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{{Use dmy dates|date=May 2019|cs1-dates=y}}
{{Use list-defined references|date=December 2021}}
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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:
Line 11 ⟶ 12:
| 00h–01h
| 2 bytes (code)
| [[CP/M-80]]-like<!-- not CP/M-86! --> exit (always contains [[DOS_API#Interrupt_vectors_used_by_DOS|INT 20h]])<ref name="Taylor_1982_Translators"/><ref name="Paul_2002_COM"/>
|-
| 02h–03h
Line 27 ⟶ 28:
| 0Ah–0Dh
| [[dword]] (4 bytes)
| Terminate address of previous program (old [[DOS_API#Interrupt_vectors_used_by_DOS|INT 22h]])
|-
| 0Eh–11h
| dword
| Break address of previous program (old [[DOS_API#Interrupt_vectors_used_by_DOS|INT 23h]])
|-
| 12h–15h
| dword
| Critical error address of previous program (old [[DOS_API#Interrupt_vectors_used_by_DOS|INT 24h]])
|-
| 16h–17h
Line 71 ⟶ 72:
| 40h–41h
| word
| DOS version to return (DOS
|-
| 42h–4Fh
Line 137 ⟶ 138:
</syntaxhighlight>
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
<syntaxhighlight lang="nasm">
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<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>
<ref name="Taylor_1982_Translators">{{cite magazine |title=Upward migration - Part 1: Translators - Using translation programs to move CP/M-86 programs to CP/M and MS-DOS<!-- this is the actual but faulty title --> |trans-title=<!-- this is how it should have been titled -->Using translation programs to move CP/M programs to CP/M-86 and MS-DOS |author-first1=Roger |author-last1=Taylor |author-first2=Phil |author-last2=Lemmons |date=June 1982 |magazine=[[BYTE]] |issn=0360-5280 |id={{CODEN|BYTEDJ}} |volume=7 |number=6 |publisher=[[BYTE Publications Inc.]] |pages=321–322, 324, 326, 328, 330, 332, 334, 336, 338, 340, 342, 344 [342, 344] |url=https://tech-insider.org/personal-computers/research/acrobat/8206-b.pdf |access-date=2020-01-15 |url-status=live |archive-url=https://web.archive.org/web/20200116024623/https://tech-insider.org/personal-computers/research/acrobat/8206-b.pdf |archive-date=2020-01-16 |quote=[…] Gaining Access to [[CP/M-86]] […] Gaining access to CP/M-86 requires placing the function code in the CL register, placing the byte parameter in the DL register or placing the word parameter in the DX register, placing the data segment in the DS register (the data segment is usually not changed for a converted program), and executing a software interrupt, [[INT E0h|INT #224]]. The result is returned in the AL register if it is a byte value; if the result is a word value, it is returned in both the AX and BX registers. Double-word values are returned with the offset in the BX registers and the segment in the ES register. Conversion of programs from [[CP/M-80]] to CP/M-86, then, requires replacing the call to ___location 5 with the software interrupt INT #224. Another necessary change involves the [[warm boot]]. Under CP/M-80, the warm boot may be accessed by a system call with a function code of 0 for a jump to ___location 0. CP/M-86, however, does not support the jump to ___location 0. As a result, you must change this program exit in the translated program if the program is to run correctly. Provided that the call to ___location 5 is replaced with INT #224, that the warm boot change is made, and that the registers are mapped correctly, there should be little problem in getting the translated program to access the CP/M-86 system functions. […] Gaining Access to [[MS-DOS]] […] Although MS-DOS has a "preferred" mechanism through a soft-ware interrupt, [[INT 21h|INT #33]], for accessing the system, an additional mechanism is provided for "preexisting" programs that is compatible with CP/M-80 calling conventions, at least for functions in the range of 0-36. As far as system calls within the allowed function range are concerned, the programmer doesn't have to do anything to translated programs to get them to run under MS-DOS other than to correctly map the registers. MS-DOS also supports the warm boot function of CP/M-80. A jump to ___location 0 under MS-DOS executes a software interrupt, [[INT 20h|INT #32]], which is functionally a program end and the normal way to exit from a