The _psp variable contains the segment address of the program segment prefix (PSP) for the process. It is declared in the STDLIB.H include file as follows:
extern unsigned int _psp;
The PSP contains execution information about the process, such as a copy of the command line that invoked the process and the return address on process termination or interrupt. The _psp variable can be used to form a long pointer to the PSP, where _psp is the segment value and 0 is the offset value.
Note that the _psp variable is supported only in DOS.