Lab #3: Writing and Debugging ISRs
This lab must completed by each individual in the class. For this
lab you will write the necessary code to allow a program running on
the simulator to respond to interrupts. This requires writing code in
both assembly and C for each individual interrupt that must be dealt
with. (For each interrupt, the assembly code will run first, saving the
context, and then calling the associated C function to respond to the
You are given a rather long-running program that we will consider to
be the background task. While this program is running on the
simulator, your ISRs must respond to any interrupts that occur,
perform the specified actions, and (in cases where the specified
actions do not include termination) return control to the main program
without affecting its operation in any way (other than slowing
it a bit).
With the exception of timer ticks, which the simulator generates
automatically in its normal operation, the interrupts you must deal
with are caused by pressing keys on the keyboard. Three specific
interrupts may be generated in this way:
- RESET, caused by pressing the 'reset' key (currently Ctrl-R)
- TICK, caused by pressing the 'tick' key (currently Ctrl-T, also generated automatically)
- KEYPRESS, caused by pressing any other key
You will know when your ISRs work when you can press an arbitrarily
long sequence of keys in rapid succession, generate the correct output
for each in your interrupt handlers, and not have the program hang or
crash. Because of the way the interrupt handlers are defined, this
will very likely ensure that your ISRs also handle interrupt nesting
Read the document The 8086 Interrupt Mechanism
before writing any code for this lab. A solid understanding of the interrupt mechanism can save you
from a lot of unnecessary debugging in this lab and especially in future labs.
The background task is defined by the code in the file primes.c. This
program computes and displays all primes in a particular range, but it
takes quite a bit of time in the process. You are free to look
through the source code, but you do not have to concern yourself with
any of its implementation details since it does not share data with
You are to write ISRs for each of the following interrupts:
|| Generated manually by
|| Any other key
Here is the functionality required for each ISR or interrupt handler:
Here is an example of output with functional ISRs:
- Reset. Program execution should cease. The handler should call
exit(0). This terminates the program, not the
This interrupt occurs only when the reset key (Ctrl-R) is pressed.
- Tick. The handler should increment a tick counter and display "TICK n", where
n is the total number of ticks, and then return. To keep
output consistent, make the output for this interrupt occupy a line by
Normally, this interrupt occurs automatically every 10,000
instructions when the simulator runs a program, but the frequency can
be changed with the "t n" command, where n is the number
of instructions between ticks. If n is 0 ("t 0"), then no
automatic timer ticks are generated. This tick can also be generated manually by
- Keyboard. For the press of any key other than 'd', the handler
should display "KEYPRESS (key) IGNORED" (where key is
the key that was pressed) and then return. The simulator will store
the value of the key that was pressed in the variable KeyBuffer. You
can access this variable (defined in clib.s) by including the line
extern int KeyBuffer;
near the top of your C file. For consistency,
the output should occupy a line by itself.
When the key pressed is a 'd', the handler should (1) display "DELAY
KEY PRESSED" on a line by itself, (2) increment some local variable in
a loop repeated 5000 times (just to cause some delay), then (3) output
"DELAY COMPLETE" on a line by itself. This will ensure that this
lowest priority interrupt will be interrupted by the tick interrupt
(generating its own output between the output lines of this ISR).
Your code must handle nested interrupts correctly.
This interrupt occurs whenever any keypress other than Ctrl-R or Ctrl-T
occurs, ignoring codes such as Ctrl-C, which is caught by the shell on
the underlying machine and causes the OS to terminate the simulator, and Ctrl-B,
which is caught by the simulator causing a manual program break.
Some of the tick interrupts were generated automatically and others
were generated manually. Obviously the keys '8', 'k', 'j', and 'd'
were also pressed along the way. The special case handling for the
'd' key confirms that nested interrupts work. The interrupt handler (C
code) for the 'd' keypress is running from the time that "DELAY KEY
PRESSED" is output until "DELAY COMPLETE" is output. You will notice
that it runs long enough to be interrupted twice by the timer tick.
Note that every time an interrupt handler runs, control is returned to
what was running before (either another interrupt handler or the
background task) which simply resumes where it left off. This is how
you can tell if your interrupt routines are working correctly.
2467 2473 2477 2503
2521 2531 2539
2543 2549 2551
2557 2579 2591 2593 2609 2617 2621 2633 2647
KEYPRESS (8) IGNORED
2659 2663 2671 2677 2683 2687 2689
KEYPRESS (k) IGNORED
2693 2699 2707
2711 2713 2719 2729 2731 2741 2749 2753
KEYPRESS (j) IGNORED
2789 2791 2797 2801 2803 2809 2819 2833 2837 2843
2851 2857 2861
2879 2887 2897 2903 2909 2917 2927
2939 2953 2957
DELAY KEY PRESSED
2963 2969 2971
In stress testing your code, the TA may revise the tick frequency and
press any sequence of keys. Your code must support a tick frequency
up to every 500 instructions, and it must work correctly when a key is
held down or when any key sequence is typed in rapid succession. Well
designed code will handle any sequence of interrupts. Your code
must support nested interrupts correctly.
