CS061 – Programming Assignment 2 D Solved

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To further familiarize you with the basic LC-3 instructions;

to understand the difference between numeric characters and actual numbers; to handle two’s complement conversions; and to perform basic input/output.


High Level Description

Prompt the user to input two single digit numbers.

The second will then be subtracted from the first, and the operation reported in the console:

<first number> – <second number> = <difference>

SO if the user enters 8 and 4, these two numbers will first be echoed to the console on separate lines, then the subtraction operation will be displayed:


Note: the user does not enter any newlines; all newlines are to be generated by the program itself.


LC-3 I/O

First, read this brief intro to the LC-3 BIOS (Basic Input Output System)


Low Level Breakdown

This assignment comprises five tasks:

  1. Prompt the user, and read two numeric characters ​(‘0’ … ‘9’)​ from the user using Trap x20 (GETC). Echo the characters to the console ​as they are received​ (OUT), and store them as character ​data in separate registers.
  2. Output to the console the operation being performed e.g.

5 – 7 =

(how will you print the ” – “? How will you print the ” = “? Note the double quotes!!)

  1. Once the setup is printed, convert the numeric characters into the actual numbers they represent (e.g. convert the ​ASCII code for ‘7’​ into the binary representation of the number 7​).
  2. Perform the subtraction operation ​(by taking the two’s complement of the second operand and adding)​, and determine the sign (+/-) of the result; if it is negative, determine the ​magnitude​ of the result ​(i.e. take the 2’s complement to turn it back into a positive number)​.
  3. Convert resulting ​number​ back to a printable ​character​ and output it, together with a minus sign if necessary. Remember, the number -4 when converted to text is actually two separate ascii characters, ‘-‘ and ‘4’.


Reminder: Make sure you always have your Text Window open when you run simpl – this is the only way to catch run-time (mostly i/o) errors!           

Example, with detailed algorithm (we won’t always give you this!) ● Program prompts for user input (two characters):

  • user enters ‘5’, which is echoed to console (followed by a newline) and copied to a register.
  • user enters ‘7’, which is echoed to console (followed by a newline) and copied to a different register.
  • Program outputs the text

5 – 7 =

(this will actually require at least 4 distinct output steps using OUT and PUTS)

  • Program converts ‘5’ ​(ascii code)​ into 5 ​(number)​ and stores it back in the same register.
  • Program converts ‘7’ into 7 and stores it back in the same register.
  • Program takes 2’s complement of 7, and stores the result back into the same register.
  • Program adds the contents of the two registers – i.e. it performs the operation (​5-7​) and stores the result (-2) in a third register.
  • Program recognizes that result is negative, obtains the magnitude of -2 (= 2), and outputs ‘-‘​ (minus sign).
  • Program converts 2 ​(number)​ into ‘2’ ​(ascii code)​, and stores it back in the same register.
  • Program outputs ‘2’ followed by a newline​.



Expected/ Sample output

In this assignment, your output must ​exactly​ match the following, including:

  • the prompt, ​followed by newline (provided in the starter code)
  • Each digit input “echoed” and ​followed by a newline
  • the subtraction operation, including spaces as shown, also followed by a newline​:




(Difference is Positive)



(Difference is Zero)






(Difference is Negative)


Your code will obviously be tested with a range of different operands giving all possible results. Make sure you test your code likewise!



  • All console output must be NEWLINE terminated.
  • We will test only with positive single digit numeric inputs
  • NO error message is needed for invalid input (i.e. we will not test with non-numeric inputs)​ Uhhelp?
  • Trap x20 (GETC) will ​always​ store the input character into R0.

You cannot specify any other register to receive the keyboard input.

  • Trap x21 (OUT) will ​always​ print whatever ASCII code is stored in R0. You cannot specify any other register to output to screen.
  • If the user enters ‘7’, the value stored into R0 is the ​ASCII code​ b0000 0000 0011 0111  ( = x0037 = ‘7’ ), not​  the ​number​ 7 = b0000 0000 0000 0111 (= x0007 or #7).

Likewise, to output the number 7 as a character, it has to be adjusted from x0007 to x0037 first. Go to​ ​www.asciitable.com​ and see why.


Note: you should ALWAYS refer to the ascii numeric offset as x30, NEVER as #48.

The ascii table only makes sense when understood as a binary code (readable as hex).


Conversion between a character and the number it represents and vice versa will be used repeatedly in this course, so make sure you understand how to do it now!!

  • To take the two’s complement of a number ​(i.e. make a positive number negative or vice versa)​: – Invert the bits ​(what assembly instruction does this?) – Add one
  • A neat trick in LC3 to copy the value of one register directly to another:

ADD R5, R6, #0   ; R5 ← (R6) + 0,  i.e.  R5 ← (R6)

  • If the result is negative, remember that you will have to print ​two​ characters, not one ​(there is no ASCII code for ‘-1’, right?)
  • If you are struggling with writing LC-3 code from scratch, try writing the program out in pseudo-code or even C++ first. Then, your only task is to convert the logic/code into LC-3.



Submission Instructions

Submit (“Upload”) your ​assignment2.asm​ file ​(and ONLY that file!)​ to the Programming Assignment 2 folder in Gradescope: the Autograder will run & report your grade within a minute or so.  You may submit as many times as you like – your grade will be that of your last submission. If you wish to set your grade to a previous submission with a higher score, you may open your “Submission history” and “Activate” any other submission – that’s the one we will see.



  • To pass the assignment, you need a score of >= 80%​.

The autograder will run several tests on your code, and assign a grade for each.

But certain errors ​(run-time errors, incorrect usage of I/O routines, missing newlines, etc.)​ may cause ALL tests to fail => 0/100! So submit early and study the autograder report carefully!!


  • You must use the template we provide​ – if you make ​any​ changes to the provided starter code, the autograder may not be able to interpret the output, resulting in a grade of 0.


Comics??! Sweet!!!


Source: ​http://xkcd.com/409/


  • assignment2-ypq43q.zip