Specifications

Introduction
1.5 Driver Context
In general, context is characterized by the following factors:
• Actual parameters that are passed to the routine
• The current IPL of the executing processor
• The range of IPLs that the routine can change and the required IPL on return
from the routine.
• The currently owned spinlocks of the executing processor
• The data structures available to the routine
• The ability or inability to access process space
A complete description of the context of each driver routine appears in the entry
points chapter of the OpenVMS AXP Device Support: Reference. The following
are some general observations:
• All device driver routines execute in kernel mode at an elevated IPL.
• Only driver FDT routines execute within process context and can access
process space (P0 and P1).
• The majority of driver routines execute in interrupt (or system context):
that is, in the sequence of execution that follows a processor’s grant of an
interrupt request at a given IPL. Such code can refer only to system (S0)
space. Moreover, it cannot incur exceptions, including page faults, without
causing a fatal bugcheck.
Most driver processing of an I/O request (before and after the device acknowledges
the servicing of the request by requesting an interrupt from the processor) occurs
at a fork IPL. This portion of driver code, which includes most of the start-I/O
routine, is commonly known as the driver’s fork process.
There are several instances in the processing of an I/O request when a driver
fork process must suspend execution to wait for a resource or a device interrupt.
To make the matter of saving and restoring fork process context as efficient as
possible, the operating system places a restriction on the context of a driver fork
process, in addition to those that apply to any process in interrupt context. Fork
context consists of the following:
• The fork routine parameters (FR3 and FR4)
• The fork routine address (FPC)
• A fork block (usually the unit control block that can contain additional fork
process context
The operating system places the fork block of a suspended fork process in either
a processor-specific fork queue or a resource wait queue where it waits to be
resumed. When it resumes the fork process, the operating system calls the
fork routine with the FR3 value, FR4 value, and a pointer to the fork block as
parameters.
1.5.1 Example of Driver Context-Switching
Because a device driver consists of a number of routines that are activated by
the system, the operating system determines the context in which the routines
execute.
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