Date   : Tue, 19 Aug 2003 21:41:33 +0100
From   : "Thomas Harte " <thomasharte@...>
Subject: Chasing 100% perfect emulation (monitor quesiton)

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I am easing myself back into development of my Electron emulator, ElectrEm,
by chasing the goal of "100% 
perfect emulation". Previously ElectrEm has not been perfect due to making
some assumptions very common to 
emulators - that it is okay to fetch video data a little late (all at the
end of the scanline in ElectrEm), that the 
6502 reads and writes which don't seem to achieve anything don't occur (e.g.
modify instructions will usually do 
read data, write same data back, write modified data back - so the first
write simply isn't done by the 
emulator), and that IRQ lines, etc, actually only change inbetween instructions
rather than only being 
processed between interrupts. Conversely ElectrEm already leads the way in
several areas - well in advance of 
all BBC emulators I have encountered in terms of things like tape and floppy
disc emulation, the latter being 
done at the level of magnetism polarity changes on the surface of the disc.

But that is all in the past, and all set to change in the next ElectrEm.
I have enough documentation to fix all 
internal mechanisms, but I am thinking about accurate emulation of the display.
I know that when connected to 
a PAL TV, neighbouring fields are interlaced with the usual level of afterglow
on PAL televisions - I have even 
seen 64 colour demos which take advantage of this afterglow to increase the
colour palette in software.

I am curious about how BBC type mointors work in comparison to TV screens.
First of all, do they actually 
interlace, or do they just treat the signal as 50 independant frames? If
they interlace, do they do so within 
roughly the same parameters as a typical PAL television? Maybe the hardware
does something even more clever 
that I am not thinking of? My suspicion that they don't function exactly
as televisions do is mostly sparked by 
the near complete absense of the sort of 'high resolution' and 'increased
colour palette' demos common on other 
micros versus the increased uptake of monitor ownership amongst BBC owners
+ my complete lack of 
documentaiton on monitors. Am I reading too much into the situation?

-Thomas

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