The Program Segment Prefix (PSP) is a data structure used in DOS systems to store the state of a program. It resembles the Zero Page in the CP/M operating system. The PSP has the following structure:
| Offset | Size | Contents |
|---|---|---|
| 00h-01h | 2 bytes (code) | CP/M exit (always contain INT 20h) |
| 02h-03h | word (2 bytes) | Memory size in paragraphs |
| 04h | byte | Reserved |
| 05h-09h | 5 bytes (code) | Far call to CP/M compatibility code within DOS |
| 0Ah-0Dh | dword (4 bytes) | Terminate address of previous program (old INT 22h) |
| 0Eh-11h | dword | Break address of previous program (old INT 23h) |
| 12h-15h | dword | Critical error address of previous program (old INT 24h) |
| 16h-17h | word | Caller's PSP segment (usually command.com - internal) |
| 18h-2Bh | 20 bytes | Job File Table (internal) |
| 2Ch-2Dh | word | Environment segment |
| 2Eh-31h | dword | SS:SP on entry to last INT 21h call (Internal) |
| 32h-33h | word | Max open files (Internal - see below) |
| 34h-37h | dword | Handle-entries address (Internal - see below) |
| 38h-4Fh | 24 bytes | Reserved |
| 50h-52h | 3 bytes (code) | Far call to DOS (always contain INT 21h + RETF) |
| 53h-5Bh | 9 bytes | Reserved |
| 5Ch-6Bh | 16 bytes | Unopened Standard FCB 1 |
| 6Ch-7Fh | 20 bytes | Unopened Standard FCB 2 (overwritten if FCB 1 is opened) |
| 80h | 1 byte | Number of bytes on command-line |
| 81h-FFh | 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 is passed in the DS register when the program is executed. It can also be determined later by using interrupt 21h subfunction 62h. 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 '$' to terminate string xor bx, bx mov bl, [80h] mov byte [bx + 81h], '$' ; print the string mov ah, 9 mov dx, 81h int 21h ; exit mov ax, 4C00h int 21h
If the command line length is non-zero, programs should first try to retrieve the command line from the appendage of the environment, if the segment pointer to the environment is neither 0000h or FFFFh. This way, it is possible to pass longer command lines than 127 bytes to applications than via the fixed length command line buffer in the PSP. If this appendage is not available (for example under DOS prior to 3.2) or if it turns out to be of length zero, the command line can be retrieved from the PSP itself, but possibly truncated after 127 bytes. Indicated lengths beyond this should not be trusted on.