MCS-170 The Nature of Computer Science
1. Review - Logic Circuits and Switches
Circuit -- circuit that
controls the flow of electricity
"ON" - represents a binary
value of 1, or a logical value
"OFF" - represents a binary value of 0, or a logical value of FALSE
- Logical Circuit is
essentially a sophisticated Switch
- A switching circuit
can be as
simple as a switch connected in series with a lamp.
computer (~1930) Electric
relays as switches.
0 Computers (1940's) - Vacuum
Tubes as switches.
- Generation 1
Computers (1950's-1960's) - Transistors as Switches.
Types of Transistors-- NMOS
and PMOS. Type is dependent on
material added to silicon.
added which makes material more negative (majority of free electrons)
p-type silicon: boron impurities
added which makes material more positive
and Drain are n-type and sit on a positively charged well of p-type
positive voltage (green arrow) is applied to the gate, electrons in the
p-type silicon are attracted to the area under the gate, forming an
channel between the source and the drain. This
opens the flow from the drain to the
source. In this state the
transistor is on.
2. Transistors as Building Blocks of Logic Gates
- Combinations of NMOS and PMOS transistors can be used in sequence
(connecting output of one to input of another) to produce signals that
are functions of the input.
3. From Gates to Complex Circuits
- AND, OR, and NOT gates form the building blocks of more complex
logical and arithmetic circuits.
- Adding two 1-bit numbers:
- 0 + 0 = 0
- 0 + 1 = 1
- 1 + 0 = 1
- 1 + 1 = 0 with a carry of 1
- A circuit that would implement this addition would have two
input lines, representing the two numbers to be
added, and would need two output lines, representing the sum of the
inputs (as a bit) and a
possible carry bit.
- Logical Table --
- How can we use NOT, AND, OR gates to implement this
- The carry output is
a simple AND of the two inputs.
- The sum output is
- Full-Adder: Handles a
carry-in from a previous addition
4. Exclusive OR
- The Exclusive-OR, or XOR
can be described verbally as, "Either A or B, but not both."
- This is logically the same as (Not A AND B) OR (A AND NOT
B) (Use a table to show equivalence)
- In Class -- design a circuit with 2 NOT gates, 2 AND gates and
one OR gate that will implement an XOR function