Re: "Partner 128" by Timeworks Inc.

From: Gerrit Heitsch <*gerrit_at_laosinh.s.bawue.de*> Date: Wed, 5 Oct 2022 19:15:06 +0200 Message-ID: <8e89119f-43c6-3b98-abe5-9d8355c6be4c_at_laosinh.s.bawue.de> · gerrit_at_laosinh.s.bawue.de

On 10/5/22 18:59, Marko Mäkelä wrote:
> Wed, Oct 05, 2022 at 05:41:15PM +0100, smf wrote:
>> NMI is edge triggered, IRQ is level triggered.
>>
>> If IRQ stays low then it will fire again when you RTI out of the
>> interrupt handler.
>>
>> NMI has no mask, so it only fires when it transitions from high to low &
>> there is nothing to stop another NMI from being triggered while the
>> handler is still running.
> 
> My point was that another high-to-low transition cannot occur as long as 
> something is holding the line low. The trick to "disable NMI" (including 
> the Restore key) on the Commodores is to program a PIA, VIA, or CIA to 
> pull the line low and let it remain there.

Yes, but if you have a cartridge, you can always add some logic plus 
driver to brute force NMI high for a cycle or two. That shouldn't damage 
the CIA pulling it low but lets you trigger another NMI for which you 
can then supply your own vector by brute forcing the data bus.

  Gerrit