Re: Information on the //e VBL

[jon@i-manila.com (Jon Co)]

I got a follow up to this.  I just installed a //e mouse card in slot
4.  Would it now be possible for my //e to generate vbl interrupts the
way the //c does?  what software uses this on the //c so i can verify
its functionality on my //e + mousecard.
"Ole Voss" <ole.voss@netrogue.de> wrote in message news:<b583g8$27ffop$1@ID-118921.news.dfncis.de>...
> I just thought this subject should get a thread on its own, seeing that the
> Contiki thread is getting difficult to read ;-)
> 
> Well, back in the old days I used the VBL quite extensively to get my
> scrolling routines smooth. This did, ofcourse, make them extremely slow -
> but they were elegant all the same. The programming is quite easy:
> $C019 holds the VBL bit (bit 7) When this is low (<128) the Vertical Blank
> is active (beam blacker than black returning to home position top-left). As
> soon as the VBL goes high, the screen is being drawn. The problem with using
> this as a technique for timing is obvious. First, you never know where the
> actual VBL is taking place. That is, it could be anywhere within its blank
> period (I don't recall how long that is - but maybe somebody would like to
> look that up) or anywhere within its normal 'drawing' period. Back then I
> used to poll the VBL and wait for a definite change between high and low.
> Only after the change had occurred, would I actually change screen data.
> Ofcourse this meant that I would often skip an entire cycle.
> 
> Will this help anybody?
> 
> 
> Cheers,
> 
> Ole.