Program Segment Prefix: Difference between revisions

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Revision as of 21:16, 19 December 2019

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{{Short description|data structure in DOS}}

{{Use dmy dates|date=May 2019|cs1-dates=y}}

The '''Program Segment Prefix''' (PSP) is a data structure used in [[DOS]] systems to store the state of a [[computer program|program]]. It resembles the [[Zero page (CP/M)|Zero Page]] in the [[CP/M]] operating system. The PSP has the following structure:

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The PSP is most often used to get the [[argc|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 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.<ref>{{cite web|urlname=http:"R1"//www.htl-steyr.ac.at/~morg/pcinfo/hardware/interrupts/inte8fjk.htm|title=INT 21h,62h - Get PSP address (DOS 3.x)|url-status=dead|archiveurl=https://web.archive.org/web/20120207062050/http://www.htl-steyr.ac.at/~morg/pcinfo/hardware/interrupts/inte8fjk.htm|archivedate=2012-02-07}}</ref>

Alternatively, in [[.COM]] programs loaded at offset <tt>100h</tt>, one can address the PSP directly just by using the offsets listed above. Offset <tt>000h</tt> points to the beginning of the PSP, <tt>0FFh</tt> points to the end, etc.

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<source lang="nasm">

org 100h ; .COM - not using ds

; INT 21h subfunction 9 requires '$' to terminate string

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mov bl, [80h]

cmp bl, 7Eh

ja exit ; preventing overflow

mov byte [bx + 81h], '$'

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exit:

mov ax, 4C00h ; subfunction 4C (see below)

int 21h

</source>

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<source lang="nasm">

; 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

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<source lang="nasm">

jmp start

mess1 db 'Hello world!$'

start:

mov dx, mess1

mov ah, 9

int 21h

int 20h

</source>

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== References ==

{{reflist}}|refs=

<ref name="R1">{{cite web |url=http://www.htl-steyr.ac.at/~morg/pcinfo/hardware/interrupts/inte8fjk.htm |title=INT 21h,62h - Get PSP address (DOS 3.x) |url-status=dead |archive-url=https://web.archive.org/web/20120207062050/http://www.htl-steyr.ac.at/~morg/pcinfo/hardware/interrupts/inte8fjk.htm |archive-date=2012-02-07}}</ref>

}}

==Further reading==

* {{cite web |title=Format of Program Segment Prefix (PSP) |date=2000 |work=[[INTER61]] |url=http://www.delorie.com/djgpp/doc/rbinter/it/78/13.html |access-date=2019-12-19 |url-status=live |archive-url= |archive-date=}}

==External links==

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

[[Category:DOS technology]]