GCSE Computing
Sample Assessment Material Write Up
Mr Travi
Royal Grammar School
No 1
Introduction to the Little Man Computer
Introduction
In this practical investigation I am going to look at the Littleman Computer
Simulation. The software for this is provided at
www.atkinson.yorku.ca/~sychen/research/LMC/LMCHome.html
The Littleman Computer Simulation simulates simple microprocessor and contains
the following elements
Program counter – this tells the program where the next instruction is going to be
executed from
Accumulator is a register in which intermediate arithmetic and logic results are stored
whilst the program is being assembled
Inbox – this is where the user enters data that the program is going to execute
Outbox – this is where the data that is processed and produced by the program is
going to be displayed
Using the Little Man Computer Simulation
The LMC responds to commands in the same as any high level programming language
and has its own instruction set.
Instruction
Mnemonic
Machine Code
Comments
Load
LDA
5xx
Load what is stored a location into the
accumulator in the accumulator into for
example
LDA count – this would load the contents of
whatever is store location count and place
this into the accumulator
Store
STA
3xx
Store the contents of the input box into a
storage location. For example
STA COUNT this would store the contents
into location count which would be the next
available storage location in the littleman
computer simulation
Add
ADD
1xx
Add One – add the contents of storage
location one to the accumulator
Subtract
SUB
2xx
Subtract One – Subtract the contents of
storage location one to the accumulator
Input
INP
901
Read in the contents of the input box into
the accumulator
Output
OUT
902
Display the contents of the accumulator in
the output box
End
HLT
000
End the program
Branch if zero
BRZ
7xx
This is selection / iteration statements
Branch if the contents of the accumulator
are zero -
Branch if zero or
BRP
8xx
Branch if the contents of the accumulator
are zero or positive-
positive
Instruction set Continued
Branch always
BRA
Data storage
DAT
6xx
Branch always if the contents of the
accumulator are not zero
DAT – declares the names of storage
locations and value
Count DAT – declares count as a location
ONE DAT 1 - declares one as a location with
a set value of 1
Task 1
As part of this assessment I am going to look at the working of 2 of the sample programs / tutorials
which came with the LMC software and explain how these operate. The two programs I have
chosen are
1. Produce a program using the LMC to input 2 integer values and output the add output the
largest then the smallest
2. Produce a program using the LMC to input 2 integer values and add them together
I am going to illustrate how the LMC with a further 3 programs as well
1. Produce a program which use the LMC instruction set to write a program to add the first 2 values
together and subtract the third integer value and store the results into the accumulator and
display it in the out box together 2 integer values and then terminate
2. Produce a program using the LMC that will output the values from 1 to 10 I am going to modify
this program so that the user can chose what number to input between 1 and 10. I will then
change the program so that it counts down
3. Produce a program which will give the user a chance of a range of numbers and output odd and
even numbers between these totals
These programs will demonstrate the use of branching statements within the LMC
Program 1 Using selection to find out which number is the largest of 2 numbers
entered
1) This represents the flowchart / algorithm for the
program to be entered into the littleman ting
2) This shows the instructions being loaded into the computer simulators memory
location now click on run
Program 1 Coded example
INP
STA first
Input first value store in location first
INP
STA second
Input first value store in location first
SUB FIRST
Subtract first from second number
BRP SECONDBIG
Is what is left a positive number if so
LDA FIRST
Load the contents of first into the accumulator and display to output
OUT
BRA PROGRAMEND
If not load the Load the contents of second into the accumulator and display to
output
SECONDBIG LDA SECOND
OUT
PROGRAMEND HLT
End the program
first DAT
Declare memory location first
second DAT
Declare memory location second
INP
STA FIRST
INP
STA SECOND
SUB FIRST
BRP SECONDBIG
LDA FIRST
OUT
BRA PROGRAMEND
SECONDBIG LDA
SECOND
OUT
PROGRAMEND HLT
FIRST DAT
SECOND DAT
The first number is loaded and then stored in
memory location first
The second number is loaded and then stored in
memory location second
The second number is subtracted from the first and
the result stored in the accumulator
Are the results in the accumulator greater than or
equal to zero? If so load the results of storage
location first and load then into the accumulator
and output them – end that part of the program
Else
Load the contents of storage location second and
load then into the accumulator and output them –
end that part of the program
Program 1 Using selection to find out which number is the largest of 2 numbers entered
In this video (called largestof2numbers.avi) I tested this program.
