mac68k: console doesn't fill screen

[Matthias Hertel <mhertel@bigfoot.de>]

Hi,

I'm using Debian on a Performa 475 with a portrait display. This means
640x870 pixels or 25x54 characters (with the 8x16 VGA font). *But*
54*16 is only 864, so there are 6 scanlines at the bottom of the
screen that the console doesn't know about. In itself, this wouldn't
be a problem, but these scanlines are *white*. And they stay white,
even when the screen blanker kicks in.

How can I set these 6 lines to black?

A different console font is not really a option, because the nearest
number that evenly divides 870 is 10, and I think 8x10 is a
not-so-great size for a console font.

TIA
Matthias