Since Google seems to have eaten this post, I'll try once again.
Paul Rosenzweig wrote:
> We who own C128's have to deal with C64 emulation as well. For
No we don't. The C128 doesn't emulate a C64. It *is* a C64 when in C64
mode, albeit with a few added features.
> some reason, C64 mode can't do some C128 operations. One of these
> is accessing BANK 1. I can think of a lot of things I could use
> that memory for. Is the BANK 1 restriction specially implemented
> by the C128 designers or is it an inherent part of the C64 design?
It's because the C128's MMU that enables switching in BANK1 in C128
isn't mapped to memory in C64 mode. To make it available, the C128
designers would have had to rewrite the Kernal and map in the MMU into
a memory area where it would have broken C64 compatibility.
The Transactor has a couple of articles on adding a second (or third,
or fourth) RAM bank to the C64. I suppose it won't work on a C128
> One can access the RGB screen and fast mode
> while in C64 emulatiomn with some restrictions.
The VDC chip that provides the RGB screen is mapped into C64 mode
memory. Fast mode is inherent to the 8502 processor. And since the
always uses the "enhanced" VIC-II (mapped to the same memory space
whether in C128 or C64 mode), you can use its processor clock rate
control register ($d030) in both modes. In his account of the C128
design process, available in the Commodore Knowledge Base, Bil Herd
says that he could have made it unavailable, but he left it in because
he thought it wouldn't break compatibility.
As it turned out, writes to $d030 probably account for nearly all
instances where C64 software won't run on a C128 in C64 mode.
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||12/7/2004 7:57:01 AM