Real-Time Operating Systems for DSP, part 1

Modern DSP applications must respond quickly to many external events, be able to prioritize processing, and perform many tasks at once. These complex applications are also changing rapidly over time, responding to ever changing market conditions. Time to market has become more important than ever. DSP developers, like many other software developers, must now be able to develop applications that are maintainable, portable, reusable, and scalable. Modern DSP systems are managed by real-time operating systems that manage multiple tasks, service events from the environment based on an interrupt structure, and effectively manage the system resources.

What Makes an OS an RTOS?
An operating system is a computer program that is initially loaded into a processor by a boot program. It then manages all the other programs in the processor. The other programs are called applications or tasks. (I'll describe tasks in more detail later.)

The operating system is part of the system software layer (Figure 1). The function of the system software is to manage the resources for the application. Examples of system resources that must be managed are peripherals like a DMA, HPI, or on-chip memory. The DSP is a processing resource to be managed and scheduled like other resources.

The system software provides the infrastructure and hardware abstraction for the application software. As application complexity grows, a real-time kernel can simplify the task of managing the DSP MIPS efficiently using a multitasking design model. The developer also has access to a standard set of interfaces for performing I/O as well as handling hardware interrupts.

Figure 1: Embedded DSP software components.

Migration to next generation processors is also easier because of the hardware abstraction provided by a real-time kernel and associated peripheral support software.

The applications make use of the operating system by making requests for services through a defined application program interface (API).

A general-purpose operating system performs these services for applications:

• In a multitasking operating system where multiple programs can be running at the same time, the operating system determines which applications should run in what order and how much time should be allowed for each application before giving another application a turn.
  
  
• It manages the sharing of internal memory among multiple applications.
  
  
• It handles input and output to and from attached hardware devices, such as hard disks, printers, and dial-up ports.
  
  
• It sends messages to each application or interactive user about the status of operation and any errors that may have occurred.
  
  
• It can offload the management of what are called batch jobs (for example, printing) so that the initiating application is freed from this work.
  
  
• On computers that can provide parallel processing, an operating system can manage how to divide the program so that it runs on more than one processor at a time.