From: Marko Mäkelä (marko.makela_at_hut.fi)
Date: 2004-08-20 08:41:39
On Fri, Aug 20, 2004 at 07:35:18AM +0200, Baltissen, GJPAA (Ruud) wrote:
> But IMHO it is the EXROM line that tells the C64 to start the module.
The C128 boot-up procedure checks twice for the C= key and the GAME and EXROM
lines. The first check is in the Z80 BIOS, and the second one is in the
C128 KERNAL. If -GAME or -EXROM is asserted or the C= key is pressed, the
system will switch to C64 mode. For more details, see my writing
"Software-switchable KERNAL on the C128" at
http://www.funet.fi/pub/cbm/documents/projects/memory/c128/kernal-switch.html
The cartridge could do its own address decoding instead of relying on -ROML
or -ROMH, but I can't see how it can be activated in the address space without
asserting either -GAME or -EXROM. One way to avoid the automatic switch to
64 mode would be to wait long enough after power-up before asserting -GAME
and thus switching to the UltiMax configuration on the C64.
Marko
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