1. Little Man Computer
When your program gets “translated to machine code” all 0’s & 1’s
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

2. 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.

3. (Central Processing Unit)
Calculations and other operations performed by:
CPU
(Central Processing Unit)

4. LMC Model

5. LMC Schematic 00
123
123 01
500 02
199 03
500 04
399
456 05 95 96 97 98
789 99
123 05

6. System Counterparts ALU Input/output interface Control unit00
123
123 01
500 02
199 03
500 04
399
456 05
Control unit 95 96 97 98
789 99
123 05
Program counter

7. Input/output interface
System Block Diagram
CPU
Memory
ALU
Input/output interface
Control unit
Program counter

8. 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

9. 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”

10. LMC Instruction Set 1 Load
Reverse of Store: walks to the mbox specified, copies the instruction on the slip of paper and takes it to the calculator and punches the number in. 2
Store
walk to calculator, reads the number there, and writes the number on a slip of paper, walks to the mbox specified in the instruction, and puts this slip of paper there (replacing any paper already there

11. LMC Instruction Set 3 Add
walk to mailbox address specified and read the number there; walk to calculator and adds it to the number already in the calculator 4
Subtract
similar to add but subtracts 5
Input
walks to the in box and picks up the slip of paper having a 3-digit number on it; walks over to the calculator and enters that number

12. LMS Instruction Set 6 Output
walk to calculator, reads the number there, and writes the number on a slip of paper, walks to the out box and puts this slip of paper there 7
Halt
stop
800
Skip if Negative
walk to calculator; if the value is less than zero then walk to prog. counter and add one to the current value

13. LMS Instruction Set 801 Skip If Zero
walk to calculator; if the value is zero then walk to prog. counter and add one to the current value
802
Skip If Positive
walk to calculator; if the value is greater than zero then walk to prog. counter and add one to the current value 9
Jump
Tells LMC to change the program counter to the number shown on the address portion

14. A program to add 2 numbers
input
store data
input
add
output
stop
data

15. A program to find positive difference of 2 numbers
00 Input
01 Store
02 Input
03 Store
04 Subtract
05 Skp if Neg 800
06 Jmp
07 Load
08 Subtract
09 Output 600
10 Halt 700
11 Data
12 Data

16. 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