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# Homework 6 Introduction to LC-3 Assembly

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CS 2110 Homework 6
Introduction to LC-3 Assembly

Contents
1 Overview 2
2 Instructions 2
2.1 Part 1: DeMorgan’s Revenge (or.asm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.2 Part 2: Arrays – Summing Negative Numbers (sum negative.asm) . . . . . . . . . . . . . . . . . 2
2.3 Part 3: Arrays – Selection Sort (select.asm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.4 Part 4: Strings – Comparing (strcmp.asm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.5 Part 5: Linked List – Finding Extrema (max min.asm) . . . . . . . . . . . . . . . . . . . . . . . . 3
3 Deliverables 4
4 LC-3 Assembly Programming Requirements 4
4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5 Hints 5
5.1 Pseudo-Ops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.2 Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.3 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6 Rules and Regulations 6
6.1 General Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.2 Submission Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.3 Submission Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.4 Syllabus Excerpt on Academic Misconduct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.5 Is collaboration allowed? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
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1 Overview
The goal of this assembly assignment is to familiarize you with coding in the LC-3 assembly language. This will
involve writing small programs, modifying memory, and converting from high-level code to assembly.
2 Instructions
2.1 Part 1: DeMorgan’s Revenge (or.asm)
For this part, notice that the LC-3 assembly language directly accommodates AND and NOT operations with
corresponding instructions but not an OR instruction.
Use your knowledge of DeMorgan’s Laws to compute A | B.
Open or.asm, and notice there are three labels. The labels A and B point to two .fill values to be OR’d.
Store the result of A | B in memory at the third label ANSWER.
2.2 Part 2: Arrays – Summing Negative Numbers (sum negative.asm)
For this part, write a program which traverses an array and sums all of the negative numbers (there will also
be positive numbers in the array).
Open sum negative.asm, and notice there are three labels. The labels ARRAY ADDR and ARRAY LEN point to the
starting address and length of the array.
Before HALTing the program, be sure to store the result at the third label ANSWER.
Note: The sum will not necessarily be negative. For example, if there is overflow, you might end up with a
positive number.
2.3 Part 3: Arrays – Selection Sort (select.asm)
For this part, write a program which sorts an array in ascending order according to the selection sort algorithm.
Consider the following pseudocode:
for i = 0 to ARRAY_LEN – 1: # Iterate through every position in the array
min_idx = i # Find the element that will move to position i
# Everything before index i is sorted, so we are
# looking for the minimum value after index i
for j = i + 1 to ARRAY_LEN:
min_idx = j
# Swap the value that belongs here with the current
# value; these might be the same value!
# (and index i might be equal to index min_idx)
Open select.asm, and notice that there are two labels. The labels ARRAY ADDR and ARRAY LEN correspond to
the starting address and length of the array, respectively.
Note: This sort should be in-place, so when your program finishes, the location in memory that contained your
original array should now contain the sorted array.
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2.4 Part 4: Strings – Comparing (strcmp.asm)
For this part, write a program which compares two null-terminated strings, or sequences of characters, according
to the following specification:
• The function should store either 1, 0, or -1 at the label ANSWER according to whether the first string is
greater than, equal to, or less than the second string
• Determine greater than, equal to, or less than by comparing their ASCII values
Consider the following examples:
strcmp(“dogcat”, catdog”) ==> 1
strcmp(“whether”, “whither”) ==> -1
strcmp(“”, “blank”) ==> -1
strcmp(“same”, “same”) ==> 0
Open strcmp.asm, and notice there are three labels. The labels ADDR STR 1 and ADDR STR 2 correspond to the
starting addresses of the first and second strings to be compared.
The process is very simple: Iterate through both strings, comparing the character values at the same indices.
When you find an inequality, or one of the two characters is a zero, stop the iteration and compute the result
based on those two characters. If both characters are zero, the strings are equal. If one character is zero, that
string comes first because it’s shorter. Otherwise, compare the differing characters, and the string with the
smaller character value comes first.
Store the result at the third label ANSWER.
Note: Strings are zero-terminated sequences of 16-bit words. Consider the string “ASSEMBLY” starting at address
x4000. The ASCII value for character ’A’ (x0041) is stored at x4000, the ASCII for character ’S’ (x0053) is
stored at x4001, etc. Finally, the value 0 is stored at address x4008. You, of course, do not need to memorize
or interpret the ASCII table to determine which value is smaller.
