Thursday, November 26, 2009


One of the biggest challenges in ECE 344 is troubleshooting on the system.

I mean walking through typing stp, then rrd when you want to read the registers.

In comes typical troubleshooting without a debugger, normally you stick some print statements in there and wait for the print fest to begin.

This bothered me because we can't really do this in our architecture very easy. The nice thing is that we learned about sub routines, and you can treat sub routines as functions if you do it right, so what did I do?

I wrote a sub routine that will print the register values at the time you branch and link, the main limitation, is that interrupts can change your register values once you've entered into the subroutine. This is where the mailbox architecture comes in, if you design your system so that the interrupts do as little as possible and leave everything as untouched as possible, you can then go into your print subroutine and it will print the register's last value.

There are probably plenty of other uses you can find for this routine, and maybe even better ways to use it, for example when you enter this subroutine it waits for a key press before you can exit. Once that happens you can return from your branch and link, this is great because now you don't just blow through your whole program before even really seeing anything.

This is untested code as I am unable to make it to the school until Monday at the earliest, but if you want to give it a try I'll link to it at the bottom.

Changes you'll want to make to allow this to integrate with your program is finding your location in memory you can stick the long ascii string I've built.

Additionally you'll need a 32 bit to ascii converting branch and link, which i haven't written, but we have written for our latest program. This will convert the actual register values to a printable value, as well as you'll need a push and pop routine at the beginning and end, with the pointer jumping by 40 each time, if you don't have this you'll have to find a way to get the data from the register's instead of memory as I'm doing.

As I have time I'll upload more files and a more complete solution but this is very far along and should only require small amounts of modification.

NOTE: As with any file you download from google documents, unfortunately just directly downloading it will have format issues. Copy and paste the contents to a new file and save as a .s file, otherwise you'll be chasing your tail trying to find problems that really don't exist.

Check out the code here: Print Function