### 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

• define correspondence between 0/1 and particular values of voltage or current
• basic element is a switch: device whereby one electrical signal can be used to control another
• switches can be used to perform logical functions (and, or, not)

#### 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.

#### Integrated Circuit Fabrication  (text, sec. 1.4 - 1.5)

Current computer technology is based on VLSI:  very large scale integration.  This involves building a circuit with millions of transistors on top of a small piece of silicon.  The ability to build many of these circuits on a single silicon wafer keeps their cost down;  the ability to make smaller and smaller transistors leads to a steady increase in circuit speed and complexity.

#### 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 (because they had no mechanical moving parts), 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.

#### Technology Trends

We continue to learn, at a steady pace, how to fabricate smaller and smaller transistors. This allows for
•      a higher level of integration (currently, millions of gates on a chip)
•      faster circuits (compare the initial PC of 5 Mhz with current PCs of 2 GHz)
The cost of a chip has remained (very roughly) constant, so price/performance has been rapidly decreasing. How should this extra circuitry and extra performance be used? We shall return to this issue after discussing processor design.

## Logic Design: Combinatorial Circuits

(Text: Appendix B.2 and B.3)

Combinational circuits and gates

combinational circuits: no memory -- output is only a function of current inputs

gates are basic combinational circuits: AND gate, OR gate, inverter (complement), etc.

realization of gates from switches:

OR gate from parallel circuit
AND gate from series circuit

#### Representation of combinational circuits

•      by Boolean formulas
•      by truth tables
•      by logic diagrams
•      translating between these representations

#### Examples of combinational circuits

•      NAND and NOR gates
•      exclusive-OR gate (half-adder)
•      multiplexors
•      decoders
•      full adders