Analog Devices EE249v01 Application Notes

Engineer-to-Engineer Note EE-249

a

Technical notes on using Analog Devices DSPs, processors and development tools

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Implementing Software Overlays on ADSP-218x DSPs with VisualDSP++®

Contributed by Ramesh Babu and Aseem Vasudev Prabhugaonkar Rev 1 – October 4, 2004

Introduction

This EE-Note discusses the implementation of
software overlays on ADSP-218x DSPs. A
simple code example, demonstrating the software
overlay technique for ADSP-218x DSP, is
discussed later in this document.

Early versions of the VisualDSP® toolset used a
different scheme to support software overlays as
discussed in the legacy EE-Note Everything You

Always Wanted to know about Overlays - *But
were afraid to ask (EE-67). With VisualDSP++®

3.5 a new approach has been enabled.

What are Software Overlays?

Due to limited address space, many ADSP-218x
derivatives provide multiple memory pages,
called hardware overlays. The software overlay
technique is a completely different scheme to
populate the content of physical SRAM with
different data or code on demand.

“live address space”, and the memory address
where the program is executed in internal
memory is called “run address space”. The code
responsible for transferring the data / program
code from external memory to internal memory
at runtime is called an overlay manager. It
typically resides in a reserved space of the DSP’s
internal memory.

Software Overlays on ADSP-218x DSPs

Software overlays can be implemented using the
ADSP-218x DSP's external byte-wide memory
space. A byte-wide memory can be interfaced
through the processor's BDMA port. The overlay
code/data resides in the external byte-wide
memory. The overlay manager is responsible for
initializing byte DMA (BDMA) to transfer the
data/code from external memory into internal
memory when required.

VisualDSP++ Support

The software overlay technique is commonly
used when a processor is does not have sufficient
internal memory to hold all of the application's
data / program code. In these situations, part of
data / program code is loaded into internal
memory during the booting process and the
remaining part is placed in external memory.
When program code (or data) in external
memory has to be executed, it is loaded into

The VisualDSP++ linker automatically generates
overlay constants, which configure the BDMA
parameters in the overlay manager. Each overlay
has a word size, run-time address and live
address used by the overlay manager to
determine where the overlay resides and where it
is executed.

The linker-generated constants (where N is the
ID# of the overlay) are:

internal memory and executed.
The memory address range where the overlay

 _ov_startaddress_N (live address space)

function resides in the external memory is called

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 _ov_word_size_run_N

p

A

 _ov_word_size_live_N
 _ov_runtimestartaddress_N (run addr. space)

The linker is also responsible for resolving the
symbol addresses of overlay data and labels.

Example Program

This section shows a simple example program to
illustrate the software overlay implementation on
ADSP-218x DSPs. The assembly program
provided with this EE-Note toggles a flag pin at
two different speeds. This program uses two
software overlay functions which exist in the
external byte memory.

Internal PM

Memory

0x0000

Main ( )

Overlay_Manager ( )

0x2000

Figure 1. Simple Memory Overlay Example

0x1000

0x2000

ace

Run S

The main program calls the two software overlay
functions in an infinite loop. One of the software
overlay functions (

Fast-LED-Blink()) toggles

the LED at a faster rate; the other (

Blink()

) toggles the LED at a slower rate.

Figure 1 illustrates where the overlay functions,

overlay manager, and the main function are
stored.

The

main() and Overlay_Manager() functions

are loaded into the internal RAM of the ADSP­218x during the booting process. The

Blink()

and Slow-LED-Blink() overlay

External Byte

Memory

Fast-LED-Blink ( )

Slow-LED-Blink ()

Slow-LED-

Fast-LED-

functions are saved in external byte memory.
During runtime, when an overlay functions is
called, the function is loaded into internal
memory and then executed. Accessing code
and/or data overlays dynamically provides
greater flexibility toward managing your DSP’s
internal memory.

Linker Definitions

Let's look into the declarations required in the
Linker Description File (.LDF) for the example
program explained above.

// "run" space for PMOVLAY pages
mem_pmovly {
TYPE(PM RAM)
START(0x002000)
END(0x003fff)
WIDTH(24)
}

// "live" space for PMOVLAY pages
mem_pmpage1 {
TYPE (BM RAM)
START(0x001000)
END(0x001fff)

Live Space

WIDTH(8)
}
mem_pmpage2 {
TYPE(BM RAM)
START(0x002000)
END(0x002fff)
WIDTH(8)
}

Listing 1. Run and Live Space Declaration in .LDF

The statements in Listing 1 in the MEMORY {}
section of the
(i.e., the run space and live space addresses of
the overlay program code).

Live space. which is specified in the byte
memory (BM), helps the linker generate the
overlay constants.

The statements in Listing 2, which are specified
in
code and data to the physical memory of a
processor in a DSP system. In Listing 2, the
commands
(e.g.,

.LDF file define the target memory

PROCESSOR { } section of the .LDF file, map

tell the linker that a specific section

pm_ovlay_1) from a specified input file

Implementing Software Overlays on ADSP-218x DSPs with VisualDSP++® (EE-249) Page 2 of 6