Date   : Fri, 10 Aug 2007 00:56:42 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: Emulating Econet hardware?

>Message-ID: <Marcel-1.53-0806200156-bc8xSBG@...>
 
Johan Heuseveldt <johan@...> wrote:
> OTOH there is the SJ Research design - in its MDFS - with only a
> Z80 SIO, and lots need to be done in software. I'm not familiar with
> that serial device, so don't know how much it can do in hardware, and
> what's left to be done in software.
 
The Z80 SIO does more or less everything the 6854 does. It is
interfaced 8-style (IORQ, WR, RD), rather than 6-style (CS, R/W).
 
A useful extra the Z80 SIO has is that it can match the first byte
in the packet itself and reject the frame if it's not destined for
me. With the 6854 the CPU has to do that.
 
> It /is/ known that the MDFS is very fast, and I can fully confirm
> that! :-)
 
A small, but significant part of that speed, is hardware rejection
of frames not for me.
 
> Which makes it very intriguing: how can a software solution be
> faster than its hardware version that it is emulating. Processor
 
It's not software, it's hardware. Serial<->Parallel conversion and
on-the-fly CRC calculation and checking is eminantly suitable to
building out of hardware, and needs lots of brute force to do in
software.
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
BBC BASIC for Windows and Internationalisation
  See http://mdfs.net/Software/BBCBasic/Windows/ProgTips