Mondrian memory protection (MMP) is a fine-grained protection scheme that allows multiple protection domains to flexibly share memory and export protected services. In contrast to earlier pagebased systems, MMP allows arbitrary permissions control at the granularity of individual words. We use a compressed permissions table to reduce space overheads and employ two levels of permissions caching to reduce run-time overheads. The protection tables in our implementation add less than 9% overhead to the memory space used by the application. Accessing the protection tables adds less than 8% additional memory references to the accesses made by the application. Although it can be layered on top of demandpaged virtual memory, MMP is also well-suited to embedded systems with a single physical address space. We extend MMP to support segment translation which allows a memory segment to appear at another location in the address space. We use this translation to implement zero-copy networking underneath the standard read system call interface, where packet payload fragments are connected together by the translation system to avoid data copying. This saves 52% of the memory references used by a traditional copying network stack.

Many modern systems, including web servers, database engines, and operating system kernels, are using language-based protection mechanisms to provide the safety and integrity traditionally supplied by hardware. As these language-based systems become used in more demanding situations, they are faced with the same problems that traditional operating systems have solved|namely shared resource management, process separation, and per-process resource accounting. While many incremental changes to language-based, extensible systems have been proposed, this thesis demonstrates that comprehensive solutions used in traditional operating systems are applicable and appropriate. This thesis describes Alta, an implementation of the Fluke operating system's nested process model in a Java virtual machine. The nested process model is a hierarchical operating system process model designed to provide a consistent approach to user-level, per-process resource accounting and control. This model accounts fo...

Since the time of D.ck Tracy cartoons, the idea of wearable videoconferencing devices has been a desirable but unachievable goal. Current technology is now on the verge of making this dream a reality. Existing and future low bit-rate video coding standards such as H.263 and MPEG-4 may require specialised hardware for real-time handheld video conferencing. This paper evaluates the performance of software-based videoconferencing on widely available mobile handheld devices using a range of both traditional and original video coding schemes. Our results demonstrate that wearable videoconferencing can no longer be relegated to the realm of science fiction as practical first generation devices are feasible today.

committee and by majority vote has been found to be satisfactory.

Recent improvements in the technology of handheld computers and mobile communications have made the task of exploring handheld mobile video conferencing feasible. Handheld mobile video conferencing introduces a number of constraints that affect the type of coders and protocols that are used. This research project has explored audio and video coders that are suitable for use in a handheld wireless environment. Six different video coders, both existing and new, were evaluated in the areas of computational complexity, stability of bit-rate and quality. Of these, two coding schemes that have not previously been applied to video coding were designed and evaluated. The first is based on the forest of Quadtrees coding technique which improves the stability of the bit-rate of the Quadtree coding technique. The forest of Quadtrees offers low computational complexity making it suitable for handheld computing. The second is a block-based conditional replenishment SPIHT coding scheme that uses th...