Hardware Reference
In-Depth Information
Object module A
Object module B
400
600
CALL B
300
500
CALL C
200
400
MOVE PTOX
300
MOVE QTOX
100
BRANCH TO 200
200
0
100
Object module C
500
BRANCH TO 300
0
400
CALL D
Object module D
300
300
200
MOVE STOX
MOVE RTOX
200
100
100
0
0
BRANCH TO 200
BRANCH TO 200
Figure 7-13.
Each module has its own address space, starting at 0.
300. In fact, all memory reference instructions will fail for the same reason.
Clearly something has to be done.
This problem, called the
relocation problem
, occurs because each object mod-
ule in Fig. 7-13 represents a separate address space. On a machine with a seg-
mented address space, such as the x86, theoretically each object module could
have its own address space by being placed in its own segment. However,
OS/2
was the only operating system for the x86 that supported this concept. All versions
of Windows and
UNIX
support only one linear address space, so all the object
modules must be merged together into a single address space.
Furthermore, the procedure call instructions in Fig. 7-14(a) will not work ei-
ther. At address 400, the programmer had intended to call object module
B
, but be-
cause each procedure is translated by itself, the assembler has no way of knowing
what address to insert into the
CALL B
instruction. The address of object module
B
is not known until linking time. This problem is called the
external reference
problem. Both of these problems can be solved in a simple way by the linker.
The linker merges the separate address spaces of the object modules into a sin-
gle linear address space in the following steps:
