## Input/Output Device Driver Reference

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### Hardware Interrupt (8259) Interface

When a hardware interrupt is pending on an interrupt level owned by the parallel port device driver, the hardware interrupt manager calls the parallel port device driver's hardware interrupt entry point for that interrupt level. PRINT01.SYS runs parallel port devices using hardware interrupt levels IRQ 7 and IRQ 5. PRINT02.SYS runs parallel port devices on hardware interrupt level IRQ 7. The parallel port device driver for Micro Channel machines shares the hardware interrupt level IRQ 7.

Each of the parallel port device driver's hardware entry points call a parallel port device driver general interrupt routine. This routine sends the next character to the port, issues an EOI (End-Of-Interrupt), and completes a strategy request when the last character has been received by the device.

The hardware interrupt manager saves all registers before calling the parallel port device driver interrupt entry points. On return to the hardware interrupt manager, the parallel port device driver's general interrupt routine clears the carry flag, signalling the interrupt manager that the parallel port device driver owns the interrupt. If the parallel port device driver does not own the hardware interrupt, it sets the carry flag. The parallel port device driver's hardware interrupt routine follows the Call/Return Far model and does not handle nested interrupts.