Fix NMI servicing reading the vector twice, and characterize JMP ($xxFF) behavior - #285
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Summary
Three small NMOS-baseline correctness items (groundwork for the upcoming CPU model architecture work):
ProcessInterruptsfetched the NMI vector once purely for a debug log line, thenProcessHardwareNMIfetched it again — two real reads of$FFFA/$FFFBper serviced NMI where hardware performs one. The reads go through the current memory mapping, so the duplicate was observable by mapped handlers. The vector is now read exactly once; the log line runs after entry and reusesPC(the vector target).LogDebugcall and itsstring.Joinallocation now only run when debug logging is enabled, matching the existing unknown-opcode guard pattern.JMP ($xxFF). The emulator currently resolves indirectJMPlinearly (CMOS/65C02-style); real NMOS hardware has the page-wrap bug. A new test pins the current behavior and documents it as a known NMOS deviation, so the planned per-model fix lands as a visible, deliberate change instead of a silent one.Tests
NMI_Servicing_Reads_Each_Vector_Byte_Exactly_Once(countingMapReaders on the vector bytes; fails against the previous code).JMP_IND_With_Pointer_At_Page_End_Currently_Reads_Linearly_Known_NMOS_Deviation.