Книга: Distributed operating systems


Mach has a powerful, elaborate, and highly flexible memory management system based on paging, including features found in few other operating systems. In particular, it separates the machine-independent parts of the memory management system from the machine-dependent parts in an extremely clear and unusual way. This separation makes the memory management far more portable than in other systems. In addition, the memory management system interacts closely with the communication system, which we will discuss in the following section.

The aspect of Mach's memory management that sets it apart from all others is that the code is split into three parts. The first part is the pmap module, which runs in the kernel and is concerned with managing the MMU. It sets up the MMU registers and hardware page tables, and catches all page faults. This code depends on the MMU architecture and must be rewritten for each new MMU Mach has to support. The second part, the machine-independent kernel code, is concerned with processing page faults, managing address maps, and replacing pages.

The third part of the memory management code runs as a user process called a memory manager or sometimes an external pager. It handles the logical (as opposed to physical) part of the memory management system, primarily management of the backing store (disk). For example, keeping track of which virtual pages are in use, which are in main memory, and where pages are kept on disk when they are not in main memory are all done by the memory manager.

The kernel and the memory manager communicate through a well-defined protocol, making it possible for users to write their own memory managers. This division of labor gives users the ability to implement special-purpose paging systems in order to write systems with special requirements. It also has the potential for making the kernel smaller and simpler by moving a large section of the code out into user space. On the other hand, it has the potential for making it more complicated, since the kernel must protect itself from buggy or malicious memory managers, and with two active entities involved in handling memory, there is now the danger of race conditions.

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