#### Transcript Lecture 4 - ODU Computer Science

```CS149D Elements of Computer
Science
Ayman Abdel-Hamid
Department of Computer Science
Old Dominion University
Lecture 4: 9/5/2002
Lecture 4: 9/5/2002
CS149D Fall 2002
1
Outline
•Storage of Bits
•Main Memory
Should cover sections 1.1 and 1.2 of Brookshear Text
Lecture 4: 9/5/2002
CS149D Fall 2002
2
Boolean Operations
Manipulate True and False:
Boolean Operations
The NOT operation
NOT 1 = 0
NOT 0 = 1
Exclusive OR
Output of 1 when
exclusively one of its
inputs is TRUE
Lecture 4: 9/5/2002
CS149D Fall 2002
3
Gates1/3
•Device implementing Boolean operation
•Current technology implements gates as small electronic circuits
•Building blocks from which computers are constructed
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CS149D Fall 2002
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Gates2/3
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Gates3/3
Which Input bit patterns will cause the following circuit to
produce an output of 1?
Inputs
XOR
AND
Lecture 4: 9/5/2002
CS149D Fall 2002
Output
6
Flip Flop
Circuit (set of connected gates) which the output will flip or flop between
two values under control of external stimuli
•If both inputs are 0,
output will not change (A
= 0, B = 0)
A
•Placing 1 on upper input
forces output to be 1 (A =
1, B= 0)
•Placing 1 on lower input
forces output to be 0 (B =
1, A = 0)
•Flip Flop can be used to
store value of a bit
Try Sequence?
B
A
B
1
0 then
0
0
Lecture 4: 9/5/2002
CS149D Fall 2002
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Other Bit Storage Techniques
•1960s: Rings of magnetic material (Cores) threaded on wires
•Electric current through wires  core magnetized in one of two directions
•Recently: Capacitor
•Metallic plates positioned // to each other with a small distance between them
•Connect voltage source and charge plates and then remove voltage source
•Capacitor is either charged or discharged. Two states (can represent 0 and 1)
•Technology can place millions of capacitors on a single chip
•Storage volatility
•Core: retain magnetic field after machine turned off
•Flip flop: looses data when power source turned off
•Charges on capacitors dissipate, and have to be replenished (need for refresh circuitry)
•Computer memory based on such technology called Dynamic Memory
Lecture 4: 9/5/2002
CS149D Fall 2002
8
Main Memory
•Memory cells (words). Typical cell size is 1 byte
•Often measured in MB (megabyte 220), KB (Kilobyte 210), GB (Gigabyte 230)
•Each cell has an address (0, 1, ….)
•Each cell can be accessed individually (random access memory RAM)
•Dynamic memory technology (DRAM)
•Bits arrangement within a cell
Lecture 4: 9/5/2002
CS149D Fall 2002
9
Main Memory
How many bits would be in the memory of a computer with 4KB memory?
Lecture 4: 9/5/2002
CS149D Fall 2002
10
```