program. […]}} [https://archive.org/details/byte-magazine-1982-06] [https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Byte-IDX/IDX/80s/82-83/Byte-1982-06-OCR-Page-0323.pdf][https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Byte-IDX/IDX/80s/82-83/Byte-1982-06-OCR-Page-0324.pdf][https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Byte-IDX/IDX/80s/82-83/Byte-1982-06-OCR-Page-0326.pdf][https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Byte-IDX/IDX/80s/82-83/Byte-1982-06-OCR-Page-0328.pdf][https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Byte-IDX/IDX/80s/82-83/Byte-1982-06-OCR-Page-0330.pdf][https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Byte-IDX/IDX/80s/82-83/Byte-1982-06-OCR-Page-0332.pdf][https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Byte-IDX/IDX/80s/82-83/Byte-1982-06-OCR-Page-0334.pdf][https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Byte-IDX/IDX/80s/82-83/Byte-1982-06-OCR-Page-0336.pdf][https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Byte-IDX/IDX/80s/82-83/Byte-1982-06-OCR-Page-0338.pdf][https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Byte-IDX/IDX/80s/82-83/Byte-1982-06-OCR-Page-0340.pdf][https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Byte-IDX/IDX/80s/82-83/Byte-1982-06-OCR-Page-0342.pdf][https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Byte-IDX/IDX/80s/82-83/Byte-1982-06-OCR-Page-0344.pdf][https://www.americanradiohistory.com/hd2/IDX-Consumer/Archive-Byte-IDX/IDX/80s/82-83/Byte-1982-06-OCR-Page-0346.pdf] (13 pages)</ref>
<ref name="Schulman_1994_Undocumented-DOS">{{cite book |author-first1=Andrew |author-last1=Schulman |author-first2=Ralf D. |author-last2=Brown |author-link2=Ralf D. Brown |author-first3=David |author-last3=Maxey |author-first4=Raymond J. |author-last4=Michels |author-first5=Jim |author-last5=Kyle |title=Undocumented DOS: A programmer's guide to reserved MS-DOS functions and data structures - expanded to include MS-DOS 6, Novell DOS and Windows 3.1 |publisher=[[Addison Wesley]] |edition=2 |date=1994 |orig-year=November 1993<!-- first printing --> |isbn=0-201-63287-X
<ref name="Necasek_2011_CALL5">{{cite web |title=Who needs the address wraparound, anyway? |date=2011-09-13 |author-first=Michal |author-last=Necasek |work=OS/2 Museum |url=http://www.os2museum.com/wp/who-needs-the-address-wraparound-anyway/ |access-date=2020-02-19 |url-status=live |archive-url=https://web.archive.org/web/20200219004644/http://www.os2museum.com/wp/who-needs-the-address-wraparound-anyway/ |archive-date=2020-02-19 |quote=[…] [[86-DOS]], and hence [[PC DOS]]/[[MS-DOS]], used a clever trick. The byte at offset 5 of the PSP contained a far call opcode (9Ah); the word at offset 6 of the PSP contained the appropriate value to indicate program segment size, and also the offset part of the far call. The word at offset 8, which served as the segment part of the far call, was crafted such that when combined with the offset, it would wrap around (a well understood feature of the [[8086]] CPU) and point to address 0:C0h, which contains interrupt vector 30h. […] the [[CALL 5 (DOS)|CALL 5]] interface works even in DOS emulation under Windows NT and OS/2, and those systems most certainly cannot run with the [[A20 line]] disabled. How does that work then? […] Rather than chopping off address bits, the system mirrors the five bytes at 0:C0h at 1000C0h. The same technique had been in fact used in DOS 5 and above running with [[DOS (CONFIG.SYS directive)|DOS=HIGH]]. In that case, DOS makes sure that linear address 1000C0h contains the appropriate far call. […]}}</ref>
<ref name="Paul_1997_MSDOS">{{cite book |title=MSDOSTIPs — Tips für den Umgang mit MS-DOS 5.0-7 |work=MPDOSTIP |author-first=Matthias R. |author-last=Paul |date=1997-07-01 |orig-year=1994-01-01 |language=de |url=http://www.antonis.de/dos/dos-tuts/mpdostip/html/msdostip.htm |access-date=2013-10-25 |url-status=live |archive-url=https://web.archive.org/web/20170822214030/http://www.antonis.de/dos/dos-tuts/mpdostip/html/msdostip.htm |archive-date=2017-08-22}} (NB. MSDOSTIP.TXT is part of MPDOSTIP.ZIP, maintained up to 2001 and distributed on many sites at the time. The provided link points to a HTML-converted older version of the MSDOSTIP.TXT file.) [https://web.archive.org/web/20190601152204/https://www.sac.sk/download/text/mpdostip.zip<!-- A yet older version 155 from 1997-05-13 of the 1997-07-15 distribution archive. -->]</ref>
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