Specifications

Writing an Interrupt Service Routine

When a device generates a hardware interrupt, it requests an interrupt at the

appropriate device interrupt priority level (IPL). Either the device or its adapter

requests a processor interrupt at that IPL. When the processor executes at an

IPL below that device IPL, interrupt dispatching begins.

The mechanism of interrupt dispatching has no direct bearing on the contents of

a driver’s interrupt service routine. Its implementation varies according to the

AXP system and I/O subsystem in use.

For most device drivers, the driver prologue table (DPT) contains, in the

reinitialization section established by the DPT_STORE_ISR macro, the address

of one or more interrupt service routines. Each interrupt service routine

corresponds to an interrupt vector on the I/O bus. You specify the interrupt

vector using the SYSMAN command IO_CONNECT.

Most device interrupt service routines perform the following functions:

• Locate the device’s unit control block (UCB)

• Determine whether the interrupt was solicited

• Reject or process unsolicited interrupts

• Activate the suspended driver to process solicited interrupts

The remaining sections of this chapter describe the handling of solicited and

unsolicited interrupts in further detail.

7.1 Servicing a Solicited Interrupt

When a driver’s fork process activates a device and expects to service a device

interrupt as a result, the fork process suspends its execution and waits for an

interrupt to occur. The suspended driver is represented only by the contents of

the fork block in the device’s UCB and the stack, which contain the following

information:

• A description of the I/O request and the state of the device

• The contents of R3 and R4

• The implicit contents of R5 (the address of the UCB fork block)

• The address at which to return control to the driver

• The implicit address of a timeout handling routine

When the interrupt service routine returns control to the main line of driver

processing, it has only restored the contents of R3, R4, R5, and the PC.

7–1