vm02: calling all IIcs - Take 2

[David Schmenk <dschmenk@YUCH.gmail.com>]

Thanks again to Tom for trying this out.  Maybe I can convince him to 
try this one out:

http://schmenk.is-a-geek.com/tarfiles/VM02ALPHA3.DSK.zip

I also discovered a couple of other issues now fixed.  Certain ROM 
routines will disable interrupts.  Some of my sample classes that call 
ROM routines for low res and hi res graphics all of the sudden stopped 
responding to interrupts.  I now re-enable interrupts after every native 
method.  I also discovered that my comparison opcodes would adjust the 
stack pointer, then compare the values that were now above the stack. 
Tsk, tsk.  When an interrupt would occur during the comparison, the 
values would get trashed.

Glen Bredon wrote a nifty little TSR style program that would hook the 
VBL interrupt to keep time.  His notes mentioned problems with various 
IIc models.  He actually didn't bother calling the mouse firmware to 
service the interrupt on a IIc.  The VBL interrupt would continue 
interrupting anyway.  So I now look to see if any hardware claimed the 
interrupt.  If not, I assume it's the VBL interrupt.  Hope this works,

Dave...