Transcript Mastering LMC Coding Part #1 • • • • • • • • • Introduction to Low Level Languages Introduction to Little Man computer Simple examples (demos) with video tutorials included Further explanation.

Mastering LMC Coding Part #1
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Introduction to Low Level Languages
Introduction to Little Man computer
Simple examples (demos) with video tutorials included
Further explanation and examples of LMC working examples
Demonstration of INP, OUT, HLT
Demonstration of LDA, STA (and how they work)
Making sense of the numbers as instructions
How to LOAD and SAVE
How to ADD and SUBTRACT
What is LMC? (Little Man Computer)
The Little Man Computer (LMC) is an
instructional model of a computer, created
by Dr. Stuart Madnick in 1965. assembly
code
The LMC is generally used to teach
students, because it models a simple von
Neumann architecture computer - which
has all of the basic features of a modern
computer. It can be programmed in
machine code (in decimal rather than
binary) or in assembly code.
REALLY exploring the inside of a computer
Getting to grips with the LMC will help you
really understand what goes on inside a
computer processor – so it’ll be worth it!
You’d be programming in a low level
language (much closer to machine code;
in fact each line translates directly into
machine code) so it’ll look a little different!
Von Neumann Architecture?
You may want to view the presentation on
this topic, but as a brief summary, Von
Neumann architecture describes the
design of a computer that consists of:
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2.
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7.
A processing unit
..which contains an ALU* and registers
Control Unit (containing instruction register)
Program Counter
A Memory to store both data and instructions
External mass storage
Input and output mechanisms
*The definition has evolved to simply mean any stored-program computer
in which an instruction fetch and data operation cannot occur at the same
time (as they share the same bus).
Why is it called “Little man computer”?
The LMC model is based
on the idea of a little man
shut in a small room (or a
computer, in this scenario)
At one end of the “room”,
there are 100 mailboxes
(memory), numbered 0 to
99, that can each contain
a 3 digit instruction or data
(ranging from 000 to 999)
Furthermore, there are two mailboxes at the
other end labelled INBOX and OUTBOX which
are used for receiving and outputting data.
This is the LMC Model we will be using for the purposes of this tutorial. Please follow the link to try the tasks and
check it out for yourself: http://ssjx.co.uk/games/educational/lmc.php
Accumulator and Program Counter?
Think of the Accumulator like a ‘work area’ which contains a simple calculator. The program counter is
a resettable counter and it holds the address of the next instruction that the little man will carry out.
This Program Counter is normally incremented by 1 after each instruction is executed, allowing the
Little Man to work through a program sequentially.
What about instructions and doing stuff!?
Branch instructions allow iteration (loops) and conditional programming structures to be incorporated
into a program. The latter would be implemented by setting the Program Counter to a non-sequential
memory address if a particular condition is met (typically the value stored in the accumulator being
zero or positive).
What do we need to look out for?
Remember in Von Neumann
architecture, memory contains
both instructions and data.
Care therefore needs to be
taken to stop the Program
Counter reaching a memory
address containing data or the
Little Man will attempt to treat it
as an instruction
How to use the LMC?
To use the LMC the user loads
data into the mailboxes and
then signals the Little Man to
begin execution, starting with
the instruction stored at
memory address zero
Resetting the Program Counter
to zero effectively restarts the
program (potentially in a
different state.
Decimal or Binary …
While the LMC does reflect the actual workings of binary processors,
the simplicity of decimal numbers was chosen to minimize the
complexity for students who may not be comfortable working in
binary/hexadecimal.
Instruction Set in LMC
Some LMC simulators are programmed directly
using 3-digit numeric instructions and some use 3letter mnemonic codes and labels. In either case,
the instruction set is deliberately very limited
(typically about ten instructions) to simplify
understanding
If the LMC uses mnemonic codes and labels then
these are converted into 3-digit numeric instructions
when the program is assembled. The first digit of a
numeric instruction represents the command to be
performed and the last two digits represent the
memory address of the mailbox relevant to that
command.
Some typical instructions
A closer look at an instruction in LMC
Things we can do in the LMC
The Instruction set may seem limited, but you can do pretty much everything in the LMC
Here are some of the things we will be looking at in these tutorials:
Input and Output
Using Memory
Adding and Subtracting
Making Decisions
Multiply two numbers
This PowerPoint contains
information on how to do all of
the things in Red
Watch Part 2 to find out
how to use LOOPS in LMC,
make decisions and multiply
two numbers.
