RE: Stashing data at $ff00-$ff04 in 128 mode?

From: Ray Bryan (
Date: 2001-07-26 00:36:44

I think from my reading of the docs that wrtiing to the $ff00 to $ff04 address
would write to registers in the MMU -- and 0FF00 - 0ffffh are always used byt he 
8502 even in cp/m mode.  (8502 is the out/in put processor)

On Wed, Jul 25, 2001, at 11:07 AM, Nate Dannenberg wrote:

> > Hello, CBM-Hackers;
> >
> > I sent this quite some time ago, and now I'm reposting here.
> >
> > Hello, CBM-Hackers;
> >
> > I was wondering if there is any way a program can stash ordinary data
> onto
> > addresses $ff00-$ff04 in 128 mode of the c128 line of computers? I want
> to
> > put data there and have the VIC-II reference it in displaying a bitmap,
> for
> > example. But, the MMU registers are always there? Is there a way of
> blocking
> According to the C128's reference guide, you're stuck - $FF00 is always
> occupied by the MMU.  A simple test I just did shows that the MMU only
> occupies $FF00-$FF04; the rest of the space around it is RAM/ROM,
> thankfully.
> So as a workaround for the affected cell (row 24 column 32 if I'm right),
> set
> the background color of the cell as you normally would, and set the
> foreground
> color of that cell to the same value, to render the MMU's register data
> invisible.  Now place a sprite over that cell and set it's color(s) to the
> actual foreground color(s) you wanted to use in that cell.  Then just make
> sure you plot to the sprite when you need to plot to the blocked cell.
> Make sure the sprite-to-background priority for the sprite you use is set
> to
> place the sprite on top of the (invisible) data instead of under it.
> You may wish to set up a raster split if you need all 8 sprites elsewhere,
> or
> if they need to be multicolor and your colors don't match what the blocked
> cell needs.
> > Can I enter Z80 mode and then stash data there at $ff00-$ff04, or does
> the
> > Z80 use the MMU as well?
> The MMU is what decides which processor is running, so I would guess its
> registers also show up in the Z80's memory map, but I'm not certain at all.

|Raymond C. Bryan  651-642-9890 vox | The battle is sometimes |
|Raymond Computer  651-642-9891 fax | to the small for        |              
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