From: Ullrich von Bassewitz (uz_at_musoftware.de)
Date: 2002-10-22 19:02:36
On Tue, Oct 22, 2002 at 11:50:06AM -0500, Brian Bagnall wrote:
> I was curious to see the Plasma demo does not need to alter the c64.cfg file
> in order to store a character set and 2 screens of data in the upper memory
> (E000 and higher). Does this mean that CC65 allows a programmer to use the
> last 8kb of memory as he wishes? In the c64 memory map it looks like this is
> used by the kernal, but possibly that's wiped out when CC65 takes over?
It is common C64 knowledge that there is memory "below" the kernal:-)
The VIC does always access the RAM below the kernal and the I/O area (if the
16K VIC bank is at $C000). cc65 cannot use this RAM without serious problems,
because it needs the kernal for several things (IRQ handling is an example) so
using the RAM below the kernal would mean banking it in and out - something
that is slow and error prone. As a consequence, the standard C64 memory map
for cc65 does not use this RAM.
So you can it for your own purpose. Some important things to consider:
* Writing to the kernal ROM area will write "through" the ROM, so you can
write to the RAM without banking out the kernal (this is what is done in
the plasma demo).
* To read the RAM, you will have to bank it in, but since the kernal is
used for interrupt handling, you will have to disable interrupts when
doing so.
* The graphics driver will place the high res screen below the kernal ROM
and the attribute RAM below the I/O area. This means that your program
will not work with this driver. Presumably, this is not a big deal for
you.
* I cannot remember testing this explicitly, but I think I've checked the
library code for it: If you set the zeropage cells that contain the screen
address managed by the kernal, even the conio module should work with a
changed screen address. This is untested however, any feedback would be
appreciated.
Regards
Uz
--
Ullrich von Bassewitz uz_at_musoftware.de
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