Re: DMA successes with Verilog

From: silverdr_at_wfmh.org.pl
Date: Fri, 15 Jun 2018 00:36:47 +0200
Message-Id: <8542BF2E-2674-4879-963E-681B4068E657@wfmh.org.pl>
> On 2018-06-15, at 00:27, Nejat Dilek <imruon@gmail.com> wrote:
> 
>>> Do CIA chips really keep the NMI low if they don't get an ACK? I don't
>>> think so.
>> 
>> This is a known (Marko did it first?) software technique for preventing NMIs coming e. g. off the RESTORE key on a 64 and spoiling the precise raster synchronisation.
> 
> I already know it but I'm not talking about software. I'm talking
> about hardware and my comments were to the statement "which would hold
> the NMI line down, preventing NMI from working". I shared my doubt as
> to the "line being held low" or "hardware generating nmi just waiting
> for an ACK and not generating any more nmi interrupts.". I think the
> latter is correct.

NMIs (active LO) in a 64 can be generated by the CIA and by RESTORE key. In order to prevent unwanted NMIs coming off keypresses, the CIA is "tricked" not to release the line, rendering any other NMI sources unable to trigger further NMIs.

> Since NMI to be generated by this special hardware, running software
> on the c64 can't prevent it happening. Marko Makela's finding is only
> relevant for the stock hardware.

It is valid and relevant for the stock CIA having its interrupt output connected to NMI line.

-- 
SD! - http://e4aws.silverdr.com/
Received on 2018-06-15 01:01:38

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