Can anyone explain this PET command?

[necronom]

I got a PET 4032 about a month ago (used these at school and have always wanted one), and I've noticed that a few programs make the screen larger vertically. I'd just re-sized the screen with the pot on the screen's board, then I saw this happen with some software (luckily, it's only the corners of the corner characters that are slightly off the screen when expanded like this), but I wondered what was going on.

I've just found a game that does this and found the command:
PRINT "NH"
The N and H are in reversed video. Usually these are created by opening quotes then doing a control character, but I wasn't sure what creates those, so just types a reversed NH, then put the print around it and the screen flickered and went bigger.

What exactly is this? Is it a documented feature, a hack, something related to the killer poke, an equivalent to a normal poke command?
Is it safe to use?
Is it something that was meant for a different model PET?

If there is a PET forum somewhere that anyone knows of, then please direct me to it.

[tokra]

This is what I found in the PET-FAQ:

http://www.zimmers.net/anonftp/pub/cbm/ ... 20FAQ.html

"The 12" 4000/8000 series PETs allow you to change case by printing
a control character: CHR$(14) - Text Mode CHR$(142)-Graphics Mode

When you issue a CHR$(14) on a 4000/800 series PET the newer display
controller will be adjusted so there is a pixel or two gap between
screen lines. If you do not wish this gap in text mode just
POKE 59468,14 instead of printing CHR$(14)
(if you want the gap in character mode you can issue a ? CHR$(14)
and then POKE 59468,12 to produce the desired effect.)"

The gap ist intentional so text-mode is more readable. Effectively a reverse N is the same as CHR$(14). Not sure what the CHR$(8) (reverse H) does. On the VIC-20 and C64 it disables C=/Shift to switch between charsets, but the PETs do not have a C= key.

[necronom]

Thanks for the reply. I've been away, so I've just seen this now.

I've been using 59468,12 or 14, but didn't know about this special text mode. I'll have a look at the link and the other info in the FAQ.