1. 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

2. Should cover sections 1.1 and 1.2 of Brookshear Text
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

3. Boolean Operations The NOT operation NOT 1 = 0 NOT 0 = 1
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
Copyright © 2003 Pearson Education, Inc.
Lecture 4: 9/5/2002
CS149D Fall 2002

4. Gates1/3 Device implementing Boolean operation
Current technology implements gates as small electronic circuits
Building blocks from which computers are constructed
Copyright © 2003 Pearson Education, Inc.
Lecture 4: 9/5/2002
CS149D Fall 2002

5. Gates2/3 Copyright © 2003 Pearson Education, Inc. Lecture 4: 9/5/2002
CS149D Fall 2002

6. Gates3/3
Which Input bit patterns will cause the following circuit to produce an output of 1?
XOR
Inputs
AND
Output
Lecture 4: 9/5/2002
CS149D Fall 2002

7. 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)
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 A
Try Sequence?
A B
1 0 then
0 0 B
Copyright © 2003 Pearson Education, Inc.
Lecture 4: 9/5/2002
CS149D Fall 2002

8. 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

9. 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, ….)
Operations: read, write
Each cell can be accessed individually (random access memory RAM)
Dynamic memory technology (DRAM)
Bits arrangement within a cell
Copyright © 2003 Pearson Education, Inc.
Lecture 4: 9/5/2002
CS149D Fall 2002

10. Main Memory
How many bits would be in the memory of a computer with 4KB memory?
Lecture 4: 9/5/2002
CS149D Fall 2002