Re: Apple video question?

[mjmahon@aol.com (Michael J. Mahon)]

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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