Structuring LC3 Assignments

To make grading of LC3 assignments, I have found the following structure for LC3 assembly language assignments to be useful. This lends itself to assignments that do not do I/O, but simply take parameters from predefined locations (labels) and leave results in predefined locations (labels).

The idea is to be able to use diff on memory dumps to view and grade the students submission. To make this easier, all parameters and outputs are placed at fixed addresses that are independent of the students code. To accomplish this, the students are given a skeleton which looks like the following: 

;File:        --> fill this in
;Description: --> fill this in
;Author(s):   --> fill this in
;Date:        --> fill this in

; ********************** BEGIN RESERVED SECTION ********************** 
; You may initialize values here (i.e., change the value of a .FILL
; statement), but you may *not* add or remove instructions or perform
; any edit which changes the address of a label in this section
	
               .ORIG x3000      ; DO NOT change any code before label END_DATA
               BR BEGIN_CODE    ; jump to beginning of code
BEGIN_DATA     .FILL xABCD      ; start of provided variables

PARAM1         .FILL xxx        ; these are assignment specific
PARAM2         .FILL yyy
RESULT1        .BLKW 1
RESULT2        .BLKW 1
RESULT_ARRAY   .BLKW 20

END_DATA       .FILL xFECD      ; end of provided variables

; ********************** END RESERVED SECTION ********************** 
;---------you may add more variables or code below this line --------

BEGIN_CODE                      ; your code begins here

The labels BEGIN_DATA and END_DATA allows the grading script to simply dump memory from BEGIN_DATA and END_DATA. This will then dump all relevant values. It also has the benefit of (possibly) detecting overwrites in case a block of data (an array) is part of the output.

Testing the assignment can be performed using the scripting capability of lc3sim. Specifically, a command file like the following is used: 

    file assigX.obj
    memory PARAM1 testValue1
    memory PARAM2 testValue2
    continue
    list BEGIN_DATA END_DATA
    quit

This set of commands

  1. loads the file assignX.obj
  2. sets two input values (PARAM1 and PARAM2)
  3. runs the program
  4. dumps the results (and inputs)
To see a list of commands avalable in lc3sim, simply start the simulator (lc3sim NOT lc3sim-tk) and type help.

Because the addresses are predefined by the provided code, the list shows the same addesss for every students code. Only the values will change. Thus, diff will correctly detect differences.

To execute lc3sim with a script file use the syntax lc3sim -s scriptFile.

When you want to do character input in a simulator run (i.e. use the LC3 IN instruction), I have found that a executing lc3sim -s scriptFile < fileOfInput works.

Fritz Sieker - Jan 2012