Re: C64 MMU POC

On Mon, Jul 08, 2019 at 03:44:21PM +0200, Mia Magnusson wrote:
>I don't know of any C64 software that won't run on a C128. My 
>conclusion is that it should be safe to map registers in the I/O areas 
>that differs between a real C64 and a C128 in C64 mode.

I remember one game (possibly it was Monty Mole) that would initialize 
the VIC-IIe to a mode where the picture is not visible, by copying a 
table from RAM to $d0xx, past the end of the last documented register. I 
would assume that any major piece of software that was released after 
the C128 became widely available would be free from such problems.  OK, 
maybe some small publisher may have skipped some compatibility testing.

It could be advisable to have a mode where the MMU hides the registers 
until the next hardware reset, or some "port knocking sequence" that 
would activate it again. The "knocking" technique was used on the C65, 
whose hardware would actually start up in C64 mode.

	Marko