Re: Interesting graphics hacks?

[mab22@po.CWRU.Edu (Michael A. Balfour)]

In a previous article, pDOTblokland@spcgroup.nl (Pim Blokland) says:

>Michael A. Balfour wrote:
>
>> Has anybody ever hacked around with the graphics ROM in their Apple II?
>
>What exactly do you mean by "the graphics ROM"?
>

Here's a brief description of how the Apple IIe produces everything you
see:

1)  You store bytes into video memory.  For simplicity, let's say that
$400-$800 is the video memory for both text and lo-res graphics, and
$2000-$4000 is the video memory for hi-res graphics.

2)  At regular intervals, each byte of this video memory is sent through
the IOU, which is a special processing chip.  This chip determines what
the current graphics mode is, and does special funky things to each byte
as it passes through the IOU to reach a graphics ROM.

<a brief digression follows>
As we all know, ROM stands for "read-only memory".  A graphics ROM would
therefore be graphics stored in read-only memory.  This ROM is not
directly addressable by Apple II programmers, it is only accessed by the
Apple II hardware.  The ROM contains $1000 bytes.  The first $800
contain the bit patterns used to draw text characters.  The last $800
contain the bit patterns used to draw the graphics.
<end digression>

3)  Each byte that passes out of the grapihcs ROM is loaded into a
bit-shifter.  These bits are shifted one at a time in time with the
video clock.  This is how images are formed.


My question about hacking the graphics ROM specifically refers to the
last $800 bytes.  $400 of these are used for lo-res, and $400 for
hi-res.  Changing these bytes could change the way things are drawn,
what colors are used, etc.

Mike

-- 
----------------------------------+--------------------------------
Mike Balfour, Partner             | BS/MS Graduate - ECMP
Overload Engineering              | Case Western Reserve University
"New Ideas for a Brighter Future" | Cleveland, OH