Cool AppleSoft BASIC trick I never thought of before

From: Derek Peschel <dpeschel_at_eskimo.com>
Date: Thu May 18 19:58:36 2000

Eric Smith wrote:
> Sellam wrote:
> > The Apple ][ has an address that you can monitor that tells you when it's
> > in the vertical blanking interval.
>
> Must be something they added for the //e, the ][ and ][+ certainly don't
> have that.

No they don't, but they do have a way to achieve a similar effect. (Sellam,
you might remember that we talked about this when I visted your house last
September.)

Because of the way the bus is constructed, you can read some address or
other and get the current byte that's being sent to the screen. By itself,
that won't give you the current screen location, but if you use a series of
marker bytes in a certain place (and make sure they ONLY appear in that
place), you can get the screen location. You may be able to store the
marker bytes in the "holes" in the screen memory map (that don't correspond
to any pixels).

This trick was described in the magazine _Computist_. There are one or two
"Best of Computist" books but they are out of print (I passed up the
opportunity to buy one). However, the magazines themselves have now been
scanned. Sorry, I don't have the URL.

Look for "Vapor-Lock". I think the name was created like this: Your
routine locks onto a certain part of the screen, but it uses fake, or
vaporous, information about how to do it. The fact that "Vapor-Lock" is
also a brand of plastic bag is merely coincidental. :)

-- Derek
Received on Thu May 18 2000 - 19:58:36 BST