Watch the
video on
the right
and let’s
analyse
what is
happening
LMC Part #1 Video 1
Analysis of the code and execution
What you should do
1.Click on the "LMC Simulator Applet" link to start the LMC simulator.
2.Clear the Message Box and all of the LMC mailboxes -- click the "Clear Messages" button and the "Clear"
button if necessary.
3.Copy the three-line program (INP, OUT, HLT) and paste it into the Message Box
4.Click on the "Compile Program" button.
5.Click on the "Run" button.
6.When prompted, enter a three-digit number in the "In-Box", and press the "Enter" button.
What you should see
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INP
OUT
After the program is compiled, you should see 901 in mailbox 0 and 902 in mailbox 1. The Program
Counter should start at 0 (click on "Reset" if necessary).
When you click on "Run" or "Step", the Message Box will describe the actions of each instruction.
After the INP instruction, the Accumulator has a copy of the value in the In Box.
After the OUT instruction, the Out Box has a copy of the value in the Accumulator.
The final value of the Program Counter is 2. This mailbox holds 0 - the machine code instruction that tells
the LMC to stop executing your program.
A few notes
Program Information
Running Program--------------Program Counter : 0
Getting INPUT from user Input :
3--Program Counter : 1
OUTPUTing accumulator (3)
--- Program Counter : 2
HALT!
The final value of the Program Counter is 2. This
mailbox holds 0  the machine code instruction that tells
the LMC to stop executing your program.
After the INP instruction, the Accumulator has a copy of
the value in the In Box.
After the OUT instruction, the Out Box has a copy of the
value in the Accumulator.
You’ll never really understand it until
you try it yourself.
LMC – how to load and save instructions
Program
INP
STA FIRST
INP
STA SECOND
LDA FIRST
OUT
LDA SECOND
OUT
HLT
FIRST DAT
SECOND DAT
What you should do
1. Click on the "LMC
Simulator Applet" link to
start the LMC simulator.
2. Clear the Message Box
and all of the LMC
mailboxes -- click the
"Clear Messages" button
and the "Clear" button if
necessary.
3. Copy the eleven line
program above and paste
it into the Message Box
4. Click on the "Compile
Program" button.
5. Click on the "Run" button.
6. When prompted, enter
three-digit numbers in the
"In-Box", and press the
"Enter" button.
What you should see
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After the program is compiled, you should see from mailbox 0 to 7 the
instructions 901, 309, 901, 310, 509, 902, 510, 902. The Program
Counter should start at 0 (click on "Reset" if necessary).
DAT is the tenth and eleventh instruction of your program, so they refer to
mailboxes 9 and 10 (0-indexed counting). FIRST and SECOND are the
identifiers that have been declared to represent these mailboxes in the
assembly language program.
When you click on "Run" or "Step", the Message Box will describe the
actions of each instruction.
After the first INP instruction, the Accumulator has a copy of the first value
entered in the In Box.
After the STA instruction, the input value is copied from the Accumulator
to mailbox 9 -- 3 means STORE and 09 refers to the mailbox to store into.
After the second INP instruction, the Accumulator has a copy of the
second value entered into the In Box (use a different value than the first).
After the second STA instruction, the second input value is copied from
the Accumulator to mailbox 10.
After the LDA instruction, the Accumulator is reset to the first input
value. This value has been retrieved from mailbox 9 -- 5 means LOAD
and 09 refers to the mailbox to load from. Note: since your Accumulator
can only work on one value at a time, you have to repeatedly STORE and
LOAD values from memory to keep them from getting erased by the next
operation
After the OUT instruction, the Out Box has a copy of the value in the
Accumulator -- the first input value.
*All this will be further explained –don’t give up yet!
How to load and save: Video demo
Program
INP
STA FIRST
INP
STA SECOND
LDA FIRST
OUT
LDA SECOND
OUT
HLT
FIRST DAT
SECOND DAT
LMC Part #1 Video 2
Program with Output
1. After the program is compiled, you should see from mailbox 0 to 7 the
instructions 901, 309, 901, 310, 509, 902, 510, 902. The Program
Counter should start at 0 (click on "Reset" if necessary).
