On Saturday 07 May 2016, 14:15:52 Leif Bloomquist <firstname.lastname@example.org> wrote: > Hi all, > > I recently got one of the Dorktronic Expansion Port breakouts > (http://www.ebay.com/itm/331840785722), great way to explore the > mysteries of the C64 Expansion Port. > > I have an Arduino wired in, triggering interrupts on falling edges on > /IO1 and /IO2 incrementing simple counters (one for each). > > It works well, but here's an interesting mystery, that folks on > cbm-hackers might appreciate. > > When I do a PEEK from BASIC of the IO space (i.e 57088 for IO2), I get > a single falling edge on /IO2, as expected. > > But when I do a POKE (again from BASIC), I get *two* falling edges. > This is quite repeatable. > > That's unexpected, any theories on why this may be? > > I don't have a scope unfortunately. Had a quick browse through the > BASIC disassembly, nothing jumped out. > > Ideas? you can use VICE to examine what happens: go to monitor and put a tracepoint on $df00: (C:$e5cf) trace df00 TRACE: 2 C:$df00 (Trace load store) (C:$e5cf) x ...leave monitor, do poke 57088,123 and enter the monitor again, you see this: #2 (Trace load df00) 213 049 .C:b82a 91 14 STA ($14),Y - A:37 X:37 Y:00 SP:f9 ..-...ZC 82195204 #2 (Trace store df00) 213 049 .C:b82a 91 14 STA ($14),Y - A:37 X:37 Y:00 SP:f9 ..-...ZC 82195204 so, what you are seeing is the dummy load that happens before the actual store :) -- http://www.hitmen-console.org http://magicdisk.untergrund.net http://www.pokefinder.org http://ar.pokefinder.org Es ist schon alles gesagt worden, nur noch nicht von jedem. <Karl Valentin> Message was sent through the cbm-hackers mailing listReceived on 2016-05-07 19:00:49
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