[Coco] Re: Mocha & OS9

[Brad Grier]

Robert Gault wrote:

> Brad Grier wrote:
> 
>> I'm trying to get OS9 working in Mocha but I have a bug in the disk 
>> emulation. It *appears* the NMI interrupt is firing when it shouldn't 
>> be. My docs on the FDC are pretty sketchy - is there a flag in one of 
>> the registers that turns off the NMI?
>>
>>
>> Thanks again,
>>
>> Brad
>>
>>
> 
> Hmmm, by definition non maskable means it won't turn off. There is a 
> Coco flag at $982 which is used by the NMI routine. If set the routine 
> rti's to the address at $983, else it just does a simple rti.
> 
> The 1793 and 1773 always generate interrupts on the completion of a 
> task. Controllers for the Coco feed that interrupt to the NMI line. 
> There is no switch for this connection.
> 
> 

I downloaded the MESS source (I should have done this earlier). There's 
a comment in cococart.c that bit 5 of the control register at $ff40 not 
only controls floppy density but is an "NMI enable flag".

I made the change and things progress further - there's still a problem 
somewhere though.

Brad