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Q48241: Relationship Between Map File and Physical Memory

Article: Q48241
Product(s): Microsoft Programming Utilities
Version(s): 3.x,4.x,5.x
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 23-OCT-1999

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The information in this article applies to:

- Microsoft LINK for MS-DOS, versions 3.x, 4.x, 5.x 
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SUMMARY
=======

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.

MORE INFORMATION
================

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)
  ---------
  2BB9:05A0

And _errno will be located as follows:

  0192:00E3   -- address of _funct in the link map
  2BB5        -- start segment address (PSP + 10h)
  ---------
  2D47:00E3

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.

Additional query words: no32bit kbinf LinkIss

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbZNotKeyword3 kbLINKSearch kbLINK3xDOSSearch kbLINK4xDOSSearch kbLINK5xDOSSearch
Version           : :3.x,4.x,5.x

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