A dummy read occurs when the processor still needs to calculate some addresses or data before it can be written. For example, if the addressing mode abs,x crosses a page boundary, it first does a dummy read on an address where the low address byte is added to, but the high address byte is not yet incremented. So if you read from $00f0,x with x equals $12, there first is a dummy read from $0002, and then the actual valid transfer (read or write) on $0102. With the indirect indexed addressing mode (zp),y it is similar. The dummy read goes to the address stored in zp with the low byte increased by the value of Y, but the high byte not yet incremented. André Am 9. Mai 2016 21:58:17 schrieb Leif Bloomquist <leif@schemafactor.com>: > Thanks. This is a new topic for me - what's a dummy read? Something > that happens when doing an STA on a referenced address? > > The VICE monitor has come a long way since I last used it! Usually > on real hardware. :-P > > -Leif > > > -- > Leif Bloomquist | leif@schemafactor.com | +1 416-737-2328 | Check out > my blog! http://www.jammingsignal.com > > "Every choice, no matter how small, begins a new story." - xkcd > > > On Mon, May 9, 2016 at 1:51 PM, Kajtár Zsolt <soci@c64.rulez.org> wrote: >>> 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 >>> :) >> >> Apart from the wrong cycle numbers it's just the regular dummy read for fixing >> up the high part of address after indexing. It's expected as it's not optional >> for a write. >> >> -- >> -soci- >> > > Message was sent through the cbm-hackers mailing list Message was sent through the cbm-hackers mailing listReceived on 2016-05-09 21:00:10
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