Accessing Command Line Arguments

• Microsoft Macro Assembler for MS-DOS, versions 5.1, 6.0, 6.1, and 6.11

SUMMARY

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

Method One

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

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.

REFERENCE

For more information on the PSP and the command tail, please see the PSP table in the "Microsoft Macro Assembler Reference."