Controlling Segment Ordering (123686)

SUMMARY

This article discusses the segment ordering directives and explains how to get segments into a desired order.

The GROUP directive does not control segment ordering. When you create a group by using the GROUP directive, you are telling the linker to treat multiple logical segments as one physical segment, known as a group. Placing two logical segments into a group forms one physical segment in the resulting executable that is composed of the two logical segments. However, the logical segments are not placed in any specific order.

The segment ordering in an executable is determined by the segment ordering directive you use. The three segment ordering directives are: .ALPHA, .SEQ, and .DOSSEG. However, the linker can use only one segment ordering in a given executable.

MORE INFORMATION

If you use the .ALPHA directive, the linker places segments into the executable in alphabetical order. In this case, segment ABC would be placed before segment DEF.

If you use the .SEQ directive or if you don't specify any of the directives, the linker places segments into the executable in the order they are encountered. Therefore, the linker makes the first segment it sees in the first .OBJ file into the first segment in the executable. It makes the next segment it sees in the first .OBJ into the second segment, and so on. When there are no more segments in the first .OBJ file, the linker processes the segment's second .OBJ file, and so on, until all .OBJ files have been processed.

If you use the .DOSSEG directive, specify .MODEL, or link with any high level language, such as C or FORTRAN, the linker places segments into the executable according to the segment class and sequential order. The linker looks at the segments class and if the class ends in CODE, the segment is placed towards the beginning of the executable. If the segment has class STACK or class BSS, the linker places it towards the end of the executable. Segments with other classes are placed in the middle. If there are multiple segments in any grouping of classes, the segments are placed in sequential order within that grouping.

One frequently used method to get segments into a specific order is to use the .SEQ or .DOSSEG ordering and specify all the segments at the top of each source file or in a file that you include into each source file. For example:

   header SEGMENT para public 'NOT_REALLY_CODE'
   header ENDS
   driver_code SEGMENT para public 'CODE'
   driver_code ENDS
   install_code SEGMENT para public 'DATA'
   install_code ENDS

If you use the .SEQ segment ordering directive, the linker will see these empty segment declarations first and place the segments in this order. If you use the .DOSSEG segment ordering directive, the linker will put segments header and driver_code towards the beginning of the executable because they both have class names that end in CODE. The linker then places header before driver_code because it saw header first, so again you end up with same segment ordering.

NOTE: For this technique to work, you must use a combine type of PUBLIC so that segment declarations with the same name are treated as the same segment.