Date   : Mon, 04 Dec 1995 11:41:45 +0000 (GMT)
From   : James Fidell <james@...>
Subject: Re: Screen timing/VSYNC interrupt/BeebDOS

Tom Seddon wrote:

> 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.

Well spare a thought for those of us that have to work...

> 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 :-)

I think you may be over-simplifying here, but when I start thinking about
the way that CRTs (and televisions/monitors in particular) work, my brain
shuts down for safety reasons.  To the best of my knowledge, there are
625 lines, half of which are scanned in the first field and the rest
in the second field of the frame.  These fields are interlaced and the
same data is read for the even field as the odd field.  What happens
when interlacing is turned off, I'm not sure.

If anyone can explain how it all *really* works, I'd love to know.

> 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.

I believe that the video circuitry is responsible for generating this
as an input to the system VIA on CA1 (which then generates the software
interrupt).

> 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).

In the old days, when 100K was actually quite a reasonable amount of
data to put on a 5.25" disk, the data was written in FM mode.  To increase
the amount of data further, the format was changed to MFM.  I believe
that, because it's so unusual now, many FDCs just don't support FM
mode.

ADFS disks are, as far as I know, written in MFM mode and should therefore
be readable.

> 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.

I'm not actually sure there's anyone on this list who *has* finished
their emulator :-)

James.

-- 
 "Yield to temptation --             | Work:  james@...      
  it may not pass your way again"    | Play:  james@...                 
                                     | http://www.OiT.co.uk/~james/
        - Lazarus Long               |              James Fidell