Program Segment Prefix
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:
| Offsetimages | Size | Contents |
|---|---|---|
| 00h-01h | 2 bytes (code) | CP/M exit (always contains INT 20h) |
| 02h-03h | word (2 bytes) | Segment of the first byte beyond the memory allocated to the program |
| 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 | Parent's PSP segment (usually COMMAND.COM - internal) |
| 18h-2Bh | 20 bytes | Job File Table (JFT) (internal) |
| 2Ch-2Dh | word | Environment segment |
| 2Eh-31h | dword | SS:SP on entry to last INT 21h call (internal) |
| 32h-33h | word | JFT size (internal) |
| 34h-37h | dword | Pointer to JFT (internal) |
| 38h-3Bh | dword | Pointer to previous PSP (only used by SHARE in DOS 3.3 and later) |
| 3Ch-3Fh | 4 bytes | Reserved |
| 40h-41h | word | DOS version to return (DOS 4 and later, alterable via SETVER in DOS 5 and later) |
| 42h-4Fh | 14 bytes | Reserved |
| 50h-52h | 3 bytes (code) | Far call to DOS (always contain INT 21h + RETF) |
| 53h-54h | 2 bytes | Reserved |
| 55h-5Bh | 7 bytes | Reserved (can be used to make first FCB into an extended FCB) |
| 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 tail (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 Int 21h function 51h or Int 21h function 62h. Either function will return the PSP address in register BX.[1]
Alternatively, in .COM 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
; INT 21h subfunction 9 requires '$' to terminate string
xor bx, bx
mov bl, [80h]
cmp bl, 126
ja exit
mov byte [bx + 81h], '$'
; print the string
mov ah, 9
mov dx, 81h
int 21h
exit:
mov ax, 4C00h
int 21h
See also
References
External links
- Accessing Command Line Arguments (Microsoft.com)