Re: Apple video question?

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

I've heard some software clocks were driven by the //c's VBL.  they'd
increment the second every 60 interrupts.  What's in the //c and
missing from the //e that allowed it to have VBL interrupts?
mjmahon@aol.com (Michael J. Mahon) wrote in message news:<20030220175904.22973.00001117@mb-bd.aol.com>...
> In article <9307085f.0302200832.53ba0ee5@posting.google.com>,
> steve@dosius.zzn.com (Dosius) writes:
> 
> >Holger Picker <pickerh@uni-muenster.de> wrote in message
> >news:<Pine.A41.4.44.0302201035540.124000-100000@zivunix.uni-muenster.de>...
> >> On Thu, 20 Feb 2003, Dan Ritter wrote:
> >> 
> >> > Date: Thu, 20 Feb 2003 09:05:20 GMT
> >> > From: Dan Ritter <dritter@angelfire.com>
> >> > Newsgroups: comp.sys.apple2
> >> > Subject: Apple video question?
> >> >
> >> > I was wondering how the Apple2 generated video?  Is it all software
>  driven?
> >> > Because I remember playing some really good games on my old Apple.  The
> >> > video seemed very fast for software generated.
> >> >
> >> Hello,
> >> 
> >> yes, the graphic is all software driven. There is no hardware support for
> >> the programmer: no sprites (as found on the C64), hardware acceleration
> >> (like blitter), no raster interrupt etc. 
> >
> >(But the //e had something like the VBL - although AFAIK it worked
> >only on the //e and not even on the //c.  It was fake, however - there
> >was no real VBL. ;))
> 
> <snip>
> 
> Yes, there was.  VBL is generated at the start of the vertical blanking
> interval, so that programs can synchronize with the video refresh to
> minimize flickering.  With single hi-res pages, this can be done by
> doing all the updates during the blanking interval (if they'll fit), or with
> double-buffered hi-res page switching, by flipping pages during vertical
> blanking.
> 
> On the //e, VBL is a level that must be polled, while on the //c it is
> a selectable source of interrupts.
> 
> -michael
> 
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