Analog Devices VisualDSP++ 3.5 Manual page 224

Memory Management Using Overlays
This example uses the concept of overlay function loading and executing.
A function
load
memory but not execute the function. A function
to execute an overlay function that may or may not be in internal memory
at the time of the execution request. If the function is not in internal
memory, a transfer must occur before execution.
In several circumstances, an overlay transfer can be in progress while the
core is executing another task. Each circumstance can be labeled as deter-
ministic or non-deterministic . A deterministic circumstance is one where
you know exactly when an overlay function is required for execution. A
non-deterministic circumstance is one where you cannot predict when an
overlay function is required for execution. For example, a deterministic
application may consist of linear flow code except for function calls. A
non-deterministic example is an application with calls to overlay functions
within an interrupt service routine where the interrupt occurs randomly.
The example provided by the software contains deterministic overlay
function calls. The time of overlay function execution requests are known
as the number of cycles required to transfer an overlay. Therefore, an over-
lay function load request can be placed to complete the transfer by the
time the execution request is made. The next overlay transfer (from a load
request) can be enabled by the core, and the core can execute the instruc-
tions leading up to the function execution request.
Since the linker handles all overlay symbol references in the same way
(jump to PLIT table and then overlay manager), it is up to the overlay
manager to distinguish between a symbol reference requesting the load of
an overlay function and a symbol reference requesting the execution of an
overlay function. In the example, the overlay manager uses a buffer in
memory as a flag to indicate whether the function call (symbol reference)
is a load or an execute request.
The overlay manager first determines whether the referenced symbol is in
internal memory. If not, it sets up the DMA transfer. If the symbol is not
in internal memory and the flag is set for execution, the core waits until
5-18
is a request to load the overlay function into internal
VisualDSP++ 3.5 Linker and Utilities Manual
is a request
execution
for 16-Bit Processors