### Lecture 1: Historical Background

Text: Chapter 1

#### 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.