Date   : Thu, 28 Jul 1994 08:56:17 WET DST
From   : Bonfield James <jkb@...>
Subject: Re: Video display.

Stephen Quan writes:

>Hi everyone!  I tested refresh rates on my BBC model B, and found : 
>
>    * modes 0..6 adhere to following rule of thumb, (despite differing memory).
>    * rule of thumb is 1 scan line per 128 cycles.
>        (or 1 scan line per 64 NOP instructions)
>    * nb. different modes have different total # of scan lines.
>    * nb. different modes have different total bytes/scan line.

You may wish to emulate at a different level - emulating the 6845 chip.
The registers for this chip (see the new advanced user guide) control much of
the above, so the default modes alone aren't emulation enough (sorry for the
bad English there).

>Has anyone got a copy of that hack program that puts you into an
>MODE 8?  I recall reading from a beeb magazine that you can take

Mode 8 is just a term that someone used for a new mode. "Mode 8" could be ANY
deviation from the default supplied 6845 register sets (ie the modes). Note
that many games, for speed, shrink the screen width slightly so that there's
no longer 320 bytes per 8-high row, but 256. This naturally makes arithmetic
easier (lookup table for hi byte, then only need to adjust lo byte - ideal for
sprites where we're plotting on a 8 pixel grid), whilst also having the effect
of shrinking the screen memory used and hence speeding up screen writing.

As for the jaggedness, I agree in that it's not worth emulating this. Some
games go to pains to hide it, by swapping colours or modes in a conveniently
blank strip. Most, I'm sure, would swap on a single scan line and remove the
jaggedness if they could.

       James