Date   : Tue, 06 Jan 2015 12:55:30 +0000 (WET)
From   : bbcmicro@... (Peter Coghlan)
Subject: Serial console active at power-on

>
>>
>> Something to be careful about - if your remotely-controlled Beeb issues 
>> a VDU 14 you will have to somehow manually press the SHIFT key to get 
>> the display to scroll! It's worth putting an intercept on WRCHV to 
>> always disable VDU 14.
>>
>I'm with you on that one, but my concern was about the dodgy re-entry
>qualities of the BBC OS.
>Or put another way - if am the currently selected language I can call
>any OS entry with impunity because I'm not executing any OS routines,
>but if I am running a OS routine (which a service routine in a sideways
>ROM is),
>I can't be certain that I can legitimately make whatever OS calls I like
>(basic OSBYTE calls would be OK, but trying to run a file system command
>whilst handling an FSCV routine could end in tears.)
>

It's almost impossible to be prepared for every possibility.  If someone did
an *SPOOL or *EXEC the potential for problems increases greatly whether you
are in the current language or not.

>
>Regarding interception of WRCHV, I guess this could be set up from the
>service entry of a language ROM.
>(AFAIK the only BBC language ROM that doesn't officially have a service
>entry is BASIC).
>

Also, be careful not to just discard every attempt to send 14 through WRCHV.
It could be part of a multi byte VDU sequence such as VDU 1 or VDU 31 for
example.  Unfortunately, it is necessary to keep track of which VDU sequence
is being sent and ignore the correct number of bytes for the sequence in
question.

Regards,
Peter Coghlan.