2. A Brief History of Computing and the LMC

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2. A Brief History of Computing and the LMC
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Review Course Concepts
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Wiki Weekly Comment Demo

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Learning objectives

• Know something about the evolution of computing
• Understand how computer operates at lowest level
• Provides analogies for understanding
  representation, architecture, operating systems
  in future classes

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A Brief History of Computing

• In which we find that computers are misnamed.
• For a more complete history, see Campbell-Kelly,
  M., Aspray, W. (1996). Computer a history of
  the information machine. ( ed.). New York, NY
  BasicBooks.

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At the start, it was a good name

• The word computer was first used to refer to a
  person employed to make calculations.
• The meaning was extended to include mechanical
  calculating devices, such as slide rules, adding
  machines, etc.
• Inputs
• Compute
• Outputs
• Examples
• Solving complex equations
• Calculating trajectories

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From Computer to Information Machine

• Today, the ability to perform calculations is
  secondary
• Electronic computers primary function is the
  storage and manipulation of information

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Storage and Interactivity

• Combining calculation and storage allows
• Aggregation of inputs over time
• Delayed output
• Human involvement closes the loop
• Input
• Output
• More input

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The Modern Electronic Computer

• Modern computers rely on this combination of
  capabilities for most tasks.
• Word processing
• Financial analysis, payroll, and accounting
• Data management
• The way that we interact with these machines has
  also evolved.

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Multi-tasking

• Early computers were very expensive
• One computer shared by many people

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Graphical User Interfaces

• GUI versus command line
• WIMP
• Windows
• Icon
• Mouse
• Pointer

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Networked Computers

• Move (communicate) data
• Using phone lines, network wires, wireless, etc.

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Current Trends

• Mobility
• Ubiquity
• Embedding

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Technical Enablers

• Better Engineering
• Moores Law
• Double the performance every 12-18 months
• Infrastructure Investment

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Individual Reflection

• What strengths (skills, experiences, etc.) do you
  bring to the class?
• What is your biggest concern regarding this
  class?
• What could you contribute to a study group?
• What would you hope to get out of a study group?

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LMC A Simplified Computer

• Input/Output (input and output conveyors)
• Memory (blackboard)
• 100 locations
• Each holds a 3-digit number and is identified by
  its address (between 0 and 99)
• Processor
• Calculator (sometimes called a register)
• Instruction Location Counter
• Keeps track of what instruction to execute next
• Two digits
• What is maximum of instructions in one
  script/program?

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The Little Man

• Little body (fits inside the computer)
• Little brain
• only simple instructions in script
• can't even add 22 without a calculator
• can't remember anything
• Big on obedience always follows instructions

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What the Little Man Does

• Look at instruction location counter
• Go to corresponding blackboard location and read
  instruction
• Perform indicated operation
• Advance instruction counter by 1
• Don't need calculator for this clicker built
  into instruction counter
• Skip this step for some instructions (branches,
  about to be introduced)
• Repeat until told to stop

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The Instructions

• Processing instructions
• Add (calculator value from memory location)
• Subtract (calculator value from memory
  location)
• Memory instructions
• Store (calc. ? memory location)
• Load (calc. ? value from memory location)
• Input/Output instructions
• Get (input belt ? calculator)
• Put (calculator ? input belt)
• STOP

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Programs (Scripts)

• A sequence of instructions
• So precise even the little man can follow the
  instructions
• Example Add two numbers
• Is this a program?

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From Flowchart to Program

• Add (calculator value from memory location)
• Subtract (calculator value from memory
  location)
• Store (calc. ? memory location)
• Load (calc. ? value from memory location)
• Get (input belt ? calculator)
• Put (calculator ? output belt)

For class to work on
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Revised Program

• 0. Get Get first number
• 1. Store 06 Store first number in location 6
• 2. Get Get second number
• 3. Add 06 Add second number (from location 6)
  to contents of calculator
• 4. Put Write answer on Post-It put on output
• 5. Stop

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Instructions in Memory

• Assign a number to each possible instruction
• Has to fit in 3 digits
• How many instructions could we have?
• But some instructions need to specify a memory
  location
• First digit says what to do
• Next two digits specify where
• Are two digits enough?
• How many instruction types can you have?

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The Instruction Codes

• Processing instructions
• 1xx Add (calculator value from memory location)
• 2xx Subtract (calculator value from memory
  location)
• Memory instructions
• 3xx Store (calc. ? memory location)
• 5xx Load (calc. ? value from memory location)
• Input/Output instructions
• 901 Get (input belt ? calculator)
• 902 Put (calculator ? output belt)
• 000 STOP

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Exercise Write the Program Using the Instruction
Codes

• 0. Get
• 1. Store 06
• 2. Get
• 3. Add 06
• 4. Put
• 5. Stop

1xx Add 2xx Subtract 3xx Store 5xx Load 901
Get 902 Put
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Revised Program

• Address Instruction Description
• 000 901 Get
• 001 306 Store 06
• 002 901 Get
• 003 106 Add 06
• 004 902 Put
• 005 000 Stop

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The LMC Simulator

• In DIAD Lab
• Install through NAL
• LMC should appear on your start menu
• Or download zip file from CourseTools

Run NAL. Download add2.lmc from CourseTools
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Branch Instructions
Spend significant time on this.

• Branch instructions change the value in the
  instruction counter
• Unconditional branch sets instruction counter to
  new value
• Branch Zero changes IC only if calculator has
  value 0
• Branch Positive changes IC only if calculator has
  value gt 0
• Calculator has a "zero" light and a "positive"
  light so little man can tell
• Branch Zero and Branch Positive increment
  instruction counter by 1 if branch not taken
• Do branch instruction codes need to save 2 digits
  for memory locations?

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The Instructions (complete)

• 000 STOP
• 1xx Add
• 2xx Subtract
• 3xx Store
• 5xx Load
• 6xx Unconditional branch
• 7xx Branch on Zero
• 8xx Branch on Positive
• 901 Get
• 902 Put

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ExerciseSum Numbers Until 0 Input

• Take 3 minutes to write out a flowchart for a
  program that adds all the numbers on the conveyor
  belt until the belt value is 0.
• The answer is on the next slide, but PLEASE
  dont look!
• If youre not ready to make a flowchart, just
  write out the steps that you can think of.
• For example, we know that the program must get
  the numbers off the belt.
• What else must it do?
• In what order must it do these tasks?

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Sum Numbers Until 0 Input
See Addall.lmc
Load 0
Save in SUM
0. 509 Load zero from 09 1. 367 Store in 67
(SUM) 2. 901 Get number 3. 707 Done if 0 4.
167 Add from 67 (SUM) 5. 367 Store result in
SUM 6. 602 Repeat 7. 567 Load from SUM 8.
902 Put 9. 000 STOP
Get
Y
0?
Load SUM
N
Add from SUM
Output SUM
Save in SUM
STOP
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Is This Data or a Program?(Look at Weird.lmc on
your own)
Weird.lmc

• 0. 901
• 1. 302
• 2. 000
• 3. 666
• 4. 565
• 5. 902
• 6. 000

• 7. 566
• 8. 902
• 9. 000
• 65.901
• 66.902
• 67.600
• Input 604 or 607
• What happens with other inputs?
• Try 665

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Preview

• Dont forget questions/comments to CourseTools by
  8PM tonight
• Section tomorrow
• Write a program together
• Very similar to the homework problem!
• LMC Homework due Tuesday
• Download assignment, LMC from CourseTools
• Submit answers to CourseTools
• Next topic
• Representation