2. DAT is the tenth and eleventh instruction of your program, so they refer
to mailboxes 9 and 10 (0-indexed counting). FIRST and SECOND are the
identifiers that have been declared to represent these mailboxes in the
assembly language program.
3. After the first INP instruction, the Accumulator has a copy of the first
value entered in the In Box
4. After the STA instruction, the input value is copied from the
Accumulator to mailbox 9 -- 3 means STORE and 09 refers to the mailbox
to store into
Look at the STA instruction more closely.
INP = 901 (this refers to the first number that will be input by the user)
STA FIRST = The code for ‘Store’ is 3. (that’s the first part of this instruction). The
second part is 09 – and this is the memory location that LMC needs to store the
INPUT number into.
In this scenario, my input number was “200” – and as you can see it has been
stored in memory location 09!
STA SECOND = This is referring to the second INPUT number that the user enters
(I had entered 100 as my second input). The 3 means ‘store’, and the second part
is ‘10’ – meaning, store ‘100’ in memory location 10 – there it is!
Making sense of LDA and OUT
After the LDA instruction, the Accumulator is reset to the first input
value. This value has been retrieved from mailbox 09
This instruction is also made of two parts. [509] The 5 means LOAD and 09
refers to the mailbox to load from. Note: since your Accumulator can only
work on one value at a time, you have to repeatedly STORE and LOAD
values from memory to keep them from getting erased by the next operation
After the OUT instruction, the Out Box has a copy of the value in the
Accumulator -- the first input value.
Remember the numbers are instructions!
English isn’t understood here! So the 5, in this case,
means ‘Store’ and the 09 is the location where the input
must be stored!
As mentioned on the previous slide, if my INPUT was
200, then you’d see the INPUT (200) stored in Location
9. There it is …
A run through demonstrating the numbers
linked to their corresponding instruction
FIRST DAT
INP
STA First
“200” is input
INP
STA Scnd
“100” is input
LDA Frst
*Load the
first Number
in 09 to the
Accumulator
OUT
200 is
output!
LDA Scnd
*Load the
second
Number in 10
to the
accumulator
OUT
HLT
SECOND DAT
Remember in Von Neumann architecture, memory contains both instructions and data.
Care therefore needs to be taken to stop the Program Counter reaching a memory address containing
data or the Little Man will attempt to treat it as an instruction. THE HLT INSTRUCTION was useful!
*try it yourself
Program
INP
STA FIRST
INP
ADD FIRST
OUT
INP
SUB FIRST
OUT
HLT
FIRST DAT
Adding in LMC
What you should do
1. Click on the "LMC
Simulator Applet" link to
start the LMC simulator.
2. Clear the Message Box
and all of the LMC
mailboxes -- click the
"Clear Messages" button
and the "Clear" button if
necessary.
3. Copy the ten line
program above and paste
it into the Message Box
4. Click on the "Compile
Program" button.
5. Click on the "Run" button.
6. When prompted, enter
numbers in the "In-Box",
and press the "Enter"
button.
What you should see (for inputs 5 + 4)
*try it yourself
Video: Adding in LMC
Watch the
video on
the right
and
analyse
what is
happening
Step 1: ADDING in LMC
Enter the code
Step 2: ADDING in LMC
Assemble to RAM
Step 3: ADDING in LMC
Run Program
Input: 5
PC: 2
INPUT:5
ACC:5
Step 5: ADDING in LMC
Second
Input: 4
PC: 5
INPUT: 4
ACC: 9
Subtracting in LMC: Method 1
INP = 5
Store 5 in 08
INP = 3
Store 3 in 00
Load 5 into the ACC
(5) Subtract second (3)
OUPUT 5 - 3
HALT
Note: First DAT = 5
stored in location 08
Subtracting in LMC: Method 2
INP = 5
Store 5 in 08
INP = 3
Store 3 in 09
Load 5 into the ACC
(5) Subtract second (3)
OUPUT 5 - 3
HALT
*The key difference is
that in Method 1,
SECOND DAT was not
specified and the
second input was stored
in 00. In this case, Input
2 is in 09.
Adding and Subtracting in LMC
You’ll note that a solution can be achieved in different
ways! Try and find the most elegant solution.
