Re: How does the apple2 RGB card work?

["mdj" <mdj.mdj@gmail.com>]

Bryan Parkoff wrote:

>     It is undocumented.  The problem is that monochrome needs more data
> because 7th bit is not available to reset/set monochrome like DHGR.  Seventh
> bit is used to switch violet/green color and blue/orange.  The only way is
> to put monochrome data in the screen hole.  It needs 960 bytes, but screen
> hole has only 512 bytes available.  The only way is to store 480 bytes
> leaving free 32 bytes in the screen hole.  It has 2 1/2 bytes or 20 bits
> each horizontial lines.  It gives monochrome in each a group of two bytes.
> It is 20 group of two bytes.  It is great memory saving.  I may be able to
> demo at later time.

It's undocumented, but relatively simple to explain:

The display is treated as 280x192 monochrome field just like composite
monochrome HR, except that 1 is treated as a 'foreground' colour and 0
treated as a 'background' colour. the high order bit is insignificant.

The corresponding byte in the auxiliary bank contains the
foreground/background colours for each byte. For example, setting each
byte in the $2000-$3FFF range in auxiliary memory to $F0 would yield an
'emulation' of monochrome hires, but you can obviously use other
colours. It's a bit restricted since each set of 7 pixels must have the
same foreground/background colours, but some interesting effects can be
achieved.

Note that this technique can be enabled in 40-column text mode too,
providing a nice 16 colour foreground/background text mode. A very
seldom used feature!

Fairly obviouisly, the RGB adapter actually has the Apple IIe in
doublehires mode while producing this mode, and likewise has it in 80
col mode while producing colour text mode

Matt