Re: Kernel uses only half of Mac IIci memory with built-in video
On Mon, Mar 15, 2021 at 08:28:37AM +1300, Michael Schmitz wrote:
> I'm pretty sure this has never worked. Though there ought to be a way to
> make use of RAM in bank 0 for video as long as the kernel is loaded at
> address 0 in that bank. Or if that's not possible, use the same trick as I
> do on Atari - ioremap some of the bank 0 memory for use by video, and make
> sure the video driver uses a dedicated low memory pool allocator for that
> ioremapped RAM.
OK, I wasn't sure if it used to work. I own both a IIci and IIsi, but I
never got Linux up and running on either. I bought both of them just
about when I stopped having as much time to work on such things.
I believe one of the headaches is that the address is non-programmable
and requires the framebuffer not just to be in bank A, but to start at
address 0 which means we can't put the kernel at address 0. So we
instead put it at the beginning of bank B if RBV video is going to be
used. The RBV chip doesn't have a way to specify the buffer address
(the official Apple documentation is quite clear that RBV has no
control over the framebuffer location). The other chip that does that
part is also the main memory mapping logic for all onboard devices and
has a completely static mapping for this sort of thing as far as I know.
The documentation I've found never explicitly says the address 0 is
hard-coded, but it never says it can be changed either.
A second headache is that we don't have real video drivers for anything
on Mac (other than valkyrie which is only found on one or two very late
m68k models and is a shared driver with some ppc models). The macfb
driver mostly just takes information passed in by Penguin and uses
that. It only has enough logic beyond that to change the color palettes
on some of the more common hardware.
On the Atari, is the rest of the low memory usable if the kernel isn't
there? I seem to recall it is only accessible by that special allocator
in that case, not as generic memory. I'm not sure of anything else we
have on a Mac that would be able to use that sort of thing.