Date   : Sat, 02 Dec 1995 17:50:32 GMT0BST
From   : "Tom Seddon" <T.W.Seddon@...>
Subject: Screen timing/VSYNC interrupt/BeebDOS

I don't have e-mail access at home, so I'm asking these questions
before I go home for Christmas and write my proper BBC emulator, so
I don't make too many mistakes.

1. Screen timing

I know that each new screen line appears every 128 cycles, and that
there are 312.5 scan lines (interlaced) or 312 (non-interlaced) on
screen. My question is: does it matter if I just stick to
non-interlaced timings at all times? I know Revs won't work :-)

2. The VSYNC interrupt

Does the 6522 just generate this every X cycles, or does it get a
signal from the video circuitry to tell it that the time has come?
I'm assuming that it gets a signal from the video circuitry
(otherwise it would start being less accurate if you switched
between interlaced and non-interlaced modes), but I'd like to check
this all the same.

3. BeebDOS

I've temporarily, uh, appropriated a copy of BeebDOS in order to read
some of my BBC disks on the PC. I heard from somewhere (although I
can't remember where...) that modern FDCs won't read the disk
format... does anybody know whether this true? (I'd like to know
before I borrow my friend's 3 1/2 disk drive and before I drag my 2nd
BBC out of the attic to use as a power supply).

Sorry about all the questions I've asked, but I seem to be the only 
person on this mailing list who hasn't actually finished their 
emulator... I'm very grateful for any help.

--Tom
     .                 ** Tom Seddon **                 .
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