Class 30: Modeling Computers

cs1120: Introduction to Computing
Explorations in Language, Logic, and Machine
University of Virginia, Spring 2016

http://xplorecs.org/class30

Class 30:
Modeling Computers
What makes a good model?
Turing's Model
Finite State Machines

Transcript

1. Class 30: Modeling Computers Introduction to Computing: Explorations in Language, Logic, and Machines cs1120 Spring 2016 David Evans University of Virginia
2. Remaining Topics Computability – What computers can and cannot do – Read Chapter 12 of coursebook Project 6: Interpreters – Read Chapter 11 of coursebook Classes this week and next week will interleave these topics
3. What’s a Computer?
4. Cray-1 (1976) Pebble (2014) Apple II (1977) Palm Pre (2009) MacBook Air (2008) Surface (2016)
5. Colossus (1944) Apollo Guidance Computer (1969) Honeywell Kitchen Computer (1969) ($10,600 “complete with two-week programming course”)
6. Pre-WWII “Computers”
7. Definition of Computer Class 1:
8. Formalizing our Definition Processing Input Output Program A computer is a machine that can execute a program. A universal computer is a machine that can execute any program. Today/Wednesday: Formalize what input, output, program, and execute mean Later: Understand what are the limits of any program
9. How should we model a computer?
10. Copernicus Ptolomy Einstein Newton
11. Modeling Computers
13. (November 21) Alan Turing 1912-1954 (pardoned in 2013)
14. What All Computers Need Processing Without it, we can’t describe a problem Input Output Without it, we can’t get an answer Way to get from input to output
15. Input Engelbart’smouse and keypad (1968) Altair BASIC Paper Tape (1976) Voice (“2001”) Multitouch (2009) 0100101011010101101010110101011010101…
16. Modeling Input 0100101011010101101010110101011010101…
18. Modeling Pencil and Paper # C S S A 7 2 3 How long should the tape be? ... ...
19. Modeling “Scratch Paper” (and Input and Output)
20. Output Connection Machine CM-5 (1993) Princeton’s Scalable Display Wall (2006)
21. Modeling Output 0100101011010101101010110101011010101…
22. Modeling Processing
23. Modeling Processing Look at the current state of the computation Follow simple rules about what to do next Scratch paper to keep track
24. Modeling Processing • Evaluation Rules – Given an input on our tape, how do we evaluate to produce the output • What do we need: – Read what is on the tape at the current square – Move the tape one square in either direction – Write into the current square 0 0 1 1 0 0 1 0 0 0 Is that enough to model a computer? 23
25. Modeling Processing Read, write and move is not enough We also need to keep track of what we are doing: – How do we know whether to read, write or move at each step? – How do we know when we’re done? 24
26. Finite State Machines 1 Start 2 HALT 1 0 1 # 0 25
27. Charge • Wednesday: – Starting Interpreters – Limits of Computation • Read Chapter 11 (Interpreters) and 12 (Computability) from coursebook • Project 6: target completion by April 25 – Opportunity for Red Belt exemption