Little Man Computer

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Little Man Computer

• When your program gets translated to machine
  code all 0s 1s
• The translator must know the language of the
  program (java) as well as the language of the cpu
  the instruction set
• LMC is a simple instruction set for toy computer

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About The LMC

• It is a model/illustration of a computer
• Created by Dr. Stuart Madnick at MIT in 1965 (he
  updated it in 1979)
• A fairly accurate representation of how computers
  still work today.

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Calculations and other operations performed by
CPU (Central Processing Unit)
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LMC Model
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LMC Schematic
123
00
123
500
01
199
02
500
03
399
04
05
95
96
97
98
123
99
05
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System Counterparts
ALU
Input/output interface
123
00
123
500
01
199
02
500
03
399
04
05
Control unit
95
96
97
98
123
99
05
Program counter
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System Block Diagram
CPU
Memory
ALU
Input/outputinterface
Control unit Program counter
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LMC Instruction Set

• Layout of Instructions
• 3 digits (use decimal idea is similar in binary)
• Think of memory as storage locations mailboxes
  that can hold one slip of paper with 3 digits

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LMC Instruction Set

• Interpretation of the 3 digit
• 1st digit instruction called the operation code
  op code for short
• 2nd and 3rd digits mailbox address (for the most
  part)
• eg.
• 225 means store into mailbox 25

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LMC Instruction Set
11
LMC Instruction Set
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LMS Instruction Set
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LMS Instruction Set
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A program to add 2 numbers

• 00 500 input
• 01 299 store data
• 02 500 input
• 03 399 add
• 04 600 output
• 05 700 stop
• 99 data

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A program to find positive difference of 2 numbers

• 00 Input 500
• 01 Store 11 211
• 02 Input 500
• 03 Store 12 212
• 04 Subtract 11 411
• 05 Skp if Neg 800
• 06 Jmp 09 909
• 07 Load 11 111
• 08 Subtract 12 412
• 09 Output 600
• 10 Halt 700
• 11 Data 000
• 12 Data 000

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Fetch, (Decode,) Execute Instruction Cycle

• Fetch get the next instruction as indicated by
  the program counter goes to mbox address and
  reads the 3 digit code (the instruction) and then
  increments the program counter
• Decode electronically decode the instruction to
  determine which operation to perform (not shown
  in LMC)
• Execute perform the instruction
• Repeat cycle until halt