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Computer Architecture and Assembly Language Assignment 2

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Computer Science 230
Computer Architecture and Assembly Language

Assignment 2

Programming environment
For this assignment you must ensure your work executes correctly on Arduino
boards in ECS 249. If you have installed Microchip Studio on your own computer
then you are welcome to do some of the programming work on your machine.
However, please plan to spend a significant amount of time in the lab.

Objectives of this assignment
• Write and use functions.
• Implement parameter passing using registers and stack.
• Implement return values using registers.
• Use stack frames where needed for parameter passing.
• Use the Arduino mega2560 board in the course lab to implement a text
signalling display.
Page 2 of 7
Semaphore signalling
One form of visually communicating a message involves transforming each letter of
the message into a visible signal. That signal may be in the form of flags (as is used
in naval flag signalling). Another is to configure a set of lights into suitably different
patterns, and this something we will do for this assignment using the six LEDs on
the lab’s Arduino Mega boards.
A video has been prepared demonstrating some working code for all five parts of
this this assignment:

The video starts, however, with a demonstration of a completely working
assignment that is flashing the message HELLOWORLD. For each of the ten letters in
this message you will see is a pattern of six LEDs (some on, some off). Even more
subtly there is also the possibility of changing the duration of the light pattern.
Breaking this down even further, and representing an LED that is on by using “o”
and an LEDS that is off by using “.”, you will notice the following in that opening
sequence of the video:
• “H”: ..oo.. (short duration)
• “E”: oooooo (long duration)
• “L”: o.o.o. (long duration)
• “L”: o.o.o. (long duration)
• “O”: .oooo. (long duration)
• “W”: o.o… (short duration)
• “O”: .oooo. (long duration)
• “R”: oo..oo (long duration)
• “L”: o.o.o. (long duration)
• “D”: …..o (long duration)
Note again that letters “H” and “W” have a short duration. The encoding for all 26
letters of the alphabet can be found at the end of the starter file provided to you for
this assignment (a2-signalling.asm).
There is only one file for this assignment. Your work will be in five parts made up
of six functions, ordered from easy to more difficult, with the assumption that you
will complete earlier parts before attempting later parts. Although you do not need
to write any functions above and beyond those listed below, you are not forbidden
from doing so.
a) Write the function set_leds
b) Write the functions fast_leds and slow_leds
c) Write the function leds_with_speed
d) Write the function encode_letter
e) Write the function display_message
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Part (a): Write function set_leds
set_leds:
parameters: one in r16, pass-by-value
return value: none
The parameter to set_leds determines which of the Arduino board’s six LEDs to
turn on (i.e., the ones which you would have used during some lab exercises this
semester). For some of what appears below, you may find it helpful to review once
again materials provided for Lab #3.
Interpretation of the parameter’s value by the function is quite straightforward.
You’ll notice that the six-LED board itself on the Arduino has numbers underneath
each of the LEDs.
• bit 5 of r16 controls the leftmost LED (i.e. LED 01) – if the bit is set, the light
is to be turned on, otherwise turned off …
• bit 4 of r16 controls the second-to-left LED (i.e. LED 02) in a manner similar
to that above …
• bit 3 of r16 controls the third-to-left LED (i.e. LED 03) …
• bit 2 of r16 controls the third-to-right LED (i.e. LED 04) …
• bit 1 of r16 controls the second-to-right LED (i.e. LED 05) …
• bit 0 of r16 controls the rightmost LED (i.e. LED 06) …
• and finally, bits 7 and 6 of r16 is ignored, regardless of value.
For example, a value in r16 that would turn on the leftmost and rightmost LEDs is
0x21 (i.e 0b00100001). A value of 0x00 would turn off all LEDs.
Also please note that this function is meant to turn on/off the LEDs regardless
of any duration for this action. LEDs will stay on or stay off until changed by
another call to set_leds with a suitably different parameter value. To obtain
effects such as turning of LEDs on for a fixed period of time, you will need to use
extra code, and this you will write written later.
Some code is provided for you at the label test_a which you may use to try out your
work after writing code for the function. The video described earlier in this
document shows expected behavior of test_a. (Note that the delay functions used
in this code are provided to you already: delay_long is about one second long, and
delay_short is about one-quarter of a second long.)
Page 4 of 7
Part (b): Write the functions fast_leds and slow_leds
fast_leds:
parameters: one in r16 r17, pass-by-value
return value: none
slow_leds:
parameters: one in r16 r17, pass-by-value
return value: none
The fast_leds function will turn on LEDs using the same pattern as described in
part (a), but will leave them on for about one-quarter of a second before turning
them off. Amongst other things, you must call your code for set_leds and use the
various delay functions in the provided code (e.g. delay_short).
The slow_leds function will turn on the LEDs using the same pattern as described
in part (b), but will leave them on for about one second before turning them off.
Amongst other things, you must call your code for set_leds and use the various
delay functions in the provided code (e.g. delay_long).
