What's your favorite embedded operating system (OS)? QNX? No? WindRiver? Argh! Due to the use of different processors and varied specifications for applications, the embedded OS market has always had a plethora of products to chose from. This lack of standardization has also discouraged the larger OS companies from offering products in this market space.

With the advent of the Information Appliance age, many of these applications are standardizing upon Internet connectivity, and high volume commodity products are now enticing the big boys to come play. Recently, Microsoft, Oracle, Sun, and Red Hat have all made this market space a strategic part of their corporate visions. But the road to riches has not been an easy one for companies trying to develop the definitive embedded OS.

Within the Information Appliance space there are many segments—from set top boxes to handheld computers. Trying to address all of the features that are required for each segment has been a frustrating ordeal for the OS companies and their developers. Even Microsoft has had its share of ups and downs since it introduced Windows CE back in 1996. However, its latest release of the product, version 3.0, is expected to achieve many of the results that the company hoped to obtain when it first entered this market.

Figure 1: Sub-PC devices such as the new iPaq Pocket PC require an embedded OS with a small footprint

Some of the complaints heard about CE in the past had to do with its real-time support, megalithic software design, instant on capability, migration down from the desktop environment, requirement for a GUI, lack of support, non-open source model, and cost. While some of these complaints were ill founded, Microsoft has attempted to address the legitimate problems. The long awaited release of Windows CE version 3.0 shores up some real-time deficiencies, adds more functionality, provides a tighter integration into the developer studio environment, and incorporates some well known Microsoft applications.

In the last few years Microsoft has restructured the Windows CE organization so that a core OS group focuses on the general platform development and vertical market groups address specific features and functionality. The vertical market groups include the Pocket PC, handheld PC (H/PC), AutoPC, multimedia, mobile phone, and embedded systems groups.

Each group places a high priority on targeting issues and enhancements needed for its vertical market. As an example, the multimedia group needed DirectX technology, which was not available at that time for CE, while other groups didn't require it as a priority item. Because of the urgency for this functionality, the multimedia group did the work to modularize this software piece. It was then immediately offered as part of the package for the Windows CE product for the multimedia vertical market.

This integration work was then passed back to the core OS group so that they could refine and package it into the base product. It is now available as part of the version 3.0 release, which not only highlights the emphasis that Microsoft is placing on meeting individual market needs, but also the modular nature of the OS.

Before highlighting the improvements in version 3.0, it is important to take a look at Window CE's history. It has been widely reported and believed that Windows CE is an OS that migrated from Microsoft desktop operating systems—Windows 95 and Windows NT. Nothing could be further from the truth. While ideas were borrowed from those operating systems, Windows CE was actually developed from scratch.

Microsoft developed CE on the premise that it would have Win32 API compatibility, a small memory footprint, portable code, reliability, and leverage existing development tools (Developer Studio). Trade-offs were then made to support a subset of Win32 API functions so that the memory footprint could be minimized. The Win32 API also had to be redeveloped to achieve greater reliability and support the Windows CE's kernel. A new kernel was also designed so that CE would have real-time features and meet the criteria needed for embedded systems design.

The Microsoft design team did a remarkable job coming up with Window CE's kernel. They set in place limitations to the design of the OS before development began. The design team limited the number of processes to 32 with each process having a virtual address space limit of 32 MB. For information appliances today, this seemed to be a fair limitation. 32 slots would be available for 32 possible processes.

Slot 0 would be a special slot that designated the active process. Window CE then changed the way a process became active. Previously, processes became active by copying program and data memory to a special area in memory, but this came with the associated overhead of installing and cleaning up processes, data, and stacks.

In Windows CE, slot 0 is a special slot that designates an active process and its pointers are directed to the slot where the process actually resides. When a process switch occurs, slot 0's pointers are changed to the new active process. Instead of memory copies to activate a process, pointers are just changed, which significantly reduces the time it takes to switch processes. The kernel also incorporates Protected Server Libraries (PSL) that borrow the resources of a calling thread to minimize the amount of stack space needed and to increase efficiency when used in conjunction with critical sections.

Interrupt handling initially didn't support nested interrupts, but it does now in version 3.0. This allows interrupts with higher priorities to be serviced immediately, instead of waiting for a lower priority interrupt to be completed. Thus real-time performance is enhanced due to the deterministic nature of nested interrupts. Window CE also provides an interrupt API that allows the use of an event handle with an interrupt ID and an interrupt thread service routine. This combination allows for the speedy servicing of interrupts and the triggering of the interrupt thread by synchronization events.

There are an unlimited amount of threads per process. The thread scheduler is a priority-based, round robin scheduler that initially had eight priority levels but now has 256 in version 3.0. The scheduler handles priority inversion. Synchronization methods for threads include critical sections, events, mutexes, and semaphores (new with version 3.0).

From the start, Windows CE has been modularized so that developers could include the components they needed for their applications and eliminate the others. This reduced the size of the code and amount of resources needed for applications. This modularization has also contained the GUI. While the other Windows operating systems required a GUI, Windows CE doesn't. In the early days much software effort was devoted to abstracting the GUI code from the messaging subsystem. Although a GUI is required for debugging on the target system, Windows CE's modularization and customization provides for instant ON capabilities upon power up.

The ability to communicate with other devices is a major priority for Windows CE. The OS supports TAPI 2.1, RAS, serial, IrDA, DCOM, and TCP/IP. Its TCP/IP support includes the WinInet component for HTTP and FTP, ASP and ISAPI Web server support, and DHCP client with Autonet that assigns an IP address if a DHCP server isn't available. However, Network Address Translation (NAT) support is missing from 3.0.

Windows CE has also incorporated Microsoft's embedded applications in its latest release. These include the Mobile Internet Explorer, Pocket Word, and Pocket Inbox email. Mobile Internet Explorer is an embedded browser component that supports .gif and .wav files, frames, tables, JavaScript ActiveX controls, HTML v4.0, DHTML, and cascading style sheets.

Figure 2: Mobile Internet Explorer enables a near-desktop browsing experience on the new Pocket PC

Windows CE's development tools set it apart from most other operating systems. Microsoft's Developer Studio tools support the Window CE development. With its version 3.0 release, Microsoft has tightened the integration with the CE platform by incorporating the platform builder, wizards for the customization of the platform, hardware assisted debugging, and source level debugging.

New to version 3.0 is the Common Executable Format (CEF), which is a processor independent language. This format is built within Visual C++ in the same manner as a normal executable, but is an intermediate set of instructions instead of native machine code instructions. This allows the applications to run on different processors within the same type of platform. Therefore, when a CEF executable is built for the Pocket PC, it will run on all Pocket PCs but will not run on H/PCs.

Windows CE version 3.0 is closing the real-time performance gap with RTOSs and is adding functionality that other embedded operating systems don't provide. All in all, this release will put a lot of pressure on its competitors to keep pace. The development environment and the widespread usage of the Win32 API will allow experienced developers and third party vendors to easily create applications for CE.

The release of the Pocket PC platform platform includes the first set of products to utilize the new version of CE. Judging from the initial buzz about these devices, version 3.0 could put Microsoft back on track in the embedded marketplace.