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 "Texas checkerboard" by alternately outputing two different lines. One line will consist of the pattern "Texas" followed by seven spaces four times. The second line will consist of the pattern seven spaces followed by "Texas" three times.
Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas Texas TexasTo 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 examine the key typed to see if it is a capital letter from the English alphabet (A,B,...Z). If it is not, the service routine will simply terminate with an RTI. If it is, the interrupt service routine will, starting on a new line on the screen, print the capital letter typed ten times, followed by the lower case version of that letter ten times. For example, if someone typed a G, the service routine would print to the screen:
GGGGGGGGGGggggggggggThe service routine would then print a line feed (0xA) to the screen, and finally terminate with an RTI. Your interrupt service routine should start at the location x1500.
VERY IMPORTANT instructions for constructing your interrupt service routine:
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
Texas 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 will be to write both the user program augmented with the
interrupt enabling code described above and the keyboard interrupt
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 Texas checkerboard ... -- --- .ENDThe interrupt service routine must be named interrupt_service_routine.asm and will be of the form:
.ORIG x1500 -- --- ; 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.