I appreciate alot your comments... They are heart-warming.
Here's the complete source for those who are interested (PPMCK). It's a bit old...
Heh not, it's not my own music engine but only that PPMCK that nobody likes... So the nice sounding portamento is nothing more than a carefully calculated pitch macro that sweeps the square channels in a exact number of frames... For the bass you'll never know how long it was to make those samples, to find where it loops better and where to put the release part -- yes, you read it right, those samples have all information for a complete ADSR, well, you can play and sustain and then release...
To achieve this I used the 'y' command in PPMCK that allow to poke values where you want, so I manually changed the playback settings soon after the beginning of the DMC and to make that bass note release, I change again playback setting and I clear the loop bit in $4010. Needless to say, that's pretty inefficient (and PPMCK itself is pretty inefficient).
Bass samples are pretty big (~2K-3K) and there are 10 of them in the nsf (of which few of them are used), occupying almost the whole 32KB.