Date   : Sat, 17 Feb 1996 20:55:06 +0000 (GMT)
From   : Tom Seddon <T.W.Seddon@...>
Subject: Strange bug!!

I've discovered a really weird bug. It first came to light on my emulator 
when I let someone play with it.

If you restart the emulator (ie from DOS/Windows/whatever) and type 
OLD--you'll get a Bad Program message--followed by 10PRINT "FRED" (or 
whatever), when you press RETURN the computer will hang! This happens on 
my emulator, on WinBeebEm, and on James Bonfield's emulator as well (I 
hacked it around a bit to get a dos-compatible version).

This is really strange! Would anyone be able to offer any kind of 
explanation?! Having implemented a 'history' feature, when I terminate 
the emulator it has just been going round in an endless loop of about 
five or six instructions. I can give more details if anyone is interested.

And hile I'm on the subject of strange things, I'm implementing graphics
at the moment. I've got modes 4 and 5 working (with dodgy palette
problems--don't know what's going on there! :-) but every so often the BBC
crashes, printing a load of crap over the screen and going into graphics
cursor mode (VDU5). I think this comes from having a CA1 VSYNC interrupt,
since when I take the CA1 interrupt out the problems go away. Did anyone
else get this problem, and, if so, what caused it? If you didn't then it
doesn't matter because I'm sure I'll get to the bottom of this eventually. 
(And before you ask I *do* have IRQs and BRKs working correctly :-)

--Tom

     .                     ** Tom Seddon **                     .
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