1cal_sum:

8for_loop:

Figure 4.8: Assembly version of sum

4.6Multi-Module Programs

A multi-module program is one composed of more than one object file. All the programs presented here have been multi-module programs. They consisted of the C driver object file and the assembly object file (plus the C library object files). Recall that the linker combines the object files into a single executable program. The linker must match up references made to each label in one module (i.e. object file) to its definition in another module. In order for module A to use a label defined in module B, the extern directive must be used. After the extern directive comes a comma delimited list of labels. The directive tells the assembler to treat these labels as external to the module. That is, these are labels that can be used in this module, but are defined in another. The asm io.inc file defines the read int, etc. routines as external.

In assembly, labels can not be accessed externally by default. If a label

3; subprogram code

4leave

5ret

Figure 4.10: General subprogram form with local variables using ENTER and

LEAVE

can be accessed from other modules than the one it is defined in, it must be declared global in its module. The global directive does this. Line 13 of the skeleton program listing in Figure 1.7 shows the asm main label being defined as global. Without this declaration, there would be a linker error. Why? Because the C code would not be able to refer to the internal asm main label.

Next is the code for the previous example, rewritten to use two modules.

The two subprograms (get int and print sum) are in a separate source file than the asm main routine.

main4.asm

1%include "asm_io.inc"

2

3segment .data

5

6segment .bss

7 input resd 1

8

9segment .text

10global _asm_main

11extern get_int, print_sum

12_asm_main: