Description
Objective
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:
- 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.
- 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!!)
- 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).
- 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).
- 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!
NOTES:
- 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) 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 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.
Rubric
- 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/
