Subject: Re: Localtalk interface design ideas
To: Erik E. Fair <>
From: Jonathan Stone <jonathan@DSG.Stanford.EDU>
List: tech-net
Date: 02/13/1997 14:55:06
Localtalk is synch.  AFAIK, the traditional Mac implementations were a
``squashed stack'' that interleaved reading bytes from the SCC and
interpreting those bytes as Localtalk.  Interrupts were effectively
disabled for an entire packet time.  Given that LocalTalk is
synchronous and _needs_ to preserve packet boundaries -- i.e., AFAIK,
one can't leave the serial line idle, as one can with asynch protocols
-- there may be no *alternative* to disabling interrupts for an entire
packet time, at least on transmit.

Erik Fair writes:

>My bet for the wide range of platforms that NetBSD runs on is that you will
>have do to it:

Well, you forgot

4) NetBSD platforms with `standard' memory-mapped 8530 chips and drivers,
   that *also* have motherboard DMA engines that can feed entire packets
   to the 8530, as long as the host CPU isn't also trying to drive the
   serial chip.

Also: What's the interrupt latency on the hardware for which Bill
wants to implement LocalTalk? How deep are the transmit and receive
FIFOs on the serial hardware on those platforms (3 chars comes to
mind.)  Is it feasible to keep an SCC running flat-out, using
interrupt-driven code, even with PDMA?

I don't know, and so I'm asking.  I suspect the answer on
68k Macs may be `no'.