Date   : Sat, 11 Aug 2007 00:43:05 +0200 (BST)
From   : johan@... (Johan Heuseveldt)
Subject: Emulating Econet hardware?

Hi Jonathan,

On Fri 10 Aug, Jonathan Graham Harston wrote:
> Jules Richardson <julesrichardsonuk@...> wrote:

Actually, it was me :-)

> > >  * Several configurations for special control fields at the start
> > >      of a frame/packet, which can be set to one or two bytes.
> > >    Econet only uses a single 8 bits 'Address' field (station number)
>  
> Econet uses a 16-bit 'address' - net.stn - but as to the ALDC,
> everything after the opening flag is just 'data', it's irrelevant
> what that data is.

No, it isn't.
The first byte is the Address field for the 68B54. The first NMI handler
set up is expecting that, and tests for it.
Normally the data is received per two bytes - CtrlReg 2, bit1, and
interrupts are prioritized - CtrlReg 2, bit0. The Address field has a
higher priority than normal data, so this very first byte - after the
opening flag - causes an Address field interrupt.

> It's only the Z80 SIO that has the capability
> of treating the first byte as an address and doing a destination
> match on it. With the 6854 it's all in software.

No, something in between, but still very efficient though:
It's detected by the ADLC, and informs the CPU about it. See above.
The CPU checks it with its own station number - and the net number
of course. (must be zero) [a]

If it's 'not for me', the frame will be rejected by ignoring the rest
of the frame by 'Discontinue' it - CtrlReg 1, bit5.

 [a] Another station could send to the same station number, but on a
      different net.

Also note that the Address field is an ADLC 68B54 property, and has
nothing to do with Econet. Econet uses this as a station number, which
is the logical use of it of course, and most likely meant. :-)

For the ADLC it is the only special byte in the data, and having an
interrupt of its own, as it is setup this way by the Econet software.


Johan

-- 
Johan Heuseveldt <johan@...              >
  aka  waarland

  The best place is a Riscy place
 
When choosing between two evils I always like to take the one
I've never tried before. - Mae West, in "Klondike Annie" 1936