The Program Segment Prefix (PSP) is a data structure used in
DOS
DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems.
DOS may also refer to:
Computing
* Data over signalling (DoS), multiplexing data onto a signalling channel
* Denial-of-service attack (DoS), an attack on a communicat ...
systems to store the state of a
program
Program, programme, programmer, or programming may refer to:
Business and management
* Program management, the process of managing several related projects
* Time management
* Program, a part of planning
Arts and entertainment Audio
* Programm ...
. It resembles the
Zero Page in the
CP/M
CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/ 85-based microcomputers by Gary Kildall of Digital Research, Inc. Initi ...
operating system. The PSP has the following structure:
The PSP is most often used to get the
command line arguments
A command-line interpreter or command-line processor uses a command-line interface (CLI) to receive commands from a user in the form of lines of text. This provides a means of setting parameters for the environment, invoking executables and pro ...
of a DOS program; for example, the command "FOO.EXE /A /F" executes FOO.EXE with the arguments '/A' and '/F'.
If the PSP entry for the command line length is non-zero and the pointer to the environment segment is neither 0000h nor FFFFh, programs should first try to retrieve the command line from the
environment variable
An environment variable is a dynamic-named value that can affect the way running processes will behave on a computer. They are part of the environment in which a process runs. For example, a running process can query the value of the TEMP envi ...
%CMDLINE% before extracting it from the PSP. This way, it is possible to pass command lines longer than 126 characters to applications.
The segment address of the PSP is passed in the DS register when the program is executed. It can also be determined later by using Int 21h function 51h or Int 21h function 62h. Either function will return the PSP address in register BX.
Alternatively, in
.COM
The domain name .com is a top-level domain (TLD) in the Domain Name System (DNS) of the Internet. Added at the beginning of 1985, its name is derived from the word ''commercial'', indicating its original intended purpose for domains registere ...
programs loaded at offset
100h
, one can address the PSP directly just by using the offsets listed above. Offset
000h
points to the beginning of the PSP,
0FFh
points to the end, etc.
For example, the following code displays the command line arguments:
org 100h ; .COM - not using ds
; INT 21h subfunction 9 requires '$' to terminate string
xor bx,bx
mov bl, 0hcmp bl,7Eh
ja exit ; preventing overflow
mov byte x+81h'$'
; print the string
mov ah,9
mov dx,81h
int 21h
exit:
mov ax,4C00h ; subfunction 4C
int 21h
In DOS 1.x, it was necessary for the CS (Code Segment) register to contain the same segment as the PSP at program termination, thus standard programming practice involved saving the DS register to the stack at program start (since the DS register is loaded with the PSP segment) and terminating the program with a RETF instruction, which would pop the saved segment value off the stack and jump to address 0 of the PSP, which contained an INT 20h instruction.
; save
push ds
xor ax,ax
push ax
; move to the default data group (@data)
mov ax,@data
mov ds,ax
; print message in mess1 (21h subfunction 9)
mov dx,mess1
mov ah,9
int 21h
retf
If the executable was a .COM file, this procedure was unnecessary and the program could be terminated merely with a direct INT 20h instruction or else calling INT 21h function 0. However, the programmer still had to ensure that the CS register contained the segment address of the PSP at program termination. Thus,
jmp start
mess1 db 'Hello world!$'
start:
mov dx,mess1
mov ah,9
int 21h
int 20h
In DOS 2.x and higher, program termination was accomplished instead with INT 21h function 4Ch which did not require the CS register to contain the segment value of the PSP.
See also
*
Zero page (CP/M)
*
CALL 5 (DOS)
The A20, or address line 20, is one of the electrical lines that make up the system bus of an x86-based computer system. The A20 line in particular is used to transmit the 21st bit on the address bus.
A microprocessor typically has a number ...
*
Stack frame
In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, program stack, control stack, run-time stack, or mach ...
(Unix)
*
Process directory
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
*Business process, activities that produce a specific se ...
(Multics)
*
Process identifier
In computing, the process identifier (a.k.a. process ID or PID) is a number used by most operating system kernels—such as those of Unix, macOS and Windows—to uniquely identify an active process. This number may be used as a parameter in vario ...
(PID)
*
this (computer programming)
this, self, and Me are keywords used in some computer programming languages to refer to the object, class, or other entity of which the currently running code is a part. The entity referred to by these keywords thus depends on the execution co ...
*
Self-reference
Self-reference occurs in natural or formal languages when a sentence, idea or formula refers to itself. The reference may be expressed either directly—through some intermediate sentence or formula—or by means of some encoding. In philoso ...
References
Further reading
* (41 pages)
*
* {{cite book , title=DOS 5 für Programmierer: Die endgültige Referenz , language=de , chapter=Kapitel 5: EXEC im Detail - Program Segment Prefix (PSP) , author-first=Arne , author-last=Schäpers , date=1991 , edition=1 , publisher=
Addison Wesley (Deutschland) GmbH , isbn=3-89319-350-2 , pages=148–151, 971–972 (1123+v pages, foldout, 5.25"-floppy)
External links
Accessing Command Line Arguments(Microsoft.com)
DOS technology