2.5 Part 5: Linked List – Finding Extrema (max min.asm)
For this part, write a program that traverses a singly-linked list of nodes and finds both the maximum and
minimum elements, where each node is comprised of a next address that points to the next node and an integer
data. Consider the following pseudocode:
if node == NULL: # For an empty list, set some invalid values:
max = MIN_INT # – The smallest possible number for max
min = MAX_INT # – The largest possible number for min
else: # For a non-empty list, initialize with the head node’s data:
max = mem[node + 1] # Remember: mem[node + 1] is the node’s integer data
min = mem[node + 1]
node = mem[node] # Remember: mem[node] is the node’s next address
while node != 0: # For all remaining nodes in the list:
if mem[node + 1] > max: # Check for a new max
max = mem[node + 1]
if mem[node + 1] < min: # Check for a new min
min = mem[node + 1]
node = mem[node] # How do we get the address of the next node?
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Once again, each node of this list is comprised of two consecutive 16-bit values in memory: The address of the
next node and the data being stored.
———+———-+————–
x4000 | x4004 | Node 1: next
x4001 | x0035 | Node 1: data
x4002 | x345A | …
x4003 | x7441 | …
x4004 | x4008 | Node 2: next
x4005 | x0101 | Node 2: data
x4006 | x0000 | Node 4: next
x4007 | x9040 | Node 4: data
x4008 | x4006 | Node 3: next
x4009 | x7000 | Node 3: data
Observe that Node 1 points to Node 2 (at address x4004). Node 2 points to Node 3 (at address x4008). Node
3 points to Node 4 (at address x4006). Finally, Node 4 points to address x0000 (i.e. NULL), signaling the end
of the list!
Our autograder will, in general, store nodes near x4000, so please avoid writing any instructions or storing any
data near there.
Note: The grader will only test lists with nonnegative integer data! (i.e. 0x0000 · · · 0x7FFF)
3 Deliverables
1. or.asm
2. sum negative.asm
3. select.asm
4. strcmp.asm
5. max min.asm
Be sure to check your score to see if you submitted the right files, as well as your email frequently
until the due date of the assignment for any potential updates.
4 LC-3 Assembly Programming Requirements
4.1 Overview
1. Your code must assemble with NO WARNINGS OR ERRORS. To assemble your program, open the
file with Complx. It will complain if there are any issues. If the code in this file does not assemble,
you WILL get a zero for that file.
2. Comment your code! This is especially important in assembly, because it’s much harder to interpret
what is happening later, and you’ll be glad you left yourself notes on what certain instructions are contributing to the code. Comment things like what registers are being used for and what less intuitive lines
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of code are actually doing. To comment code in LC-3 assembly just type a semicolon (;), and the rest of
that line will be a comment.
3. Avoid stating the obvious in your comments; it doesn’t help in understanding what the code is doing. Try
Good Comment
ADD R3, R3, -1 ; counter–
BRp LOOP ; if counter == 0 don’t loop again
ADD R3, R3, -1 ; Decrement R3
BRp LOOP ; Branch to LOOP if positive
4. DO NOT assume that ANYTHING in the LC-3 is already zero. Treat the machine as if your
program was loaded into a machine with random values stored in the memory and register file.
5. Following from 3. You can randomize the memory and load your program by doing File – Randomize and
7. Test your assembly. Don’t just assume it works and turn it in.
5 Hints
5.1 Pseudo-Ops
Pseudo-Ops are directions for the assembler that aren’t actually instructions in the ISA.
1. .orig: Tells the assembler to put this block of code at the desired address.
Example: .orig x3000 will put the block of code at address x3000.
2. .stringz “something”: Will put a string of characters in memory followed by NULL (which is a single
ASCII character with the value 0).
Example: .stringz “sanjay” will put the ASCII code for the letter ‘s’ in the first memory location,
the code for ‘a’ in the second memory location, and so on until putting ‘y’ in the next-to-last position
and NULL (which again, has the ASCII code 0) in the last memory location as the terminator.
3. .blkw n: A pseudo-op that will allocate the next n locations of memory for a specified label.
4. .fill value: A pseudo-op that will put the value in that memory location.
5. .end: A psuedo-op that denotes the end of an address block. Matches with an .orig.
5.2 Traps
Traps are subroutines built into the LC-3 to help simplify instructions. Some helpful TRAPS include:
1. HALT: HALT is an alias for a TRAP that will stop the LC-3 from running
2. OUT: OUT is an alias for a TRAP that will take whatever character is in R0 and print it to the console
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3. PUTS: PUTS is an alias for a TRAP that will print a string of characters with the starting address saved in
R0 until it reaches a NULL (0) character
4. GETC: GETC is another TRAP alias that will take in a character input and store it in R0
Being an alias for a TRAP instruction means that the assembler will convert them to TRAP instructions
at assembly time. For example, if you write HALT in your code, the assembler will convert it to the
instruction, TRAP x25 for you.
