Understanding nestest STA to $4015
Posted: Mon Sep 03, 2018 3:22 pm
Hi all, long time lurker here!
Been hacking for a while, but now I'm in dire need for some help about something I can't wrap my head around regarding nestest.log.
On line 8981 the rom does a STA, where A is 02, to $4015. My emu writes 02 to this address (which should be APU status reg?), but according to the log it looks like it should be writing FF somehow? Up until that point everything matches to my emu.
Am I missing something fundamental about the log or the behaviour of the CPU/APU? Up until this point I have been assuming that " = XX" in the log indicates what value is being written.
Thanks for a great community and resource!

On line 8981 the rom does a STA, where A is 02, to $4015. My emu writes 02 to this address (which should be APU status reg?), but according to the log it looks like it should be writing FF somehow? Up until that point everything matches to my emu.
Code: Select all
...
C689 A9 02 LDA #$02 A:00 X:FF Y:15 P:27 SP:FB CYC: 80 SL:212
> C68B 8D 15 40 STA $4015 = FF A:02 X:FF Y:15 P:25 SP:FB CYC: 86 SL:212
C68E A9 3F LDA #$3F A:02 X:FF Y:15 P:25 SP:FB CYC: 98 SL:212
...
Thanks for a great community and resource!