Re: 65816 and I/O

From: Marko Mäkelä (
Date: 2001-01-22 17:36:09

On Mon, 22 Jan 2001, MagerValp wrote:

> RB> Thanks for all who answered. Reading the answers I have the
> RB> feeling that my question is misunderstood. Maybe I can ask it in
> RB> this way: How does the Super-CPU manipulate the I/O registers of
> RB> the original 6510 at the addresses $0000 and $0001?
> Dude. Yes, how the hell does it do that? :)

Does it need to do that?  JiffyDOS doesn't support the tape drive.  So,
the only reason for accessing $0000 and $0001 is altering the status of
the CHAREN, LORAM and HIRAM lines.  This can be achieved (to some extent)
with the -EXROM and -GAME lines on the cartridge port.

If the SuperCPU really boots up the 6510 before halting it and giving all
control to the 65816, it can write something to $01 that allows the
SuperCPU to control the memory configurations.  I think that the only
thing SuperCPU needs to control is whether writes to $d000 go to RAM or to
the I/O area.  The SuperCPU doesn't need to read anything from the C64 RAM
or ROM; it only needs to read from the I/O space.

[Oh, now I see that Nicolas Welte wrote basically the same answer.]

BTW, I managed to sort out most of the /pub/cbm/incoming.old files on  The remaining files are for the PET and a collection of
plus/4 games in disk images.  The new uploads (since about 1998) are
roughly 8 megabytes, half of which is for the c64.


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