[Simh] simh on RaspBerry Pi

[Thomas Merritt]

> On Feb 15, 2016, at 8:37 AM, Johnny Billquist <bqt at softjar.se> wrote:
> 
> On 2016-02-15 16:15, Paul Koning wrote:
>> 
>>> On Feb 15, 2016, at 9:01 AM, Wilm Boerhout <wboerhout at gmail.com> wrote:
>>> 
>>> Will Senn schreef op 15-2-2016 om 14:26
>>> 
>>> [snip]
>>> 
>>>> Are you documenting the setup process for your endeavors, or just blogging about the result? I think it would be interesting to see how you clustered those Pi Vaxen as much as to know it was possible. I've got a few Pi around looking for something to cluster around...
>>> There are three parts to a successful setup:
>>> 
>>> 1. Since each Pi has only one Ethernet interface, make sure you use a
>>>   wired connection (wireless isn't real Ethernet)
>> 
>> Well, Wireless is 802.11 which indeed isn't 802.3 / Ethernet.  But that's not really relevant.  The question is whether it uses Ethernet addressing and offers an Ethernet-compatible MAC layer API, and 802.11 certainly does.  You can run SimH Ethernet just fine over a wireless LAN.  I've run PDP11 SimH that way with no problems.
> 
> Actually, I have run into problems. The broadcast domain, as well as the Unicast have slight differences to Ethernet, which makes it sometimes fail in subtle ways.
> Having a second mac address on a WiFi interface, one that is used by simh, though tun/tap, does not work that well with WiFi. Unfortunately.
> 
> I've definitely had problems keeping it working under OS X at least. And I'm pretty sure I've read of others having the same problem.
> 
> 	Johnny
> 
> 
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This is a feature of 802.11.  The packet format over the air for 802.11 has two MAC addresses in it to in theory enable bridging.  One for the radio and one for the sender.  Most 802.11 firmware just stuffs the radio's MAC address in both and ignores the MAC address in the original frame when sending, and when receiving the senders radio MAC address is used in the frame propagated to the computer by the adapter.  The result is that everything is sent as if from the host and not the VM and replies are delivered back to the host and not the VM.  This can be worked around in the hypervisor as VirtualBox does for IPv4 and to some degree IPv6.  But don't expect a SimH machines to be able to communicate over WiFi without a lot of work in SimH’s networking implementation.

-- TJ