After verifying that your code works, the TA will examine your ISR
code (assembly), your interrupt handler code (C), and your makefile. You
are required to use a makefile for this lab.
You must also do the reading about the 8086 interrupt mechanism.
To pass off this lab, you must have done the reading about the interrupt
mechanism. Then, you must demonstrate to a TA the correct
operation of your ISRs. Since you must demonstrate working code to
a TA, please consider their lab schedule well in advance.
Important Notes, Hints, and Recommendations
Where do I start?
- First, be sure you've read about and thoroughly understand the material in The 8086 Interrupt Mechanism, especially the section on ISR actions. You will have a much easier time in
the next couple of labs if you understand the sequence of events that
takes place when an interrupt occurs. For this lab, you do not need to perform
the ISR steps of calling OS functions on entry and exit since we don't
have an OS to call yet.
- Look at the library header file clib.h to see what
library functions are
available to you. You will need them for string output and program termination
in your interrupt handlers.
- Remember that the interrupt vector table needs to be initialized
with the beginning address of each ISR (the routine written in
assembly). This table is actually located in the clib.s file which you
must modify. Use the "dd label" directive to set
the contents of an entry to the address of label, where
label should be the entry point to your ISR routine. Using
"dd" will create a doubleword (32 bits), setting the
segment for the entry to 0 and the offset to label. You can get
a fresh copy of clib.s by clicking here: clib.s.
- You may want to write the reset ISR first. Since you never return
from this ISR, it doesn't have to be tidy about saving context, so it
isn't a bad place to start. You don't technically need all the
pieces, but it is recommended that you write an ISR in assembly and a
simple handler in C. Make sure you update the interrupt vector
table with the address of your ISR before testing it. (If you test it
before writing the other ISRs, you must ensure that no tick or
keyboard interrupts occur while the task executes; until you install
meaningful ISRs for the other interrupts, the system will respond to
those interrupts in ways that can mess up the processing of all
interrupts. You can stop the system from generating tick interrupts in
Emu86 by typing "t 0". You'll avoid generating keyboard
interrupts if you don't press other keys. For the curious, this was
not a problem in the first two labs only because we didn't press keys
during execution and the code didn't run long enough to have an
automatically generated timer tick.)
- The remaining ISRs will be a bit more involved since they must
save and restore context as well as inform the PIC when the ISR is
complete. Save the context by pushing registers ax, bx, cx, dx, si,
di, bp, es, and ds onto the stack. DO NOT push or pop sp or ss
since this is unnecessary and "push sp" behaves differently on some
8086 clones. It is also unnecessary to save ip, cs, or the flags
since these are saved automatically by the CPU when the interrupt
occurs. Restore the context by popping from the stack (in reverse
order) the registers you previously pushed. Always use iret at the end
an ISR to restore execution.
How do I organize my files?
- For this lab you should probably have one .s file with all of your
ISR code and one .c file with all of your interrupt handling code.
Please note that these files are separate from and in addition to
the primes.c application code and the clib.s file. There should be
no main() in either of the new files you create since the main()
routine is found in primes.c and that is where execution will begin.
- Don't forget that clib.s has to be listed first when the files are
assembled so that the vector table gets placed at address zero.
- The details of preprocessing, compiling, concatenating, and assembling files
to create a single .bin file should be thought
about carefully when creating your makefile. You can review the document on
Building Programs with the Tools for a discussion on makefiles.
Be careful not to use the same
filename for different purposes in the makefile. Once the makefile is complete, you
shouldn't have to worry about it again for this lab. Trying to do this lab or
any future labs without makefiles is not something you should
Here is a sample Makefile that can serve as a
starting point. It assumes that your ISR code (assembly) is in a file
named myisr.s, that your interrupt handler code (C) is in a file named
myinth.c, and that you want your output file to be named lab3.bin. These names can
be changed easily if you wish to name your files differently.
- Although you do not need to explicitly modify the PIC's registers for this lab or any future labs,
being aware of the IRR and ISR register contents can be very helpful in debugging. You can view the
contents of these registers using the 'r' command in Emu86.
- To help debug your tick ISR, it is sometimes helpful to turn off the automatic generation of ticks and use only manually generated
- Note that the interrupt behavior is defined only when pressing Ctrl-R, Ctrl-T, and standard alpha-numeric keys found on any keyboard.
However, the function keys on keyboards and other key sequences can generate a
series of multiple interrupts, as though multiple keys had been pressed.
Stick to standard keys to get the expected behavior.
Last updated 27 September 2017