Student "edition" found at {csi dot journalspace dot com}.

Maybe I shouldn't have started this blog now, not with everything that's been going on.

For the first meeting of my Integrated Computer Systems class for the thirteenth week of the second term, they continued with their last experiment.

One group, in fact, thought that they had finished. The external subtracting circuit was working (which means that from the sample input they gave, they got the correct results displayed), and the data was being passed correctly through the parallel port. But they weren’t sending the two four bit numbers that were to be subtracted to the parallel port correctly.

One student thought at first that it was as easy as adding the two numbers then sending it to the data address, but it was not. Adding made the two values indistinct.

So he ended up wanting to work on that the long way, extracting each bit then placing it in a separate variable, when I hinted to them that there was a shorter way.

By providing four examples using small numbers (including zero) they were able to arrive for themselves that to obtain the composite number.

All they had to do was multiply the number to be placed in the higher bits by sixteen and add it to the other number. This way, the external circuit could still extract each four bit number separately.

Besides that, another problem they were encountering was with the inversion of one of their outputs as they passed the result from the external circuit to the status pins of the parallel port.

They had already made this work in the previous experiment, but it was a bit (pun intended) complicated to remember which pins had to be inverted all the time. It might have been easier to just let the software convert rather than always adding inverters to their external circuit, since all they had to do then was copy a few lines of code to each program they made that required reading four bits from the status byte through the parallel port.

Hopefully everything will be finished by the last (and I do mean last) meeting.

Session 897 received wrong value through the parallel port. Class dismissed.