Date   : Fri, 22 Feb 2008 08:25:15 +0000 (GMT)
From   : info@... (Sprow)
Subject: BASIC error message - "Mis..."?

In article <9FDBD87B3DB3A033E79EDBB6@...[10.37.129.2]>,
   Joel Rowbottom <joel@...> wrote:
> --On 21 February 2008 21:16:20 +0000 Jonathan Graham Harston 
> <jgh@...> wrote:

> > That's paged mode, selected with VDU 14, cancelled with VDU 15.
> > Both LEDs come on steady when waiting for SHIFT to continue
> > scrolling; a full buffer wibbles the LEDs, so makes them look as
> > though they are half on.
>
> Curiosity strikes - was this intentional?

Absolutely. It's done in the internal buffer write routine, LEDs on for C=1
on exit from INSV, then it checks the keyboard for escape (which turns the
LEDs off again). This causes the half lit effect.

Note that if you call the INSV vector yourself you don't get the funky LED
side effect. ie. it's not part of the buffer handler it's only used when the
MOS calls itself. I found this out when writing my second serial port driver
  http://www.sprow.co.uk/bbc/extraserial.htm
as I had expected the lights to come on,
Sprow.