Q47959: Description of a .MAP File’s "Program Entry Point"

Article: Q47959
Product(s): See article
Version(s): 5.00 5.10 | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 16-JAN-1990

The "Program entry point" is the last item in a .MAP file. It refers
to the segment:offset address of the first instruction of the program
to be executed, relative to the lowest memory address in the .EXE load
image (the program loaded into RAM). A more detailed description and
the use of entry point(s) are noted below.

The load address can be obtained by adding 10h to the segment address
of the PSP. (In C, this is available in the variable _psp.)

An entry point is where code in a module actually begins execution
via the loading of the CS:IP (CodeSegment:InstructionPointer --
segment:offset) registers with the entry point address. The entry
point could be the actual beginning of a program executed by the
operating system, or it could be an exported function from an
OS/2 or DOS Windows dynamic link library (DLL).

The "Program entry point" specified at the end of a .MAP file is where
the operating system will begin executing the program, RELATIVE to the
lowest memory address where the program's code will be loaded. In DOS
or real-mode OS/2, the lowest memory address will be the lowest
physical address in RAM. In protected-mode OS/2, the lowest relative
address will be segment selector 1, offset 0.

The lowest address your code occupies in RAM is usually the first
routine (function, procedure, or main program body) defined at the
beginning of your main source file, and it is assigned a RELATIVE
address of 0000:0000 [or, typically, 0:10 for main()] in DOS or
real-mode OS/2, or an address of 0001:0000 in protected-mode OS/2.

For our current high-level languages (except Microsoft QuickPascal
1.00), the entry point is at the beginning of the routine "__astart".
Our high-level languages require initialization code before executing
C's main() function or the main program code for other languages such
as Microsoft Pascal 3.3x or 4.00, and currently __astart is where the
initializations begin.

The "Program entry point" is a segment:offset address above the
program's relative address of 0:0. For a C program, the entry point
will be __astart. If the entry point is something like 2:0abc hex, the
absolute DOS physical address or the virtual OS/2 address where
__astart will be located can be calculated as follows:

     lowest address of your program's load image  1234:0400
   + relative entry point                         +  2:0abc
   ---------------------------------------------  ---------
   = DOS absolute or OS/2 virtual address of      1236:0e34
     the entry point (__astart)

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.