Hardware Reference
In-Depth Information
20K
16K
12K
Symbol
table
8K
Call
stack
S
ource
text
Parse
tree
4K
Constant
table
0
Segment
0
Segment
1
Segment
2
Segment
3
Segment
4
Figure 6-8.
A segmented memory allows each table to grow or shrink indepen-
dently of the other tables.
A segmented memory has other advantages besides simplifying the handling of
data structures that are growing or shrinking. If each procedure occupies a sepa-
rate segment, with address 0 as its starting address, the linking up of procedures
compiled separately is greatly simplified. After all the procedures that constitute a
program have been compiled and linked up, a procedure call to the procedure in
segment
n
will use the two-part address (
n
, 0) to address word 0 (the entry point).
If the procedure in segment
n
is subsequently modified and recompiled, no
other procedures need be changed (because no starting addresses have been modi-
fied), even if the new version is larger than the old one. With a one-dimensional
memory, the procedures are normally packed tightly next to each other, with no ad-
dress space between them. Consequently, changing one procedure's size can affect
the starting address of other, unrelated, procedures. This, in turn, requires modi-
fying all procedures that call any of the moved procedures, in order to incorporate
their new starting addresses. If a program contains hundreds of procedures, this
process can be costly.
Segmentation also facilitates sharing procedures or data among several pro-
grams. If a computer has several programs running in parallel (either true or simu-
lated parallel processing), all of which use certain library procedures, it is wasteful
of main memory to provide each one with its own private copy. If we make each
procedure a separate segment, they can be shared easily, thus eliminating the need
for more than one physical copy of any shared procedure to be in main memory.
As a result, memory is saved.
Because each segment forms a logical entity of which the programmer is
aware, such as a procedure, or an array, or a stack, different segments can have dif-
ferent kinds of protection. A procedure segment could be specified as execute
only, prohibiting attempts to read from it or store into it. A floating-point array
could be specified as read/write but not execute, and attempts to branch to it would
be caught. Such protection is frequently helpful in catching programming errors.
