Programming Assignment 02 Solution

Objective

To further familiarize you with the basic LC3 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, then the subtraction will be displayed:







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:




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 input, and store them as character data in separate registers.



Output to the console the operation being performed e.g.
5 - 7 =




Using the same registers, 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).
Perform the subtraction operation (by taking the two's complement of the second operand and adding) ,and determine the sign (+/-) of the result; if negative, determine the magnitude of the result (i.e. take 2's complement to turn it back into a positive number)



Convert resulting number back to a printable character and print it, together with minus sign if necessary.



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 and copied to a register.



user enters ‘7’, which is echoed to console and copied to a different register.



Program outputs
5 - 7 =

(this will actually require at least 4 distinct output steps)

Program converts ‘5’ into 5 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, converts -2 to 2, and outputs '-' (minus sign)



Program converts 2 into '2', and stores it back in 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
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!

NOTE :

All console output must be NEWLINE terminated.
We will test only with positive single digit numeric input



NO error message is needed for invalid input (i.e. we will not test with non-numeric inputs)
Uh…help?
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 tp 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 (= #7). Go to www.asciitable.com and see why.



(conversion between a character and the number it represents 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. to 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:



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 LC3 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 LC3.






Submission Instructions

Submit to GitHub (pull, add, commit, push)




Comments/Feedback

Do a "Git pull" to download the results.html file for detailed feedback.




Rubric




The autograder will attempt to assign partial credit for each test (like those described in the expected output section above), and will report which tests passed and which failed.
To pass the assignment, you need a cumulative score of = 8/10.

HOWEVER :after certain errors in your output (run-time errors, missing newlines, etc), the autograder will be unable to proceed, resulting in a grade of 0/10, with no partial credit.




This should not be a problem :the problematic output will usually be clearly highlighted in the feedback, so you can fix & resubmit, and hopefully get past the blockage.




(Unless, of course, you waited until one hour before the deadline to submit, in which case you're stuck with the 0/10 - but you would never do that, would you?)




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 ouput, resulting in a grade of 0..
 
Comics??! Sweet!!!









Source: http://xkcd.com/409/