Re: Apple IIGS bank latching

[Daniel Kruszyna <dan@krue.net>]

Marc S Ressl <mressl@macgui.com> wrote:
> * The New Video "memory linearization" appears to not scramble the contents
> of video memory (I'm still having a hard time understanding this, as I
> believed it would).

When linearization is enabled, e1/2000..9fff is remapped.

x = linear_address - $2000
y = x[15]:x[0]:x[14..1]
physical_address = y + $2000

in other words:

linear $2000 maps to physical $2000
linear $2001 maps to physical $6000

$2002 -> $2001
$2003 -> $6001

the pattern continues...

It just so happens that the mapping between main/aux bank and physical
chips on the board is swapped for addresses $6000..$9fff. These two
hacks together let the VGC access two contiguous bytes of SHR data
simultaneously.

-- Daniel