Segments

Segments are individual chunks of continuous address space, generally with separate characteristics (such as belonging to a particular file). Each segment structure ( struct seg ) contains a base address and a size , links to other segments that belong to the same process, and a pointer to a list of functions. This list, defined in the seg_ops structure, contains addresses of functions that perform a certain set of operations specifically for the given type of segment. These operations include things like:

• Unmapping a portion of a segment

• Removing an entire segment and any hardware mappings for the pages

• Handling a page fault that occurs inside this segment

• Performing a sync

• Returning a pointer to the vnode that is associated with this segment

Some actual functions for the different segment types are:

• segvn_unmap() ” Do an unmap operation on a seg_vn type of segment. A seg_vn is one that refers to a file (a vnode). A memory-mapped file would be a seg_vn type.

• segdev_free() ” Free a segment that referred to a device. A frame buffer would be handled as a seg_dev type of segment.

There are also some generic segment handling functions that deal with the segment structures themselves .

• seg_alloc() ” Allocate a segment for the specified address range and attach it to the list of segments for a particular address space structure.

• seg_attach() ” Insert a segment structure into a list of segments. Called by seg_alloc() to actually do the work.

• seg_unmap() ” Is a front end to the segment-specific unmap operation.

• seg_free() ” Remove a segment structure from the list and actually free up the structure. This function also calls the segment type-specific free function to take care of releasing any resources used by the segment.

Segments are another "virtual" layer, like vnodes, to allow a range of addresses to refer to many different things (a file, a device like a frame buffer, anonymous memory) but still be handled at a high level in a standardized way.