Re: DMA successes with Verilog

On Thu, Jun 14, 2018 at 8:02 AM, Jim Brain <brain@jbrain.com> wrote:
> On 6/13/2018 3:47 PM, smf wrote:
>>
>> On 13/06/2018 18:53, Jim Brain wrote:
>>>
>>>
>>
>> I would have thought that there would be a safe point if you trigger an
>> nmi and switch to ultimax mode when it fetches the vector, have the ultimax
>> nmi handler just return and switch out of ultimax mode.
>
> If you can guarantee the NMI will occur (I think Marko showed that you can
> trigger it and not ack an NMI, which would hold the NMI line down,
> preventing NMI from working), then your idea would work.
>

This is wrong, this is just the behaviour of the chipset in the C64.
And I don't think NMI line is held low that way because NMI is edge
triggered unlike IRQ which is level triggered.
It's up to the hardware to care or not care for any acknowledgement
from the cpu. NMI line to work, it's just a matter of triggering that
line high to low and back to high. Yes CIAs care for the ACK but other
hardware may not.

Do CIA chips really keep the NMI low if they don't get an ACK? I don't
think so.

Regards,

Nejat