Q123729: Accessing Command Line Arguments
Article: Q123729
Product(s): Microsoft Macro Assembler
Version(s): 5.1,6.0,6.1,6.11
Operating System(s):
Keyword(s):
Last Modified: 04-FEB-2002
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The information in this article applies to:
- Microsoft Macro Assembler (MASM), versions 5.1, 6.0, 6.1, 6.11
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SUMMARY
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In C, you access command line arguments via the argc and argv parameters in your
main function. MASM doesn't have a similarly built-in feature in a main
function, but there are two ways to get to the command line arguments for your
program.
MORE INFORMATION
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Method One
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The first way to get the command line arguments for a MASM program is to access
the command tail in the Program Segment Prefix (PSP). The command tail is one or
more bytes of ASCII text representing a program's command line arguments. When
your program starts, the ds and es segment registers initially point to the PSP.
The byte value at PSP+80h is the length of the command tail. Therefore, you can
access the command tail length through either ds:80h or es:80h. The command tail
itself begins at PSP+81h.
The command tail is so named because it contains everything that appears in the
command line after the command name. The arguments are unformatted; they appear
exactly as the user typed them in, including any extra spaces. It is up to you
to parse the command string. Typically, if you use COMMAND.COM to load the
program, the first character of the command tail is the space that separated the
command name from the first argument. If your program was called from another
program, the command tail may not contain the leading space.
NOTE: 80h-FFh in the PSP is the default Disk Transfer Area (DTA). Therefore, if
you use MS-DOS calls that use the DTA, such as file I/O, the command line
arguments will be overwritten. To avoid this problem, either extract the command
tail before you do anything else in your code or set up a custom DTA.
Method Two
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The second way to get the command line arguments for a MASM program is to access
the command line at the end of the environment block. The environment block
contains zero-terminated ASCII strings, each of which represents the name and
value of an environment variable. After the last string in the environment
block, there are two zero terminators instead of one. These are followed by the
full command line, including the command name that was used to start your
program. The segment for the environment block can be found at offset 2Ch in the
PSP. Therefore, you will need to load this segment value into a segment
register, such as es, and access the environment block from there.
REFERENCES
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For more information on the PSP and the command tail, please see the PSP table
in the "Microsoft Macro Assembler Reference."
Additional query words: kbinf 5.10 6.00 6.10
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Keywords :
Technology : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM600 kbMASM610 kbMASM611
Version : :5.1,6.0,6.1,6.11
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