Department of Electrical and Computer Engineering

The University of Texas at Austin

EE 306, Fall 2011
Programming Assignment 5
Due: 2 December, 5:00 pm
Yale N. Patt, Instructor
TAs: Faruk Guvenilir, Milad Hashemi, Jennifer Davis, Garrett Galow, Ben Lin, Taylor Morrow, Stephen Pruett, Jee Ho Ryoo
You must do the programming assignment by yourself. You are permitted to get help from ONLY the TAs and the instructor.

The purpose of this assignment is to show how interrupt-driven Input/Output can interrupt a program that is running, execute the interrupt service routine, and return to the interrupted program, picking up exactly where it left off (just as if nothing had happened). In this assignment, we will use the Keyboard as the input device for interrupting the running program.

The assignment consists of three parts:


The user program.

Your user program will consist of continually producing the "UT checkerboard" by alternately outputing two different lines. One line will consist of the pattern "UT followed by four spaces" eight times. The second line will consist of three spaces, and the pattern (UT followed by four spaces) seven times. 

UT    UT    UT    UT    UT    UT    UT    UT    
   UT    UT    UT    UT    UT    UT    UT       
UT    UT    UT    UT    UT    UT    UT    UT    
   UT    UT    UT    UT    UT    UT    UT       
UT    UT    UT    UT    UT    UT    UT    UT    
   UT    UT    UT    UT    UT    UT    UT       
UT    UT    UT    UT    UT    UT    UT    UT    
   UT    UT    UT    UT    UT    UT    UT
To ensure the output on the screen is not too fast to be seen by the naked eye, the user program should include a piece of code that will count down from 2500 each time a line is output on the screen. A simple way to do this is with the following subroutine DELAY: 

 
DELAY   ST  R1, SaveR1
        LD  R1, COUNT
REP     ADD R1,R1,#-1
        BRp REP
        LD  R1, SaveR1
        RET
COUNT   .FILL #2500
SaveR1  .BLKW 1


The keyboard interrupt service routine.

The keyboard interrupt service routine will simply write to the screen ten times whatever key the person sitting at the keyboard typed, followed by a linefeed (x0A).

VERY IMPORTANT: You may not use any TRAP instructions in your interrupt service routine. To display a character to the screen, you must poll the DSR and then write to the DDR, you may not call TRAP x21 (OUT), or use any of the other TRAP routines. If you use TRAP in the interrupt service routine or if you do not properly poll the DSR before writing to the DDR, your program is not correct and will fail our testing even though it may appear to work when you test it. You may use any TRAP instructions you wish in your user program.

Hint: Don't forget to save and restore any registers that you use in the interrupt service routine.


The operating system enabling code.

Unfortunately, we have not YET installed Windows or Linux on the LC-3, so we are going to have to ask you to do the following three enabling actions in your user program first, before your user program starts outputing the UT checkerboard. Normally, these would be done by the operating system.

1. Normally, the operating system would have previously set up some stack space so that the PC and PSR can be pushed when an interrupt is encountered. (As you know, when the service routine executes RTI, both PC and PSR will be popped, returning the machine to the interrupted program.) Since there is no operating system, please initialize R6 to x3000, indicating an empty stack.

2. Also, normally, the operating system establishes the interrupt vector table to contain the starting addresses of the corresponding interrupt service routines. You will have to do that for the keyboard interrupt. The starting address of the interrupt vector table is x0100 and the interrupt vector for the keyboard is x80. It is necessary for you to only provide the one entry in the interrupt vector table that is needed for this programming lab assignment.

3. Finally, normally, the operating system would set the IE bit of the KBSR. You will have to do that as well.


Your job.

Your job will be to write both the user program augmented with the interrupt enabling code described above and the keyboard interrupt service routine.

The user program must be named user_program.asm and will be of the form:

                .ORIG    x3000
                  --      ---     ; initialize the stack pointer
                     ...
                  --      ---
                  --      ---     ; set up the keyboard interrupt vector table entry
                     ...
                  --      ---
                  --      ---     ; enable keyboard interrupts
                     ...
                  --      ---     
                  --      ---     ; start of actual user program to print UT checkerboard
                     ...
                  --      ---
                .END
The interrupt service routine must be named interrupt_service_routine.asm and will be of the form:

                .ORIG     x2000
                  --     ---      ; the code
                     ...
                  --     ---
                 RTI
                  --     ---      ; buffer space as required
                      ...
                  --     ---
                .END

We have provided a screenshot as an example of how the console should look when you run the program.

Notes: