Date   : Wed, 22 Mar 2006 10:48:13 +0000
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: Advanced BASIC pretending to be the MOS!

Steve Fewell <kranser@...> wrote:
> Checked, are you are absolutely correct. OS 1.20 Just
> hangs instead of going into command ('*') mode. I'd
> always wondered why *FX247,76 causes BBC Bs to hang
> on Break, but Masters enter the MOS command entry
> mode.
 
*fx247,76 will cause the MOS to jump to the address set by *fx248
and *fx249 on BREAK. If you haven't set them, they will be zero,
so pressing BREAK will jump to &0000 where it will try to execute
whatever is there. As this is the language workspace, the most
likely result is a crash. There are a lot of &00s there which are
the BRK instruction, used by the BBC MOS to signal an error. On
startup, the BBC B sets BRKV to display the error message and
hang. The Master sets BRKV to display the error message and then
provide a *command prompt. Try doing Shift-Break to a medium with
no !Boot file when *OPT 4 is set. The BBC displays "File not found"
and hangs, the Master displays "File not found" and goes to the
*command prompt.
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
In 1939 $50 of groceries would fill three station wagons. Today I
can lift $50 of groceries with one hand. I must have got stonger.