The EA bit can be used to disable interrupts temporary. But it have no influence to the interrupt handler currently in progress. So you can clear it and set it also inside an interrupt handler.
E.g. if you must prevent a code sequence inside a low level interrupt from interruption by a high level interrupt, you can do it with the CLR EA command at the beginning and SETB EA at the end.
Means, in exact the same manner, if critical code was encapsulated in the main program.
During this all interrupt events can still set the appropriate interrupt flags and after SETB EA these interrupts are also executed in the given priority chain.

The 8051 interrupt logic is very clear, since every interrupt event have its own interrupt flag and so no interrupt can be lost.
Also its easy to generate software interrupts by setting an interrupt flag in software.
Its also easy to poll these flags to handle events.


Peter