Date   : Wed, 06 Feb 2002 19:59:42 GMT
From   : pete@... (Pete Turnbull)
Subject: Re: Firetrack

On Feb 6, 15:04, Richard Gellman wrote:

> 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)

Not quite.  According to my original Motorola 6845 data sheet, it's R3 that
holds the width, not R2, and it does hold both widths in all but the first
release batch of 6845's.  All later ones and all 68A45 and 68B45 varients
hold both (HSW in the lower 4 bits as you say).

On the earliest 6845's, the top 4 bits are zeros, and the VS width (scan
lines) is 16.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York