Program Segment Prefix
|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|
|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)|
|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)|
|40h-41h||word||DOS version to return (DOS 4 and later, alterable via SETVER in DOS 5 and later)|
|50h-52h||3 bytes (code)||Far call to DOS (always contain INT 21h + RETF)|
|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.
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 '$' to terminate string xor bx, bx mov bl, [80h] cmp bl, 7Eh ja exit mov byte [bx + 81h], '$' ; print the string mov ah, 9 mov dx, 81h int 21h exit: mov ax, 4C00h int 21h
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 to the stack at program start (since the DS register is loaded with the PSP segment) and terminating the program with a RETF instruction, which would pop the saved segment value off the stack and jump to address 0 of the PSP, which contained an INT 20h instruction.
push ds xor ax,ax push ax mov ax,@data mov ds,ax mov dx,mess1 mov ah,9h int 21h retf
If the executable was a .COM file, this procedure was unnecessary and the program could be terminated merely with a direct INT 20h instruction or else calling INT 21h Function 0, however the programmer still had to ensure that the CS register contained the segment address of the PSP at program termination. Thus,
jmp start mess1 db 'Hello world!$' start: mov dx,mess1 mov ah,9 int 21h int 20h
In DOS 2.x and higher, program termination was accomplished instead with INT 21h Function 4Ch which did not require the CS register to contain the segment value of the PSP.
- Accessing Command Line Arguments (Microsoft.com)