Date   : Fri, 07 Aug 2009 12:02:01 +0100
From   : michael.firth@... (michael.firth@...)
Subject: Acknowledging 1770 NMI - was Re: BBC B+

> -----Original Message-----
> From: 
> bbc-micro-bounces+michael.firth=bt.com@... 
> [mailto:bbc-micro-bounces+michael.firth=bt.com@...
> .uk] On Behalf Of Rick Murray
> Sent: 07 August 2009 00:45
> To: BBC MailList
> Subject: Re: [BBC-Micro] Acknowledging 1770 NMI - was Re: BBC 
> B+ Econetweirdness
> 
> Michael Firth wrote:
> 
> > The question now is whether anyone can point me in the 
> direction of the 
> > minimum code needed to clear / acknowledge an interrupt 
> from the disk
>  > controller so that it won't hold the NMI line active on power on.
> 
> According to a text copy of the datasheet [*]:
>    The 177x interrupts the CPU: *each time it finishes a 
> command, *if a
>    Force Interrupt condition is true, or *if the FDDC tries 
> to write to a
>    write-protected disk.  The 177x's interrupt output pin 
> resets when the
>    CPU reads the status register or writes to the command register,
>    except after a Force Interrupt (immediate) command.
> 
> 
> What I am wondering is if it is normal for it to want to interrupt on 
> start-up?
> 
> 
> It's a shame you can't just fiddle with link 9, but it must 
> be *CLOSED* 
> to disable the disc NMI so I think that could be 'unpleasant' for the 
> FDC? Probably pulls it to a certain logic level, which the 
> FDC will then 
> want to override when asserting an interrupt... Mmmm...
> 
> 
Thanks Rick! (and thanks to Jason too, for suggesting the same thing)

A read of &FE84 clears the problem. I've now added a read of this location to my
custom startup ROM, so hopefully I won't have any more Econet problems.

Like everyone else, I'm not sure why the 1770 is interrupting on power on
- perhaps it starts up thinking it is executing a command some times, so
gives a 'command finished' interrupt after the reset?

Fiddling with the NMI link isn't a good idea because:
A) It would involve opening the case every time I wanted to use / not use
the disk interface
B) There are dire warnings not to have the disable NMI link made if a disk
controller is fitted
C) I'm not sure there even is a NMI disable link on the B+ - the service 
manual lists S5 as "allows disc filing system to disable NMI. This feature 
is not supported by current disc software", which doesn't sound the same
as what S9 does on a BBC B. S9 on the B+ has a completely different function
(selecting between 16K and 32K for one of the ROMs)

Regards

Michael