Method 1
Adding and Subtracting in LMC
Running Program--------------Program Counter : 0
Getting INPUT from user Input : 5--Program Counter : 1
STore Accumulator contents (5) in box 14--Program Counter : 2
Getting INPUT from user Input : 4--Program Counter : 3
STore Accumulator contents (4) in box 15--Program Counter : 4
LoaD Accumulator with contents of box 14 (5)--- Program
Counter : 5 ADD contents of box 15 (4) to the accumulator--Program Counter : 6 OUTPUTing accumulator (9)--- Program
Counter : 7 STore Accumulator contents (9) in box 0--Program Counter : 8
Getting INPUT from user Input : 3--Program Counter : 9 STore Accumulator contents (3) in box
16--Program Counter : 10
LoaD Accumulator with contents of box 0 (9)--Program Counter : 11
SUBtract contents of box 16 (3) from the accumulatorProgram Counter : 12 OUTPUTing accumulator (6)--Program Counter : 13 HALT!
*try it yourself
Program
INP
STA FIRST
INP
ADD FIRST
OUT
INP
SUB FIRST
OUT
HLT
FIRST DAT
Adding and Subtracting in LMC
What you should do
1. Click on the "LMC
Simulator Applet" link to
start the LMC simulator.
2. Clear the Message Box
and all of the LMC
mailboxes -- click the
"Clear Messages" button
and the "Clear" button if
necessary.
3. Copy the ten line
program above and paste
it into the Message Box
4. Click on the "Compile
Program" button.
5. Click on the "Run" button.
6. When prompted, enter
three-digit numbers in the
"In-Box", and press the
"Enter" button.
What you should see
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2.
3.
4.
5.
6.
7.
8.
9.
After the program is compiled, you should see from mailbox 0 to 7 the
instructions 901, 309, 901, 109, 902, 901, 209, 902. The Program
Counter should start at 0 (click on "Reset" if necessary).
DAT is the tenth instruction of your program, so it refers to mailbox 9 (0indexed counting). FIRST is the identifier that has been declared to
represent this mailbox in the assembly language program.
When you click on "Run" or "Step", the Message Box will describe the
actions of each instruction.
After the first INP instruction, the Accumulator has a copy of the first value
entered in the In Box.
After the STA instruction, the input value is copied from the Accumulator
to mailbox 9.
After the second INP instruction, the Accumulator has a copy of the
second value entered into the In Box (try values that will lead to three and
four digit sums).
After the ADD instruction, the Accumulator value represents the sum of
the two input values -- 1 means ADD and 09 refers to the mailbox where
the value to be added to the Accumulator is stored.
After the third INP instruction, the Accumulator has a copy of the third
value entered into the In Box (try values that will lead to positive and
negative results).
After the SUB instruction, the Accumulator value represents the difference
between the two input values -- 2 means SUBTRACT and 09 refers to the
mailbox where the value to be subtracted from the Accumulator is stored.
Method 1
Program showing the Output
Program
INP
STA FIRST
INP
ADD FIRST
OUT
INP
SUB FIRST
OUT
HLT
FIRST DAT
ADD the first INPUT
INP
Store INP in
Location 09
INP
Questions and Answers
Question 1
If
INPUT 1 = 3
and
INPUT 2 = 4
What would the
output of this program
be?
0
Answer
Load Second (4)
4 SUB Second =
4–4
=0
Question 2
Can you tell from
analysing the LMC,
what the INPUTS to
this program are?
Answer
INPUT 1: 12
INPUT 2: 13
See Memory Box 1
and 3, which give
their storage locations
Question 3
Can you tell from
analysing the LMC,
what the INPUTS to
this program are?
Answer
INPUT 1: 12
INPUT 2: 13
See Memory Box 1
and 3, which give
their storage locations
Question 4
What are the
numbers 901 and 902
referring to?
Answer
901 is referring to the
instruction: INP (input)
902 which we see in
location 6, is referring
to OUT (output)
Question 5
What is the meaning
of the number 309 in
memory location #3?
Answer
It is an instruction in
two parts. The 3
means ‘store’ and the
09 is referring to
where the number
should be stored – in
this case location 09.
What’s next? #Part 2 LMC
We’ve looked at commands like INP, HLT, OUT, STA, and LDA
But using only the ones above don’t allow us to do everything we need.
Looping (as you know from ordinary programming) is an important tool.
In the next session we will look at MAKING
DECISIONS in the LMC and how to Multiply
(which is a great example of using a loop)