Program Segment Prefix

From Wikipedia, the free encyclopedia

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:

Offset	Size	Contents
00-01	2 bytes (code)	CP/M exit (always contain INT 20)
02-03	word (2 bytes)	Memory size in paragraphs
04	byte	Reserved
05-09	5 bytes (code)	Far call to CP/M compatibility code within DOS
0A-0D	dword (4 bytes)	Terminate address of previous program (old INT 22)
0E-11	dword	Break address of previous program (old INT 23)
12-15	dword	Critical error address of previous program (old INT 24)
16-17	word	Caller's PSP segment (usually command.com - internal)
18-2B	20 bytes	Job File Table (internal)
2C-2D	word	Environment segment
2E-31	dword	SS:SP on entry to last INT 21 call (Internal)
32-33	word	Max open files (Internal - see below)
34-37	dword	Handle-entries address (Internal - see below)
38-4F	24 bytes	Reserved
50-52	3 bytes (code)	Far call to DOS (always contain INT 21 + RETF)
53-5B	9 bytes	Reserved
5C-6B	16 bytes	Unopened Standard FCB 1
6C-7F	20 bytes	Unopened Standard FCB 2 (overwritten if FCB 1 is opened)
80	1 byte	Number of characters on command-line
81-FF	127 bytes	Command-line (terminated by a 0Dh)

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.^[1]

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 '$' to terminate string
xor   bx, bx
mov   bl, [80h]
mov   byte [81h+bx], '$'

; print the string
mov   ah, 9
mov   dx, 81h
int   21h

; exit
mov   ax, 4C00h
int   21h