CSC103 How Computers Work

Notes4

1. Switches → Logic Gates

2. Switches
   • Electromechanical relays are switches that are flipped on/off by electricity.
   • Transistors are electronic switches.
   • Tinkertoy Computer!
3. Analogy:
   • electrical voltage = water pressure.
   • switch = valve
   • wire = pipe
   • output 1 = water flows out
   • ouput 0 = no water comes out
   • Think of switch opened by water pressure.
4. Normally closed (n.c.)switch: acts like a NOT gate
5. • 

6. A: input	Output
   0 (leave closed)	1 (water out)
   1 (pop open)	0 (nothing out)

   • 0 in, 1 out; 1 in, 0 out
   • Called NOT because false → true, true → false.
7. Normally open (n.o.) switch: acts like an AND gate.
   • 

   • A: input	B: input	Output
     0 (leave open)	0 (leave open)	0 (nothing out)
     1 (shut closed)	0 (leave open)	0 (nothing out)
     0 (leave open)	1 (shut closed)	0 (nothing out)
     1 (shut closed)	1 (shut closed)	1 (water out)

   • Reorganized AND table:

     A: input	B: input	Output
     0	0	0
     0	1	0
     1	0	0
     1	1	1

8. Can make all logical operations from NOT and AND. In particular, OR
   • Example in English:
     • If you have either your license or your passport
     • Then it is not the case that you are missing both.
     • I.e., it is not the case that you both do not have your license and you do not have your passport.
     • license or passport = not ( (not license) and (not passport)).
   • OR table:

9. A: input	B: input	Output
   0	0	0
   0	1	1
   1	0	1
   1	1	1

• If you not-ed A and B in the AND table, you'd get the right output but with 0/1 reversed. So if you not that output, you get the OR table.

Logic Gates→ Arithmetic

10. {NOT, AND, OR}→ One Bit Adder

• A: input	B: input	A+B: output
  0	0	0
  0	1	1
  1	0	1
  1	1	0

• 
• Missing the carry: Carry only when both A and B are 1.
• 
• String many of these circuits together (usually 32) to perform binary addition.

11. Arithmetic → CPU: Central Processing Unit

12. Addition circuitry: one little piece of the CPU (in ALU: Arithmetic/Logic Unit)
    • Not just adding number for computation, also address computations: find address after file which is 4567B long, etc.
    • Also: Multiplication circuitry.
13. CPU: Fetch/Execute cycle
• Controlled by clock: 1GHz = one billion clock ticks per second.
• 
• Instruction in "machine language," language specific to the CPU.
• One is: ADD # in Register 1 to # in Register 2.

14. CPU → Motherboard

15. PC: Motherboard; Apple: Logic board; sometimes abbreviated mobo
16. Includes
    • CPU
    • BIOS
    • Clock
    • CPU heat sink
    • Memory chips; Slots for more memory chips
    • 
17. Labeled Motherboard Image
18. CPU Heat
    • Cause: electrons colliding
    • Problem: switches start failing if too hot
    • Heat sink: increase surface area for more efficent cooling
    • 
    • CPU fans.