Program 1 Using selection to find out which number is the largest of 2
numbers entered test plan and table
Test
Data
Expected Outcome
Actual Outcome
1
I am going to enter 65 as the first
number 64 as the second number
65 should come out as the largest
As expected
2
I am going to enter 64 as the first
number 65 as the second number
65 should come out as the largest
As expected
The program loads the instruction set
64 is entered - stored in the first memory location
65 is entered – stored in the second memory
location
INP opcode 901 stored in 0
STA opcode 312 result stored in 12 on the LMC
stored in 1
INP opcode 901 stored in 2
STA opcode 313 result stored in 13 on the LMC
stored in 3
Sub FIRST opcode 212 stored in 4
BRP SECONDBIG opcode 809 – branch if positive
stored in 5
LDA FIRST – load the result of first and place into
accumulator stored at 6
Out – display results of accumulator to output box
stored at 7
Branchalways if value in accumulator is not postive
stored at 8
LDA second load the result off first and place into
accumulator stored at 9
Out – display results of accumulator to output box
stored at 10
End program
Produce a program using the LMC to input 2 integer values and add them together
inp
Input a value into the input and copy this into the accumulator
sta first
Store this value in store location called first LMC allocates this to the first memory location available
inp
Input a value into the input and copy this into the accumulator
sta second
Store this value in store location called first LMC allocates this to the next memory location available
add first
Add the contents of the accumulator to what is stored in memory location first and return these values to the
accumulator
out
Output these values
first dat
Define memory location first dat
Second dat
Define memory location Second dat
Test
1
Data
I am going to enter4 as the
first number 4 as the second
number
Expected Outcome
The result in the accumulator
should be 6
Actual Outcome
As expected
2) Produce a program using the LMC to input 2 integer values and add them together
In this video (called adding2nos.avi) I tested this program. I have enclosed the video as a
standalone file in case this does not load and shows the test mentioned above
3) Produce a program using the LMC that will output the values from 1 to 10 counting up I
am going to modify this program so that the user can chose what number to input
between 1 and 10. I will then change the program so that it counts down
Start
INP
STA COUNT
OUT
LOOPTOP LDA COUNT
ADD ONE
OUT
STA COUNT
SUB TWENTY
BRP ENDLOOP
BRA LOOPTOP
ENDLOOP HLT
ONE DAT 001
TWENTY DAT 020
COUNT DAT
Is the result positive?
Input a number and store in
count
Load the result into the
accumulator
Add the contents of
memory location 1 to the
accumulator
Subtract the contents of
memory location 1 to the
accumulator (20)
Output the results to the
accumulator
No
Yes
End
3) Produce a program using the LMC that will output the values from 1 to 10 counting up I am
going to modify this program so that the user can chose what number to input between 1 and
10. I will then change the program so that it counts down test plan
Test
1
Data
I am going to 4 into the
accumulator
2
I am going to 1 into the
accumulator
Expected Outcome
The program will output all
the numbers between 4 and
10 in increments of 1
The program will output all
the numbers between 1 and
10 in increments of 1
Actual Outcome
As expected
As expected
In this video (called countingprog1to10) I tested this program. I have enclosed the video as a
standalone file in case this does not load and shows the test mentioned above
Video Counting prog and testing counting up 1 to 10
3) I am going to modify the program in 3a so that it counts down. I will then change the program so that it counts down test plan
Test
1
Data
I am going to enter 19 as the
first number
2
I am going to enter 2 as the
first number
Expected Outcome
19 should come out and the
program should count down
in increments of 1 until it
reaches 0
2 should come out and the
program should count down
in increments of 1 until it
reaches 0
Actual Outcome
As expected
As expected
INP
Input value
STA COUNT
store value in memory location count
OUT
output value
LOOPTOP LDA
COUNT
load the value into the accumulator
BRZ ENDLOOP
is the value 0 if so go to endloop sub if not
SUB ONE
subtract 1
OUT
load the value into the accumulator
STA COUNT
store value in memory location count
SUB TWENTY
subtract 20 from the value in count
BRA LOOPTOP
go back to loopcount
ENDLOOP HLT
end the program
ONE DAT 001
one declared as memory location with a value of 1
TWENTY DAT 020
COUNT DAT
twenty declared as memory location with a value of 20
count declared as variable
Video Counting prog and testing counting down 20 to 1
I have saved this as countprog20to1.avi and this will show the program working counting
back from 19 in increments of 1 to 0 and from 2 in increments of 1 to 0
Produce a program which will give the user a chance of a range of numbers and output odd and
even numbers between these totals
Test
1
Data
I am going to enter 1 as the
first number
2
I am going to enter 57 as the
first number
Expected Outcome
1 should come out and the
program should count down
in increments of 2 until it
reaches 99
57 should come out and the
program should count down
in increments of 1 until it
reaches 99
Actual Outcome
As expected
As expected
INP
Number initally input
LOOP STA ODD
start loop save this number in storage location ODD located at 111
LDA NUM
load the contents of number into the accumulator
SUB ODD
subtract from the contents of number the vale in ODD
BRP MORE
Branch if the number is postive
END HLT
end the program
MORE LDA ODD
if not load the contents of odd
OUT
send this to display in the out box
ADD TWO
add the value of storage location two (which is 2) and loop again
BRA LOOP
TWO DAT 2
Declaration of variable TWO with value 2
ODD DAT 1
Declaration of variable ONE with value 1
NUM DAT 99
Declaration of variable NUM with value 99
Video Counting prog and testing counting up from a user defined no to 99 in
increments of 2
I have produced a video called countprog1TO99ODDNOS which I have embedded here and
have included with this portfoilio
For the second task I have been given a number of programming tasks to tackle
1)
Write programs to run in LMC:
a) Produce a multiplication table from 1 to 10 for any number input by the user
b) Modify this program so that a user can enter a value and it produces the square of that
number for example if the user entered 5 it would produce 25
c) Modify the program so that a can user can enter a value and it produces the cube of that
number for example if the user entered 2 it would produce the number 8 (which is 2*2*2)
(d) Produce evidence to show that you have planned, written and tested your code.
