Microsoft KB Archive/48241

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Relationship Between Map File and Physical Memory

Article ID: 48241

Article Last Modified on 9/30/2003


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This article was previously published under Q48241


In situations where memory is very short or where CodeView interacts with your program, it is sometimes necessary to use the DEBUG.COM program supplied with DOS.

Using DEBUG is more difficult than using SYMDEB or CodeView because DEBUG has no symbolic features. You must use the map produced by the /M option when you link with a standard DOS overlay linker (i.e., not a segmented executable linker) to locate specific parts of your program.

However, since DOS relocates programs when it loads them, the addresses given in the map need conversion before you can use them.

This conversion is simple: DOS adds the address of the start segment (defined below) to each segment address in the load map. The offsets never change from the values shown in the link map -- only the segments change.


The start segment is the base address of the Program Segment Prefix (PSP) plus the size of the PSP in paragraphs. Since the PSP is always 100h (256) bytes long, the size of the PSP is 10h paragraphs.

Note: DOS puts the base segment address of the PSP in DS and in ES when a program begins execution.

For example, assume that the link map says that the function _funct is at 0004:05A0 (all values in hex) and that the global variable _errno is at 0192:00E3. Suppose further that when the program is loaded into DEBUG, the DS and ES registers contain 2BA5 -- the segment address of the PSP. (Use the R command to display the values of the registers.)

The start segment for loading the program will be 2BB5 -- the value of the PSP base address (2BA5) + 10h to allow for the 10h paragraph length of the PSP (100h bytes).

Thus, the function _funct will be located as follows:

   0004:05A0   -- address of _funct in the link map
   2BB5        -- start segment address (PSP + 10h)

And _errno will be located as follows:

   0192:00E3   -- address of _funct in the link map
   2BB5        -- start segment address (PSP + 10h)

Use this calculation on any address in the link map to find where the symbol is located in memory when actually loaded.

The H (Hex Arithmetic) command in DEBUG can be helpful when performing these calculations, as can a hex calculator with constant feature.

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