Program Segment Prefix

The Program Segment Prefix (PSP) is a data structure used in DOS systems to store the state of a program.It has the following structure:

The PSP is most often used to get the command line arguments of a DOS program, for example the command "foo.exe -a -f" executes foo.exe with the arguments '-a' and '-f'.

The segment address of the PSP can be determined by using interrupt 21 subfunction 62. This interrupt will return the PSP address in register BX. [ [http://www.htl-steyr.ac.at/~morg/pcinfo/hardware/interrupts/inte8fjk.htm INT 21,62 - Get PSP address (DOS 3 ] ]

Alternatively, in .COM programs, one can address the PSP directly just by using the offsets listed above. 00h points to the beginning of the PSP, FFh points to the end, etc. For example, the following code displays the command line arguments:

org 100h

; int 21h subfunction 9 requires '$' to terminate stringxor bx, bxmov bl, [80h] mov byte [81h+bx] , '$'

; print the stringmov ah, 9mov dx, 81hint 21h

; exitmov ax, 4C00hint 21h

References

External links

* [http://support.microsoft.com/kb/123729 Accessing Command Line Arguments] (microsoft.com)