Analog Devices VisualDSP++ 5.0 Service Manual page 214

Introduction
deterministic as possible and to a minimum. Callbacks, on the other hand,
may be lengthy and non-deterministic. In most cases, users may prefer to
defer the execution of such callbacks to a scheduler running at a lower pri-
ority, which can be preempted by higher priority interrupts. In doing so,
the requesting ISR can complete with minimal delay.
The system services library's deferred callback manager provides this
service by managing one or more queues of deferred callbacks, such that
their invocation typically occurs within a dispatch function operating at a
lower-interrupt priority than the rest of the application's interrupt ser-
vices. Each callback entry posted to a queue comprises the address of the
required callback function along with three values (two pointers and one
32-bit unsigned integer), which are passed to the callback function upon
its (deferred) execution.
The deferred callback (DCB) manager is designed to operate as a
standalone module or in conjunction with a real-time operating system
(RTOS). Implementations of the module exist for Express Logic's
ThreadX, Green Hills Software' INTEGRITY, as well as Analog Devices
VDK.
The number of queues available and their length is determined by the cli-
ent application upon module and queue initialization. Whether the DCB
manager is implemented in standalone mode or in conjunction with one
of the above RTOSs also impacts the number and size of queues. When
implemented in conjunction with VDK, the DCB manager can support
only one queue at a fixed-priority level of IVG 14.
While only one queue is permitted per IVG level, engineers can set priori-
ties for individual callback entries by supplying a software priority level
upon posting. There is no limit to the number of software priority levels
that can be used (except for practical implications within the limits of
unsigned short values) The dispatch function attempts to execute all
higher-priority callbacks before those with lower priorities at the same
IVG level.
5-2 VisualDSP++ 5.0 Device Drivers and System
Services Manual for Blackfin Processors