(e) Produce an evaluation of your solutions.
2)
Write a conclusion about the possibility of writing effective and complex programs
with only a limited instruction set.
a) Produce a multiplication table from 1 to 10 for any number input by the user
Start
Add the contents of
number1 into the
accumulator
Is the result positive?
Input a first number and
store in Number1
Store the result into
memory location RESULT
Input a
number
Subtract the contents of
the accumulator by the
contents in memory
location ONE
Store the result of this in
memory location
number2
Load the contents of
storage location RESULTS
into the accumulator
Load the contents of
memory location TWO and
load this into the
accumulator
subtract the contents of
this by the contents in
memory location ONE
Store the result into
memory location Number2
Load the contents of result
into the accumulator and
display in the output boc
End
yes
Test
1
2
Data
I am going to enter
12 in memory
location number1
and 12 in memory
location 2
I am going to enter
12 in memory
location number1
and 12 in memory
location 2
Expected Outcome
The program will add 12 together 12 times and
will display 144 in the accumulator at the end
Accepted outcome
As expected
The program will add 9 together 8 times and
will display 72 in the accumulator at the end
As expected
INP
Input a number
STA NUMBER1
store this number in memory location number1
INP
Input a number
SUB ONE
subtract the contents of this by the contents in memory location ONE
STA NUMBER2
store this number in memory location number2
LOOPTOP LDA
RESULT
load the contents of store location result and load this into the accumulator
ADD NUMBER1 add the contents of memory location number1
STA RESULT
store the result of this into memory location result
LDA NUMBER2
load the contents of store location number2 and load this into the accumulator
SUB ONE
subtract the contents of this by the contents in memory location ONE
STA NUMBER2
store this number in memory location number2
BRP LOOPTOP
is the result postive? If yes go back to top of looptop and load result again
LDA RESULT
if no load the result into the accumulator
OUT
output this result to the accumulator
HLT
end the program
BRA LOOPTOP
ONE DAT 001
ONE memory location declared initialised
NUMBER1 DAT
Number 1 memory location declared
NUMBER2 DAT
Number 2 memory location declared
RESULT DAT 000 Result memory location declared initialised
a) Produce a multiplication table from 1 to 10 for any number input by the user
I have produced a video called mulplicationof2nos. which I have embedded here and have
included with this portfoilio
b) Modify this program so that a user can enter a value and it produces the square of that
number for example if the user entered 5 it would produce 25
Test
1
Data
I am going to enter 6
2
I am going to enter 31
INP
STA FIRST
STA SECOND
SUB ONE
STA SECOND
LDA FIRST
LOOPTOP ADD FIRST
STA RESULT
LDA SECOND
SUB ONE
STA SECOND
BRZ LOOKIFZERO
LDA RESULT
BRA LOOPTOP
LOOKIFZERO LDA RESULT
OUT
STA RESULT
HLT
FIRST DAT
SECOND DAT
ONE DAT 1
RESULT DAT
Expected Outcome
The program will add 6 together 6 times and will
display 36 in the accumulator at the end
The program will add 31 together 31 times and
will display 961 in the accumulator at the end
This is the highest that the LMC could display
using this program
Accepted outcome
As expected
As expected
Input a number
store this number in memory location FIRST
store this number in memory location Second
subtract the contents of this by the contents in memory location ONE
store this number in memory location Second
load the contents of store location First and load this into the accumulator
add the contents of memory location First
store the result of this into memory location result
load the contents of store location Second and load this into the accumulator
subtract the contents of this by the contents in memory location ONE
store this number in memory location Second
is the result zero? If no go back to top of looptop and load result again
if no load the result into the accumulator
output this result to the accumulator
if second is zero load the result into the accumulator and output
Store the contents of the accumulator into the memory location results
end the program
First memory location declared
Second memory location declared
one memory location declared and initialised
Result memory location declared initialised
b) Modify this program so that a user can enter a value and it produces the square of that
number for example if the user entered 5 it would produce 25
I have produced a video called squareof2nos. which I have embedded here and have
included with this portfoilio
c) Modify the program so that a can user can enter a value and it produces the cube of
that number for example if the user entered 2 it would produce the number 8 (which is
2*2*2)
INP
Input a number
STA FIRST
store this in memory location first
STA COUNTER
store this in memory location counter
SUB ONE
subtract this from the contents held in memory location one
STA COUNTER
store this in memory location counter
LDA FIRST
load the contents of memory location FIRST into the accumulator
LOOPTOP ADD FIRST
add the contents of memory location FIRST into what is currently in the accumulator
STA RESULT
store this number in memory location result
LDA COUNTER
load the contents of memory location Counter into the accumulator
SUB ONE
subtract this from the contents held in memory location one
STA COUNTER
store this number in memory location counter
BRZ LOOKIFZERO
is this result zero?