5.3 Example
The following small example will demonstrate the use of these Traps and pseudo-ops:
.orig x3000 ; Where the code will begin
LEA R0, HW ; Loads the address of the string into R0
PUTS ; Print the string whose address is in R0
HALT ; Stops the program from executing
HW .stringz “Hello. \n” ; Stores the word ’Hello’ in memory with ’H’ be located at
.end ; Denotes the end of the address block
6 Rules and Regulations
6.1 General Rules
1. Starting with the assembly homeworks, any code you write must be meaningfully commented. You should
comment your code in terms of the algorithm you are implementing; we all know what each line of code
does.
2. Although you may ask TAs for clarification, you are ultimately responsible for what you submit. This
means that (in the case of demos) you should come prepared to explain to the TA how any piece of code
you submitted works, even if you copied it from the book or read about it on the internet.
4. Please start assignments early, and ask for help early. Do not email us the night the assignment is due
with questions.
5. If you find any problems with the assignment it would be greatly appreciated if you reported them to the
author (which can be found at the top of the assignment). Announcements will be posted if the assignment
changes.
6.2 Submission Conventions
1. All files you submit for assignments in this course should have your name at the top of the file as a
comment for any source code file, and somewhere in the file, near the top, for other files unless otherwise
noted.
2. When preparing your submission you may either submit the files individually to Canvas/Gradescope or
you may submit an archive (zip or tar.gz only please) of the files. You can create an archive by right
files or an archive) are exactly equivalent, so choose whichever is most convenient for you.
3. Do not submit compiled files, that is .class files for Java code and .o files for C code. Only submit the files
we ask for in the assignment.
4. Do not submit links to files. The autograder does not understand it, and we will not manually grade
assignments submitted this way as it is easy to change the files after the submission period ends.
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6.3 Submission Guidelines
1. You are responsible for turning in assignments on time. This includes allowing for unforeseen circumstances. If you have an emergency let us know IN ADVANCE of the due time supplying documentation
(i.e. note from the dean, doctor’s note, etc). Extensions will only be granted to those who contact us in
2. You are also responsible for ensuring that what you turned in is what you meant to turn in. After
submitting you should be sure to download your submission into a brand new folder and test if it works.
No excuses if you submit the wrong files, what you turn in is what we grade. In addition, your assignment
must be turned in via Canvas/Gradescope. Under no circumstances whatsoever we will accept any email
submission of an assignment. Note: if you were granted an extension you will still turn in the assignment
3. There is a 6-hour grace period added to all assignments. You may submit your assignment without penalty
up until 11:55PM, or with 25% penalty up until 5:55AM. So what you should take from this is not to start
assignments on the last day and plan to submit right at 11:54AM. You alone are responsible for submitting
are to blame if you are unable to submit because you banked on your computer working up until 11:54PM.
The penalty for submitting during the grace period (25%) or after (no credit) is non-negotiable.
6.4 Syllabus Excerpt on Academic Misconduct
Academic misconduct is taken very seriously in this class. Quizzes, timed labs and the final examination are
individual work.
Homework assignments are collaborative, In addition many if not all homework assignments will be evaluated
via demo or code review. During this evaluation, you will be expected to be able to explain every aspect of
your submission. Homework assignments will also be examined using computer programs to find evidence of
unauthorized collaboration.
What is unauthorized collaboration? Each individual programming assignment should be coded by you. You
may work with others, but each student should be turning in their own version of the assignment. Submissions
that are essentially identical will receive a zero and will be sent to the Dean of Students’ Office of Academic
Integrity. Submissions that are copies that have been superficially modified to conceal that they are copies are
also considered unauthorized collaboration.
You are expressly forbidden to supply a copy of your homework to another student via electronic
means. This includes simply e-mailing it to them so they can look at it. If you supply an electronic
copy of your homework to another student and they are charged with copying, you will also be
charged. This includes storing your code on any site which would allow other parties to obtain
your code such as but not limited to public repositories (Github), pastebin, etc. If you would
like to use version control, use github.gatech.edu
6.5 Is collaboration allowed?
Collaboration is allowed on a high level, meaning that you may discuss design points and concepts relevant
to the homework with your peers, share algorithms and pseudo-code, as well as help each other debug code.
What you shouldn’t be doing, however, is pair programming where you collaborate with each other on a single
instance of the code. Furthermore, sending an electronic copy of your homework to another student for them
to look at and figure out what is wrong with their code is not an acceptable way to help them, because it is
frequently the case that the recipient will simply modify the code and submit it as their own. Consider instead
using a screen-sharing collaboration app, such as http://webex.gatech.edu/, to help someone with debugging
if you’re not in the same room.
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Figure 1: Collaboration rules, explained colorfully
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