On NMOS, the simultaneous assertion of a hardware interrupt line and execution of BRK was not accounted for in the design — the BRK instruction will be ignored in such a case. On CMOS, this situation is correctly handled by executing BRK and then servicing the interrupt.
=== Half Cycles ===
The 6502 divides each clock cycle into two phases (ϕ1 and ϕ2):