Analog Devices VisualDSP++ 3.5 Manual page 227

Memory Overlays and Advanced LDF Commands
OVERLAY_GROUP2 {
OVERLAY_INPUT
{
ALGORITHM(ALL_FIT)
OVERLAY_OUTPUT(fft_two.ovl)
INPUT_SECTIONS( Fft_ovl.doj(program) )
} >ovl_code
OVERLAY_INPUT
{
ALGORITHM(ALL_FIT)
OVERLAY_OUTPUT(fft_last.ovl)
INPUT_SECTIONS( Fft_ovl.doj(program) )
} >ovl_code
} > mem_code
The first and third overlays share one run-time location, and the second
and fourth overlays share the second run-time location.
Additional instructions are included to determine whether the function
call is a load or an execution call. If the function call is a load, the overlay
manager initiates the DMA transfer and then jumps the PC back to the
location where the call was made. If the call is an execution call, the over-
lay manager determines whether the overlay is currently in internal
memory. If so, the PC jumps to the run-time location of the called func-
tion. If the overlay is not in the internal memory, a DMA transfer is
initiated and the core waits for the transfer to be completed.
The overlay manager pushes the appropriate registers on the run-time
stack. It checks whether the requested overlay is currently in internal
memory. If not, it sets up the DMA transfer. It then checks whether the
function call is a load or an execution call.
If it is a load, it begins the transfer and returns the PC back to the instruc-
tion following the call. If it is an execution call, the core is idle until the
transfer is completed (if the transfer was necessary). The PC then jumps
the run-time location of the function.
VisualDSP++ 3.5 Linker and Utilities Manual
for 16-Bit Processors
// Overlay to live in section ovl_code
// Overlay to live in section ovl_code
5-21