[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Mix LGR and Text?

josephoswaldgg@hotmail.com wrote:

Jorge Chamorro Bieling wrote:



memory-mapped, but i can not figure out how did you manage to know when
it's vsync-time on an a2, a2+ ? I'm telling you because I tried this
long time (20 years ?) ago on an a2 with the mouse card's "vbl"
interrupts. The other way I thought it could be done (but never tried)
was finding the vsync signal in the video generator circuit (page 151,
red book, it is one of c13's inputs) and tie it to a game-connector's
switch-n input to be able to poll it reading c06n.



Don Lancaster documented just such a modification in his Enhancing the
Apple II, volume 1.
Bob Bishop discovered the "floating bus" technique, reading the video
values hanging around on the data bus, which Lancaster documented in
his Enhancing the Apple II/IIe, volume 2. Some hardware expansion cards
can load down the bus enough that the value goes away.


Ack--I just went back and re-read Don's article, and sure enough,
Bob Bishop is the discoverer, and Don read it in Softalk.  I was
under the misconception that Don had also discovered it...  ;-(.

I'll *remember* that now!  ;-)


Also, being such a time-critical loop, if not interrupt-driven, there's
not much you can do while in-the-loop, or the whole thing stops being
in-sync... ?



It depends on what you are trying to do. Lancaster showed a good
technique for getting an absolute lock on the bus-sniffing "Vaporlock"
by carefully choosing hidden hi-res video values on successive scans to
get a byte-exact lock in one video field. He also showed if you look at
just the VBL value, it takes multiple video fields with careful timing
to zero in on the transition, because the width of the VBL transition
is pretty large.


And the rep rate is only once per field, so zeroing in on the edge
takes multiple fields.  (The width really isn't the issue.)

-michael

Music synthesis for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
tool--and it is seriously underused."