Little Man Computer – Burglar Alarm

Posted on April 28, 2017 by Administrator Posted in Computer Science, Computing Concepts, Little Man Computer (LMC)

Write an LMC program to let the user enter a PIN code to deactivate a burglar alarm.

The program should let the user have up to 3 attempts to enter the correct PIN code before starting the alarm.

The correct PIN code is “123”.

If the user gets the right PIN code the program should output value 1.
If the user gets the wrong PIN code the program should output value 9 indicating that they can have another go at entering the PIN code.
If after three attempts the PIN code entered is still incorrect, the program should output value -1.

LMC Simulator

Online LMC Simulator

Solution

START      INP
           SUB PINCODE
           BRZ DEACTIVATE
           LDA WRONGCODE
           OUT
           LDA COUNTER
           SUB ONE
           STA COUNTER
           BRP START
           BRA ALARM
           HLT
DEACTIVATE LDA TRUE
           OUT 
           HLT
ALARM      LDA FALSE
           OUT
           HLT
ONE        DAT 1
COUNTER    DAT 2
PINCODE    DAT 123
TRUE       DAT 1
FALSE      DAT -1
WRONGCODE  DAT 9

START INP

SUB PINCODE

BRZ DEACTIVATE

LDA WRONGCODE

OUT

LDA COUNTER

SUB ONE

STA COUNTER

BRP START

BRA ALARM

HLT

DEACTIVATE LDA TRUE

OUT

HLT

ALARM LDA FALSE

OUT

HLT

ONE DAT 1

COUNTER DAT 2

PINCODE DAT 123

TRUE DAT 1

FALSE DAT -1

WRONGCODE DAT 9

Your Task

Copy the code provided above in the LMC simulator.

Test the code.

Answer the following questions:

What code is used to find out if the user did input the right PIN number?
What is the purpose of the Counter label?
How does the computer know if the user has used all 3 attempts?

LMC Instruction Set

Note that in the following table “xx” refers to a memory address (aka mailbox) in the RAM. The online LMC simulator has 100 different mailboxes in the RAM ranging from 00 to 99.

Mnemonic	Name	Description	Op Code
INP	INPUT	Retrieve user input and stores it in the accumulator.	901
OUT	OUTPUT	Output the value stored in the accumulator.	902
LDA	LOAD	Load the Accumulator with the contents of the memory address given.	5xx
STA	STORE	Store the value in the Accumulator in the memory address given.	3xx
ADD	ADD	Add the contents of the memory address to the Accumulator	1xx
SUB	SUBTRACT	Subtract the contents of the memory address from the Accumulator	2xx
BRP	BRANCH IF POSITIVE	Branch/Jump to the address given if the Accumulator is zero or positive.	8xx
BRZ	BRANCH IF ZERO	Branch/Jump to the address given if the Accumulator is zero.	7xx
BRA	BRANCH ALWAYS	Branch/Jump to the address given.	6xx
HLT	HALT	Stop the code	000
DAT	DATA LOCATION	Used to associate a label to a free memory address. An optional value can also be used to be stored at the memory address.