Description of the Map File Microsoft LINK Creates

MS-DOS                           | OS/2

The information in this article applies to:

• Microsoft LINK for MS-DOS, versions 3.x, 4.x, 5.0x, 5.1x, 5.2, 5.3x, and 5.5
• Microsoft LINK for OS/2, versions 5.0x, 5.1, 5.11, 5.13, and 5.15

SUMMARY

When the Microsoft LINK command line includes the /MAP option, the <mapfile> field specifies the name for the map file. The default map file name is the base name of the executable file with the extension .MAP. To suppress creating a map file, set the map filename to NUL.

The map file contains the following contents:

• For each logical segment, the Start address, Stop address, Length, Name, and Class
• Groups
• Program Entry point

• Public symbols by Value

• Contributions of each object file to a segment (this information follows the description of each logical segment)
• Public symbols by Name
• Undecorated names of C++ symbols

MORE INFORMATION

In applications developed for MS-DOS or OS/2 in real mode, the Start and Stop addresses are 20-bit hexadecimal addresses relative to the application's load image. In applications developed for Microsoft Windows or OS/2 in protected mode, the Start and Stop addresses are 32-bit addresses in segment:offset notation. In OS/2, the segment value represents a selector in the descriptor table that contains information about the segments.

A group is a collection of logical segments that total not more than 64K in size. To create a group, use the GROUP assembler directive. Microsoft language products create the group DGROUP that contains the _NULL, _DATA, CONST, C_COMMON, _BSS, and STACK segments, among others. For additional information, please see the following article(s) in the Microsoft Knowledge Base:

ARTICLE-ID: Q48241

TITLE     : Relationship Between Map File and Physical Memory

   RES --  The symbol resides in the resident portion
   OVL --  The symbol resides in an overlay
   ABS --  The symbol represents a constant value

   EXP --  If the symbol was exported
   IMP --  If the symbol was imported

In an application developed for MS-DOS, the start segment is the base address of the program segment prefix (PSP) plus the size of the PSP (100h bytes). The loader places this value into the DS and ES registers when the application begins execution. To retrieve this segment address in C, declare the following variable:

   extern unsigned _psp;

In a protected-mode environment, there is no reliable method to determine the address of a symbol because segments may move in memory or be discarded.