, and it was the first universal computing machine ever conceived. However, this work was largely forgotten for a hundred years.
• "Human computers". Before computers existed, complicated computations were carried out by hand by people who were called "computers". These human computers were replaced by the machines we now call computers.
• Relays. Some very early computers were built using mechanical switches called relays, instead of the much faster electronic switches known as vacuum tubes and transistors.
• Eckert and Mauchly. J. Presper Eckert and John Mauchly designed the first universal computing machine that was ever built, the ENIAC. They also came up with the idea of a stored program computer, and they founded the very first commercial computer company.
• The ENIAC. When the ENAIC was first turned on, on February 14, 1946, it was the first general-purpose electronic computer. Programming the ENIAC, however, involved setting thousands of switches and plugging and unplugging thousands of wires.
• John von Neumann. Von Neumann was an American mathematician who wrote a report on the idea of a "stored program computer". This report was very influencial, and such computers came to be known as "von Neumann machines".
• Alan Turing. Ten years before actual computers were invented, Turing wrote about theroetical computing machines now called Turing machines, and laid the groundwork for theoretical computer science. During World War II, he worked on the British code-breaking effort. He was perhaps the first to realize that computers could do more than arithmetic, that they could process symbols of any type. He speculated on the possibilities of machine intelligence, and came up with a test, now called the "Turing Test", to determine whether a computer is intelligent. He committed suicide at the age of 42 after prosecution under British anti-homosexual laws. The Turing Award, the highest prize in the field of computer sicence, is named after him.
• Integrated Circuits. An integrated circuit contains several--or many--transistors on a single silicon chip. Integrated circuits made small, cheap, reliable computers possible. Modern integrated circuits can contain millions of transistors on a chip less than a square inch in area. An entire CPU can be fabricated on a chip, which is known as a microprocessor. Microprocessors are the basis of the personal computer industry.

Question 2: What is the purpose of a device driver ?

Answer: A device is the software that the CPU executes in order to communicate with and control other devices, such as the keyboard, a sound card, a network interface card, etc. A device driver consists of subroutines and interrupt handlers specific to one particular type of device. Connecting a device physically to the computer is not enough to make it usable; the device driver must also be added to the system software.

Question 3: What is the purpose of a bus in a computer?

Answer: A bus is a bunch of wires that connect the CPU to other hardware devices in the system. The idea of a bus is that many devices can attach to the same set of wires. This brings order to the system and makes it possible to add new kinds of devices by plugging them into "expasion slots" on the bus rather than by redesigning the whole system.

Question 4: "Interrupts" make it possible for the CPU to communicate with and control other devices in a computer system. Outline the process that the CPU uses in order to "handle" an interrupt when one occurs.

Answer: An interrupt is a physical signal on a wire attached to the CPU. It indicates that some device needs the CPU's attention. When an interrupt occurs, the CPU first saves enough information about its current "state" to enable it to restore that state later. This generally means saving the values of important registers. Then the CPU jumps to a predertermined location in memory where an "interrupt handler" has been placed. This interrupt handler is the program that the CPU executed in order to respond to the situation that caused the interrupt. When it finishes executing the interrupt handler, the CPU restores its saved state and so returns to what it was doing before the interrupt occurred.

(by David Eck)