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RVM Segment Loader

Designed and programmed by David C. Steere

The RVM segment loader provides a mechanism that allows a load map to be created for a segment. Regions can then be mapped to specific virtual memory addresses or to virtual memory allocated by RVM in a single library call. There are only two library functions: rvm_create_segment and rvm_load_segment.

To use the segment loader, a segment header must be created by rvm_create_segment. The header, which occupies the first page of the segment, contains the load map for regions defined within the segment, and a version identifier so that rvm_load_segment can recognize suitable segments. All region lengths must be integral multiple of the page size of the machine, which is available as the macro RVM_PAGE_SIZE. Virtual memory addresses must be page-aligned.

The load map is prepared in a vector of descriptors for the desired regions (type rvm_region_def_t). The descriptors specify just two parameters: the region length and the virtual memory address where the region is to be loaded. The offsets of the regions in the segment are assigned by rvm_create_segment. If pointers are included in the data, a fixed virtual memory address must be specified so that the loader will place the region there at each loading. If there are no pointers, the segment loader can be instructed to allocate space for the region by specifying an address of zero. The segment loader will make virtual memory addressable as necessary when loading.

The virtual memory address for the first region should be specified far enough beyond the applications break point so that the program can grow during development. Otherwise, there will have to be a way to modify the pointers or rebuild the structures.

The segment header establishes the load map for only that segment. If multiple segments are used, each must have its own header, and rvm_load_segment must be called for each.

The segment loader is used by the RDS allocator (see Chapter RDS, A Dynamic Heap Allocator) to automate loading of a persistent heap region and a statically allocated region. A utility program is available to create the segment header and initialize the heap so that it is easily loaded.

The header file giving all necessary definitions for the RVM segment loader is included in Appendix C Declaration of RVM.