A.2.5.1 IF - Interrupt flag
If set, enables hardware interrupts (IRQs) to be serviced by the CPU.
Note that a MOV or POP to SS (see section A.5.156 and section A.5.244)
disables hardware interrupt servicing and Trace flag tracing until after
the next instruction, to allow atomic setting of both SS and SP at the
same time. (There is a bug that makes this interrupt lockout fail on
early 8086/8088 CPUs, which is why 8086-compatible code that sets SS and
SP usually also clears the Interrupt flag explicitly first.)