Some code is provided for you at the label test_b which you may use to try out your
work after writing code for these two functions. The video described earlier in this
document shows expected behavior of test_b.
Part (c): Write the function leds_with_speed
leds_with_speed:
parameters: one byte, pushed onto the stack by the caller, pass-by-value
return value: none
In the functions described within the previous two parts of the assignment, the byte
provided as a parameter encoded the LEDs to be turned on or off. This is indicated
by the way bits 5 through 0 of the parameter are set or unset. Bits 7 and 6 have been
ignored until now. In this function, we’ll begin to make use of those two left-over
bits.
• If the two top-most bits are set, the LED pattern is to be on for about one
second.
• If the two top-most bits are unset, the LED pattern is to be on for about onequarter of a second.
As an example, to turn on all LEDs for one second, the value pushed onto the stack
would be 0xFF (i.e. 0b11111111). However, to turn them all on for one-quarter of a
second, the value pushed would be 0x3F (0b00111111). Do not worry about the other
two patterns possible for the left-most two bits as those patterns may be ignored.
Page 5 of 7
Call your code for fast_leds and slow_leds as part of the implementation of this
function.
Some code is provided for you at the label test_c which you may use to try out your
work after writing code for these two functions. The video described earlier in this
document shows expected behavior of test_c.
Part (d): Write the function encode_letter
encode_letter:
parameters: one byte, pushed onto the stack by the caller, pass-by-value
return value: r25
This document began by describing a way of encoding letters as LED light patterns,
with the example of “HELLOWORLD” given. Although you’re not yet ready to display
the whole message, your task for this part is to take a single upper-case letter and to
convert it into the correct pattern (lights and duration).
Towards the bottom of a2-signalling.asm you will find the label PATTERNS,
followed by an upper-case alphabet – one letter per line – with information on
LED/duration encodings. For example, the first non-comment line below PATTERNS
is:
.db “A”, “..oo..”, 1
which is to be interpreted as follows:
• This line is for the letter “A”;
• LEDS 01, 02, 05 and 06 are off in this pattern;
• LEDS 03 and 04 are on in this pattern;
• The lighting pattern is to appear for about one second.
Right below appears this line:
.db “B”, “.o..o.”, 2
which is to be interpreted as follows:
• This line is for the letter “B”;
• LEDS 01, 03, 04, and 06 are off in this pattern;
• LEDS 02 and 05 are on in this pattern;
• The lighting pattern is to appear for about one-quarter of a second.
So on and so forth.
The only other entry that requires some special explanation is the very last. It
appears here as something which you may choose to check – i.e. if you are looking
Page 6 of 7
for a letter, but find the dash (“-“) then something has gone wrong with the
parameter value. (When testing your code, the evaluators will only use upper-case
letters. However, you cannot be sure how you yourself will accidentally test your
code!)
Given the letter pushed onto the stack, your function is to determine the encoding
for the letter as would be given to leds_with_speed (i.e. a byte value), and this
returned in register r25. (That is, you are not to call leds_with_speed from within
encode_letter) Make use the functions you have written earlier in this assignment
to complete the function, as well as other operations. For example, the value
returned in r25 for “A” would be 0b11001100 or 0xcc; the value returned in r25 for
“B” would be 0b00010010 or 0x12.
Some code is provided for you at the label test_d which you may use to try out your
work after writing code for these two functions. The video described earlier in this
document shows expected behavior of test_d.
Part (e): Write the function display_message
display_message:
parameters: byte address of message in program memory which is to be
displayed; high byte of address in r25; low byte of address in r24;
messages have no spaces and contain only upper-case letters;
messages are terminated with null (ASCII code 0).
return value: none
This last function of the assignment is meant to tie together all your work so far.
Again towards the bottom of the assembly file provided to you are a few other
labels. For example, there is this line:
WORD07: .db “THE”, 0
where WORD07 can be provided to the assembler as the address within program
memory to the string “THE”. Your function will move from letter to letter, encoding
each in turn by calling encode_letter and using the return value as the parameter
to call leds_with_speed.
Some code is provided for you at the label test_e which you may use to try out your
work after writing code for these two functions. The video described earlier in this
document shows expected behavior of test_e.
Page 7 of 7
What you must submit
• Your completed work in the single source code file (a2-signalling.asm);
do not change the name of this file!
• Your work must use the provided skeleton a2-signalling.asm. Any other
kinds of solutions will not be accepted.
• Nothing else is to be submitted – no ZIP files, no project files, no manifestos,
no video rants, nothing else!
Evaluation
• 3 marks: Solution for set_leds
• 2 mark: Solution for fast_leds for slow_leds
• 4 marks: Solution for leds_with_speed
• 6 marks: Solution for encode_letter
• 5 mark: Solution for display_message
Therefore the total mark for this assignment is 20.
Some of the evaluation above will also take into account whether or not submitted
code is properly formatted (i.e., indenting and commenting are suitably used), and
the file correctly named.

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