Page 121 - DCAP507_SYSTEM_SOFTWARE

Unit 8: Introduction to Linking and Loading

Nothing is wrong in it. But from assembler's point of vision while producing the code for B, as Notes
B is not defined in the segment A, the assembler can not locate the value of this symbolic
reference and therefore it will state it as an error. To conquer problem, there should be some
method by which the assembler should be explicitly learned that segment B is really defined in
some other segment C.

!
Caution Whenever segment B is used in segment A and if at all B is defined in C, then B
must be stated as an external routine in A.
To state such subroutine as external, we can use the assembler directive EXT. Thus, the statement
such as EXT B should be added at the beginning of the segment A. This really helps to notify
assembler that B is defined somewhere else. Likewise, if one subroutine or a variable is defined
in the existing segment and can be pointed by other segments then those should be stated by
means of pseudo-ops INT. Thus, the assembler could inform loader that these are the subroutines
or variables used by other segments.

Did u know? This general process of establishing the relations among the subroutines can
be theoretically known as a_ subroutine linkage.

Example:

At the beginning of the MAIN the subroutine B is stated as external. When a call to subroutine B
is prepared, before creating the unconditional jump, the recent content of the program counter
should be accumulated in the system stack preserved internally. Likewise while returning from
the subroutine B (at RET) the pop is executed to restore the program counter of caller routine
with the address of next instruction to be implemented.

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