Re: C64 MMU POC

On Mon, Sep 30, 2019 at 11:08 PM Jim Brain <brain_at_jbrain.com> wrote:
>
> Special timed writes/reads are fine to unlock the register space, but it
> would not make sense to set all of the registers in this MMU via that
> mechanism, due to the number of addresses to change and the frequency of
> changing them.
>

You could have a state machine where you only react to command writes
when the state machine is in unlocked state otherwise ignore them.
After the unlocking, calling program may start issuing commands to the
registers of the hardware. You could have a control register to
provide shut off mechanism for the calling program too when in
unlocked state.

By the way you don't have to deal with bit transfers to use such a
unlocking mechanism, in my case I was constrained to 1 bit input so I
designed a bit of time taking procedure for the unlocking mechanism.
If you can capture full byte writes then the whole process could
complete in just 30-40 cycles too. Of course then you need a litle bit
of memory buffer in the hardware. Say, getting 8 consecutive writes of
specific data like "TURN ON!" to a specified address.

Regards,

Nejat