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What is the function of an interrupt flag?
The Interrupt flag (IF) is a flag bit in the CPU’s FLAGS register, which determines whether or not the (CPU) will respond immediately to maskable hardware interrupts. If the flag is set to 1 maskable interrupts are enabled.
What is latched interrupt?
As mentioned, interrupt events are quite often latched. This means that when the event occurs, a flag is set and that flag remains set until some different action clears it. In particular, the flag remains set even when the setting event goes away.
When if interrupt flag is disabled following is true?
If a trigger flag is set, but the interrupts are disabled (I=1), the interrupt level is not high enough, or the flag is disarmed, the request is not dismissed. Rather the request is held pending, postponed until a later time, when the system deems it convenient to handle the requests.
What happens when interrupt flag is disabled?
The I flag is a global interrupt enable/disable bit. All of the interrupt sources are gated with the I flag. If the I flag is set, none of the interrupts will be seen by the processor hardware. This allows the programmer to easily disable/enable all interrupts.
Which instruction is use to disable interrupts?
The instruction PUSHF stores all flags, including IF, in the stack where they can be examined. Task switches and the instructions POPF and IRET load the flags register; therefore, they can be used to modify IF. Interrupts through interrupt gates automatically reset IF, disabling interrupts.
What happens when the interrupt flag is set to 1?
If the flag is set to 1, maskable hardware interrupts will be handled. If cleared (set to 0), such interrupts will be ignored. IF does not affect the handling of non-maskable interrupts (NMIs) or software interrupts generated by the INT instruction.
Do you need flags to decode UART interrupts?
Without the flags you do not know the exact source of the interrupts. E.g., the UART can interrupt for a number of reasons, each of which will have its own flag. Modules can share interrupts (and this run the same ISR) so, again, you need the flags to decode the source of the interrupt.
How to restore the flag from the stack?
Some old DOS programs that use a protected mode DOS extender and install their own interrupt handlers (usually games) use the CLI instruction in the handlers to disable interrupts and either POPF (after a corresponding PUSHF) or IRET (which restores the flags from the stack as part of its effects) to restore it.
How to define the exact type of interrupt you want?
That defines the exact type of interrupt you want: 0: The low level of INT0 generates an interrupt request (LOW interrupt). 1: Any logical change on INT0 generates an interrupt request (CHANGE interrupt). 2: The falling edge of INT0 generates an interrupt request (FALLING interrupt).