Re: How does the apple2 RGB card work?

["Bryan Parkoff" <none@nospam.net>]

"mdj" <mdj.mdj@gmail.com> wrote in message 
1148954474.576052.288730@y43g2000cwc.googlegroups.com">news:1148954474.576052.288730@y43g2000cwc.googlegroups.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
>
Matt,

    I do know that background and foreground data are in aux $400-$7FF.  It 
allows 40 column to have colored text in main memory while high nibble for 
background and low nibble for foreground to set one of 16 colors in aux 
memory.
    I am not sure what I understand that you are referring $2000-$3FFF in 
aux memory for HGR (not DHGR).  Can you please explain little more?

Bryan Parkoff