1. Little Man Computer

2. Components Memory (100 cells numbered 0 to 99) Program Counter
Holds machine language instructions and data
Program Counter
Holds the address of the next instruction to be executed
Accumulator
Holds the result of the most recent ADD / SUB instruction
In box
Where the computer gets its input
Out box
Where the computer sends its ouput

3. Instructions OpCode Mnemonic Description HLT Halt execution 1 ADD
HLT
Halt execution 1
ADD
Add memory cell to accumulator 2
SUB
Subtract memory cell from accumulator 3
STA
Store accumulator to memory cell 4
LDA
Load memory cell to accumulator 5
BRA
Set program counter to operand 6
BRZ
Set program counter to operand only if accumulator is zero 7
INP
Load in box to accumulator 8
OUT
Write accumulator to out box

4. LMC Machine Language
Each machine language instruction is a 3-digit number
3 1 9
The opcode specifies which instruction to execute
The operand specifies additional information needed by the instruction

5. 3 1 9 LMC Assembly Language STA 19
An assembly language instruction is a decoded machine language instruction
3 1 9
STA 19

6. Execution Cycle
Fetch => Decode => Execute

7. Example – Add 2 Numbers Get a number from input into calc
Copy that number to an unused mailbox (mailbox 99)
Get another number from input into calc
Add the number we put into a mailbox 99 to calc
Copy calc to the output
Halt

8. Example – Add 2 Numbers Address Machine Language Assembly 700 INP 1
700
INP 1
399
STA 99 2 3
199
ADD 99 4
800
OUT 5
000
HLT

9. Example – Count Down to Zero
Address
Machine Language
Assembly
700
INP 1
605
BRZ 5 2
800
OUT 3
206
SUB 6 4
501
BRA 1 5
000
HLT 6
001
DAT 1

10. Example – Read / Write Two Numbers But Swap Them
Get a number from input into calc
Copy the number to an unused mailbox (mailbox 99)
Copy the number to an unused mailbox (mailbox 98)
Copy the value from mailbox 98 to calc
Copy the value in calc to output
Copy the value from mailbox 99 to calc

11. Example – Read / Write Two Numbers But Swap Them
INP (700)
STA 99 (399)
STA 98 (398)
LDA 98 (598)
OUT (800)
LDA 99 (599)

12. Example – Multiply by 3 if Positive
INP (700)
STA 99 (399)
STA 98 (398)
STA 97 (397)
LDA 98 (498)
ADD 13 (113)
BRZ 13 (613)
LDA 97 (497)
SUB 13 (213)
BRZ 17 (617)
BRA 04 (504)
1 (this is data, not an instruction)
LDA 99 (399)
OUT (800)
HLT (000)
ADD 99 (199)