The NOP was needed, because every instruction influencing the interrupt logic cause, that minimal one additional instruction was executed before any pending interrupt was served.


And only one NOP cause in this case, that also only one pending interrupt was served. Because a RETI was also an instruction, which influence the interrupt logic

This nice feature was added on the 8051 to give the main a chance to be executed, even if interrupts faster triggered than executed.

In opposition on the ARM7TDMI (e.g. Philips LPC2106) such interrupts can block the system permanently.


Peter