Date   : Wed, 06 Feb 2002 15:04:09 -0000
From   : "Richard Gellman" <r.gellman@...>
Subject: Re: Firetrack

Richard Talbot-Watkins wrote:

>We need to find out somehow how many scanlines of the PAL screen are
>occupied with vertical retrace.

R2 holds both the vertical sync and horizontal sync widths (one per 4 bits).
I believe the bottom 4 bits is the horizontal sync width.
Note: Only certain variants of the 6845 have vertical sync width support,
I'm led to believe such a variant has been used in the bbc micro, as data
logging
from BeebEm shows some games attempting to write to the top half of this
register.

(The original Motorola 6845 does NOT have programmable vertical sync width)

The Watford Master Advanced Reference manual shows the values programmed to
R2 for
both horizontal and vertical sync widths for each screen mode.

Vertical sync width is character lines (scanlinesPerChar+1), and horizontal
sync width
is in characters. (8-mode 0 pixels for modes 0-3, and 8 mode 4 pixels for
modes 4-7
the difference is due to a 2Mhz clock in modes 0-3, and 1Mhz clock in modes
4-7)

-- Richard Gellman

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