> Hi there,
>
> I'd like to know once and for all what the exact meaning of the IE bits of
> registers #0 and #1 is. As Laurens pointed out some days ago bit #5 (IE0)
of
> regiter #1 is the enable bit for the vertical blank interrupt (ie the VDP
> reaching line 212). So what's IE0 (vertical retrace enable) for then?
IE0 is to enable/disable interrupts from a lightpen/mouse connected to the
v9938/58. However, in the MSX system the mouse is read through the PSG and a
lightpen is most of the time not connected (you'll have to solder on your
VDP to do that... there is no connector for it).
> Here's the second one: if I use interrupt mode 2 what are the contents of
> the lsb that forms (with register I being the msb) the address whose
contents
> point to the actual interrupt routine? It's supposed to be the contents of
the
> data bus at the moment of the interrupt being acknowledged. It used to be
#FF on
> the Spectrum, what about the MSX?
On MSX it is #FF by default. However, this is not specified in the MSX
system (in fact IM1 is the only official interrupt-mode, although IM2 can be
used too) so it can actually have any value because of a distortion or so...
All programs should fill the entire area from I*256 to (I+1)*256 (INCLUDING
the last byte!!!)... If you are programming an application for MSX you
should do that too (or simply hook the adress of IM1 (=#38) in
Dos-environment). If you are writing an emulator you can always assume #FF.
I wrote a very extended article about it in one of the most recent
XSW-Magazine issues... You should really take a look at that. But it's in
Dutch, though.
~Grauw
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