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Program Segment Prefix

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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 '$' to terminate string
xor   bx, bx
mov   bl, [80h]
cmp   bl, 7Eh
ja    exit
mov   byte [bx + 81h], '$'

; 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.

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