V22.0436 - Prof. Grishman

Lecture 1: Historical Background

Text: Chapter 1

(review administrative material)

Types of computers

Digital vs. analog computer systems: we will only discuss digital systems (systems where signals take on only a discrete number of states, rather than a continuous range of values)

Binary nature of essentially all hardware components: switches which are open or closed, bistable memory elements (which hold a single bit of information)

Electronic systems (as contrasted to mechanical or fluidic computers): information represented by electical signals

Time and frequency

Time is measured in seconds and in fractions of a second:

millisecond (ms): 0.001 second
microsecond (us): 0.000 001 second
nanosecond (ns): 0.000 000 001 second
(and soon we will need picoseconds (ps): 0.000 000 000 001 second)

For a repetitive phenomenon, the rate at which it repeats is the frequency, measured in cycles per second or Hertz. For higher frequencies we have

1 kilohertz (kHz) = 1 000 cycles / second
1 megahertz (MHz) = 1 000 000 cycles / second
(and soon we will need 1 gigahertz (GHz) = 1 000 000 000 cycles / second)

There is a reciprocal relationship between frequency and the time for a cycle (the 'period'):

frequency = 1 / clock period
clock period = 1 / frequency

For example, if a CPU operates at 100 Hz, its "clock cycle" is 0.01 second = 10 ms; if it operates at 100 MHz, its clock cycle is 0.000 000 01 second = 10 ns.

Switching elements and computer generations

(text, sec. 1.7)

Relays were used in the earliest equipment: electronic accounting machines, and early computers made in the 1930's and early 1940's at Bell Labs and by Zuse in Germany. Relays are mechanical and hence relatively slow --- their response time is measured in milliseconds.

Vacuum tubes were dominant about 1945 to 1960 ('first generation of computers'). The first general purpose computer was the ENIAC (1946): 18,000 tubes, 20 10-digit accumulators, 100 kHz clock, 200 microsecond add time, 200 KW power. Tubes were much faster than relays, but they were bulky, required high power, and had a relatively short life (crucial because of the large number of components).

Discrete transistors ('second generation computers') were used in computers starting around 1960. They took less power , were smaller, and had a longer lifetime than vacuum tubes.

Integrated circuits ('third generation computers') were created by fabricating several transistors on a single chip, allowing computers to be made smaller. IC's were introduced in the late 1960's, and gradually increased their level of integration (number of transistors on a chip).

Very large scale integration ('fourth generation computers') represented the ability to put more and more transistors on a chip, until an entire processor (a microprocessor) could be fabricated on a single chip. Initially VLSI was used to make personal computers (Apple II - 1977; IBM PC - 1981); now all computers are made from VLSI.