In Programmable BeamSync modes, I am seeing a problem at the bottom of the
screen in AA chips that does not exist on ECS chips.

A line at the bottom of the screen (I believe the line number is either VTOTAL
or VTOTAL + 1, where VTOTAL is the value programmed into the VTOTAL register),
is the expected black at the left hand side of the line, but at a point along
the line, the colour is changed to colour 0.

I thought this may be caused by the fact that copinit was writing to bplcon3
without the EXTBLKEN bit set, and with bplcon0 without ECSENA set. Setting these
bits in the copinit copperlist helped the problem, but did not cure it - the
line stays black for a while longer before changing to colour 0.

This behaviour is incompatible with the ECS.

FYI - VBSTRT is usually programmed with the value 0. If I poke VBSTRT to the
same value that is poked into VTOTAL, the bottom line becomes all black, as it
should be.

Spence
