Re: Kernel uses only half of Mac IIci memory with built-in video
On Sun, 14 Mar 2021, Brad Boyer wrote:
> Perhaps we could find some other use for the rest of the RAM in that
> bank (a IIsi has a fixed 1MB there, but it was possible to put a lot
> more than that in bank A on a IIci).
I didn't know that the IIsi limited video RAM to 1 MB. It's unsurprising
though, considering the drawbacks of RAM based video.
The Guide to Macintosh Family Hardware says, regarding the IIci,
The built-in video circuits use a screen buffer in RAM bank A. The
effect of the video RAM cycles can decrease the processor's access to
bank A by as little as 6% or as much as 65%, depending on the type of
video display in use: larger displays and display modes with more bits
per pixel have a greater effect.
Back in the day, IIci owners would install a NuBus video board to avoid
the performance penalty. IIci owners without a Nubus video board would
avoid putting expensive RAM in bank A.
Because Linux is so demanding, I'd expect IIci users would upgrade both
RAM and video hardware. For Linux on the IIsi, the amount of lost bank A
RAM is neglible -- probably only hundreds of KiB.
So for RBV systems with on-board video disabled, we just need Penguin to
do the right thing -- the RAM from both banks can be made available to the
kernel and macfb should use the Nubus card. I may be missing something but
I don't see why Linux wouldn't work as is.