Re: Apple video question?

[Roger Johnstone <rojaws@es.co.nz>]

In <a2c2f792.0302202102.a58029f@posting.google.com> Jon Co wrote:

> 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?

It's because of the mouse, of all things. The Apple mouse card has it's 
own processor which keeps track of the mouse movement. The card can be 
set to generate interrupts on movement, button click, or during the 
video vertical blanking (VBL).

The II, II+ and IIe have no interrupt source at all without extra 
hardware like a mouse card. Since the Apple IIc has a mouse port it also 
has the extra hardware to generate the interrupts needed.

-- 
Roger Johnstone, Invercargill, New Zealand

Apple II - FutureCop:LAPD - iMac Game Wizard
http://homepages.ihug.co.nz/~rojaws/
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