Re: Does anyone understand this...

[beejay@Micor.OCUnix.on.ca (Basil Johnson)]

gdesroch@slate.mines.colorado.edu (DESROCHERS GARY FREDERIC) writes:

> 
>      I need to know what a 
> 
> BIT $CFFF
> 
> does.  I don't need breif answers like 'It clears all I/O ROMS'
> or 'It switches out C8 ROMS.".  I can get these out of the IIc rom listing
> and II plus listing that I have.  What does it do?!?  The best answer would
> be as technical as you can give me or where I can get the answers.
> 
>      I appreciate anything you can give.
> 
> Gary
> gdesroch@slate.mines.colorado.edu
>   
> -- 
> Gary F. Desrochers
> gdesroch@slate.mines.colorado.edu

 
Page 84 of the Apple ][ reference manual says in part:
 
A peripheral card's 256-byte PROM can gain sole access to the expansion 
ROM space by referring to location $CFFF in its initialization 
subroutine.  The location is a special location, and all peripheral 
cards should recognize it as a signal to turn their flip-flops "off" 
and disable their expansion ROMs.  Of course, this will also disable 
the expansion ROM of the card which is trying to grab space, but the 
ROM will be enabled again when the microprocessor gets another 
instruction from the 256-byte driving PROM.  Now the expansion 
ROM is enabled, and its space is clear.  The driving subroutines can 
then jump directly into the programs in the ROM, where they can 
enjoy the 2K of unobstructed, absolutely located memory space. 
 
 
Basil Johnson           UUCP: micor!beejay        |Oh, Fate! ...
Nepean, Ontario         InterNet:                 |Such a stupid thing:
CANADA  K2B 8E9         beejay@micor.ocunix.on.ca |Command a bird to fly
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