1. LMC Little Moron Computer
Chapter 6
LMC
Little Moron Computer

2. von Neumann Principles (revisited)
Stored program concept - Memory & Data
Memory is addressed linearly
Memory is addressed without regard to content1
John von Neumann, 1945

3. LMC Mailboxes Calculator LM Counter 00 01 02 03 04 05 06 07 96 97 9899 LM
Counter

4. LMC Organization

5. CPU Components

6. LMC - CPU

7. LMC Instructions OPCODE OPERAND Source OPERAND OPCODE Mailbox Address

8. Address vs. Contents G Addresses are consecutive
G Contents may be data or instructions

9. The CPU - The Instruction Cycle
START
Fetch Next
Instruction &
Decode
Fetch Cycle
Execute
Instruction
Execute Cycle
HALT

10. Fetch an Instruction

11. Fetch an Instruction (continued)

12. Execute an Instruction

13. Execute an Instruction (continued)

14. LMC Instruction Set 1 xx ADD Arithmetic 2 xx SUB
3 xx STO Data Movement
5 xx LDA
6 xx BR Program Control
7 xx BRZ
8 xx BRP
IN Input/Output
OUT
HLT
DAT Data Storage Location

15. LDA INSTRUCTION 5 xx Go to mailbox with address xx
Read 3-digit number on paper in mailbox (leave paper there)
Enter 3-digit number into calculator
Advance location counter
NOTE: Accumulator changes in value. Contents of memory location xx does not change.

16. STO INSTRUCTION 3 xx Retrieve value from calculator
Go to mailbox at address xx and replace 3-digit number on paper in mailbox with value from calculator
Advance location counter
NOTE: Accumulator does not change in value. Contents of memory location xx does change.

17. ADD INSTRUCTION 1 xx Go to mailbox with address xx
Read 3-digit number on paper in mailbox (leave paper there)
Add 3-digit number to number stored in calculator
Advance location counter
NOTE: Accumulator changes in value.

18. SUB INSTRUCTION 2 xx Go to mailbox with address xx
Read 3-digit number on paper in mailbox (leave paper there)
Subtract 3-digit number from number stored in calculator
Advance location counter
NOTE: Accumulator changes in value.

19. IN INSTRUCTION 9 01 Wait for paper slip to be placed in IN Basket.
Remove paper from IN Basket, and read 3-digit number on it
Enter number into calculator
Advance location counter
NOTE: Accumulator changes in value.

20. OUT INSTRUCTION 9 02 Copy number from calculator onto paper
Place paper into OUT Basket
Advance location counter
NOTE: Accumulator does not change in value.

21. BRANCH INSTRUCTION 6 xx
Set value of location counter to xx

22. BRZ INSTRUCTION 7 xx Observe value stored in calculator
If value is 0, set value of location counter to xx

23. BRP INSTRUCTION 8 xx Observe value stored in calculator
If value is not negative, set value of location counter to xx
NOTE: 0 value is considered positive.

24. HLT INSTRUCTION 0 00 LMC takes a break.
NOTE: Make no assumptions about contents of memory or calculator.

25. ADD TWO INPUT NUMBERS 00 IN 9 01 01 STO 99 3 99 02 IN 9 01
04 OUT
05 HLT
99 DAT

26. FIND POSITIVE DIFFERENCE OF TWO INPUT NUMBERS
01 STO
02 IN 9 01
03 STO
04 SUB
05 BRP
06 LDA
07 SUB
08 OUT 9 02
09 HLT 0 00
10 DAT
11 DAT

27. Problem 6.6: Find largest of 3 numbers
Part 1: Find larger of two numbers and store its value in memory cell.
00 IN ;input three numbers and save
01 STO
02 IN 9 01
03 STO
04 IN
05 STO
06 SUB ;subtract number in 98 from that in 97
07 BRP ;number in 97 larger
08 LDA ;number in 98 larger-restore 98
09 BR
10 LDA ;restore 97
11 STO ;store larger of (97, 98) in 96

28. Problem 6.6 (continued)
12 SUB ;subtract number in 99 from larger
13 BRP ;number in 96 larger
14 LDA ;number in 99 larger--restore 99
15 BR
16 LDA ;restore 96
17 OUT
18 HLT 0 00
96 DAT
97 DAT
98 DAT
99 DAT

29. Problem 6.7 - Find Largest of N Numbers
01 STO ;store the first number in 99
02 IN
03 BRZ ;completion flag
04 STO ;store new value temporarily
05 SUB
06 BRP ;new value larger--store it in 99
07 BR ;old value in 99 is larger--get next number
08 LDA
09 STO
10 BR
11 LDA
12 OUT
13 HLT 0 00
98 DAT
99 DAT ;contains largest number currently input

30. Problem 6.14: Program Loader
00 BR ;Jump to location 86 …
86 LDA ;initialize the STORE instruction at location 92
87 STO
88 INP ;input a value
89 SUB ;subtract 999
90 BRZ ;done loading--jump to 01 for execution
91 ADD ;restore input value
92 DAT ;store value. This will contain 3XX, with XX indexed
;each time
93 LDA ;load the STORE instruction in 92
94 ADD ;increment it
95 STO ;and resave it
96 BR ;continue with the next input [NOTE: No test for exceeding ;memory]
97 DAT ;value 999 for testing
98 DAT ;initial STORE 01 instruction as a data value
99 DAT ;value 1 for indexing