Address spaces

Every process has its own address space, so every process table entry contains a pointer to an address space structure just for that process. The structure serves mostly as an anchor for a list of segments, which describe the valid ranges of virtual addresses that each individual process has. The structure contains global pieces of information, such as a flag that indicates that the entire process is locked in memory, and a count of the number of physical pages actually in use by this process (the a_rss field: the resident set size ).

A few functions deal with the address space as a whole. Some of these are:

• as_segat() ” Find and return the segment that contains the specified address.

• as_alloc() ” Allocate a new address space structure.

• as_dup() ” Create a copy of an existing structure.

• as_fault() ” Handle a fault at the given address for "size" bytes. This function is the one eventually called to handle a page fault.

• as_unmap() ” Unmap all the segments that cover the given address range.

There are more functions; they have similar names and deal with various requirements at this level: locking and unlocking address ranges, finding an empty memory range for mapping something in, adding a segment to the list, and so forth.