Segmentation

COMPUTER ORGANIZATION AND ARCHITECTURE DESIGNING FOR PERFORMANCE NINTH EDITION

There is another way in which addressable memory can be subdivided, known as
segmentation. Whereas paging is invisible to the programmer and serves the purpose
of providing the programmer with a larger address space, segmentation is usually
visible to the programmer and is provided as a convenience for organizing programs
and data and as a means for associating privilege and protection attributes with
instructions and data.
Segmentation allows the programmer to view memory as consisting of mul-
tiple address spaces or segments. Segments are of variable, indeed dynamic, size.
Typically, the programmer or the OS will assign programs and data to different seg-
ments. There may be a number of program segments for various types of programs as
well as a number of data segments. Each segment may be assigned access and usage
rights. Memory references consist of a (segment number, offset) form of address.

This organization has a number of advantages to the programmer over a non-
segmented address space:

1. It simplifies the handling of growing data structures. If the programmer does
not know ahead of time how large a particular data structure will become, it
is not necessary to guess. The data structure can be assigned its own segment,
and the OS will expand or shrink the segment as needed.
2. It allows programs to be altered and recompiled independently without
requiring that an entire set of programs be relinked and reloaded. Again, this
is accomplished using multiple segments.
3. It lends itself to sharing among processes. A programmer can place a utility
program or a useful table of data in a segment that can be addressed by other
processes.
4. It lends itself to protection. Because a segment can be constructed to contain a
well-defined set of programs or data, the programmer or a system administra-
tor can assign access privileges in a convenient fashion.

These advantages are not available with paging, which is invisible to the pro-
grammer. On the other hand, we have seen that paging provides for an efficient
form of memory management. To combine the advantages of both, some systems
are equipped with the hardware and OS software to provide both.