LDA RESULT
no load the result into the accumulator
BRA LOOPTOP
go back to looptop
LOOKIFZERO LDA RESULT
if counter = 0, load the contents in memory location result into the accumulator
OUT
and display in out box
Prog C continued
STA RESULT
LDA FIRST
STA COUNTER
LDA RESULT
STA number
LOOPTOP1 LDA result
ADD NUMBER
STA result
LDA Counter
SUB ONE
STA counter
BRZ LOOKUP
LDA RESULT
SUB NUMBER
BRA LOOPTOP1
LOOKUP LDA RESULT
SUB NUMBER
OUT
HLT
FIRST DAT
Counter DAT
ONE DAT 1
RESULT DAT
Number DAT
Store this number in memory location result
load the contents of memory location FIRST into the accumulator
store this in memory location counter
load the contents of memory location result into the accumulator
Store this number in memory location number
load the contents of memory location result into the accumulator
add the contents of memory location NUMBER into what is currently in the accumulator
Store this number in memory location RESULT
load the contents of memory location counter into the accumulator
subtract this from the contents held in memory location one
store this in memory location counter
is this result zero?
load the contents of memory location result into the accumulator
subtract this from the contents held in memory location NUMBER
go back to looptop1
if counter = 0, load the contents in memory location result into the accumulator
subtract this from the contents held in memory location NUMBER
Send this to the outbox
end the program
First memory location declared where inital number to be processed is declared
Counter memory location declared
one memory location declared and initialised
Result memory location declared initialised
Number memory location declared where result of first calculation is stored
Testing stage
Test
1
Data
I am going to
enter 2
2
I am going to
enter 9
Expected Outcome
Accepted outcome
The program add 2 to together As expected
2 times giving a result of 4 and
take that result and perform
the same calculation again
leaving a result of 8
The program will add 9
As expected
together 9 times giving a
result of 81 and will perform
the same calculation again
giving a result of 729
Modify the program so that a can user can enter a value and it produces the cube of that
number for example if the user entered 2 it would produce the number 8 (which is 2*2*2)
I have embedded a video which shows the results of the program being run testing 2 * 2 * 2
and 9 * 9 * 9. This is also available as a standalone video called cubeof1nos
(e) Produce an evaluation of your solutions and (f) Write a conclusion about the possibility of
writing effective and complex programs with only a limited instruction set. .
For each of the programs I have as produced detailed code and shown my solution through the
videos which are attached to this presentation and have been submitted as standalone
products. In each case I have tested the programs at the extreme ends of what the LMC
simulator is capable of doing.
The LMC is limited in its functionality as this will support numbers upto a 1000, will not
accept decimal or real numbers or data types and can only perform limited arithmetic
operations namely addition and division. This is a severely limiting factor on the use of the
LMC but this does highlight the basic principles of machine architecture in that it shows the
fetch execute cycle in action and the respective role of the registers.
If the LMC or programs like it were to be used for the calculations then a lot of processing is
going to be undertaken by the program and the result will not be as accurate owing to its
(the LMC’s) operation when compared to modern software applications and high level
programming compilers
Bibliography
Slide 2 Definitions and ideas taken from Wikipedia and adapted
Slide 3 Instruction set taken from
http://www.atkinson.yorku.ca/~sychen/research/LMC/LMCHome.html