Garbage collectors used in embedded systems such as PersonalJava and Inferno or in operating systems such as SPIN must operate with limited resources and minimize their impact on application performance. Consequently, they must maintain short real-time pauses, low overhead, and a small memory footprint. Most garbage collectors, including the Treadmill algorithm, are inadequate because they sacrifice space for time. We have implemented a new Treadmill variant that provides good memory utilization by using real-time page-level management techniques that reduce fragmentation. A page-wise collection is used to locate pages of free memory, which are then dynamically reassigned between free lists as needed. Virtual memory is used to dynamically remap free pages into continuous ranges, and objects are allocated from under-utilized pages when needed. Finally, the use of header compaction and arbitrary free-list sizes reduces internal fragmentation. Our experiments demonstrate that we have substantially improved memory utilization without compromising latency or overhead, and that the new collector performs very well for SPIN’s workloads and regular Modula-3 programs. 1.1 Keywords Garbage collection, real-time, Treadmill, operating systems

Many modern extensible systems, such as Java and the SPIN operating system, depend on type safety for memory protection. Unfortunately, current type-safe languages do not support systems programming well, because they do not give programmers the ability to deal with untyped data easily. In particular, they do not support the ability to cast between untyped data and language-level types. We describe a powerful, type-safe cast operator that helps programmers write low-level systems codes in type-safe languages. We have implemented this operator in Modula-3 for the SPIN operating system, and we give specific examples of how we use it in SPIN. © 1998 John Wiley & Sons, Ltd.

No abstract available.

Modern operating systems are strongly dependent on software mechanisms to protect system resources from users. This is true despite the fact that the promoters of these systems imply that their reliability and integrity derive solely from the use of a core set of protected hardware mechanisms, such as address spaces and protected supervisor mode. While typical microprocessors provide cheap and effective hardware mechanisms to protect the load word/store word interface, operating systems are forced to abstract and virtualize this interface to export a far richer set, of resources such as files, sockets, threads, and consoles. The access semantics for these resources are almost always protected by software checks and not hardware. Processor architectures simply do not provide enough fine-grained control over access to shared system resources to ensure that a program only accesses the resources to which it is allowed. Our position is that software protection mechanisms are not only necessary, but have inherent advantages over hardware for enforcing the protection requirements of an operating system. Software is flexible, explicit, precise, and in many cases, open to incredible optimizations. By contrast, hardware mechanisms are rigid, implicit, imprecise, and unoptimizable.

This paper describes the motivation, architecture and performance of SPIN, an extensible operating system. SPIN provides an extension infrastructure, together with a core set of extensible services, that allow applications to safely change the operating system's interface and implementation. Extensions allow an application to specialize the underlying operating system in order to achieve a particular level of performance and functionality. SPIN uses language and link-time mechanisms to inexpensively export fine-grained interfaces to operating system services. Extensions are written in a type safe language, and are dynamically linked into the operating system kernel. This approach offers extensions rapid access to system services, while protecting the operating system code executing within the kernel address space. SPIN and its extensions are written in Modula-3 and run on DEC Alpha workstations.

Describes a structuring mechanism for grouping objects in a distributed object-oriented language. A group structuring mechanism provides a single flexible method for managing distributed applications that involve complicated communication protocols and sophisticated structure. We have added such a mechanism to the Emerald distributed object-oriented language and its run-time system. Our group structuring mechanism fits entirely within the context of object-oriented programming, so similar mechanisms could be added to other distributed object-oriented languages.>