An End Of Interrupt (EOI) is a signal sent to a Programmable Interrupt Controller (PIC) to indicate the completion of interrupt processing for a given interrupt. An EOI is used to cause a PIC to clear the corresponding bit in the In-Service Register (ISR), and thus allow more interrupt requests of equal or lower priority to be generated by the PIC.
EOIs may indicate the interrupt vector implicitly, or explicitly. An explicit EOI vector is indicated with the EOI, whereas an implicit EOI vector will typically use a vector as indicated by the PICs priority schema. For example, the highest vector in the ISR. Also, EOIs may be sent at the end of interrupt processing by an interrupt handler, or the operation of a PIC may be set to auto-EOI at the start of the interrupt handler.