Int 21H, AH=26H

1 and higher

CREATE NEW PROGRAM SEGMENT PREFIX

      AH = 26h
      DX = segment at which to create PSP (see #01378)

Return: AL destroyed

new PSP is updated with memory size information; INTs 22h, 23h, 24h taken from interrupt vector table; the parent PSP field is set to 0

(DOS 2+) DOS assumes that the caller's CS is the segment of the PSP to copy

BUG: DR DOS 6.0 original releases 05/1991 & 08/1991 had a problem where the segment from which the PSP was copied was incorrect so that the PSP was not filled correctly and did not contain the command tail. The DR DOS 6.0 BDOS patch “PAT312” English (1992/01/07, XDIR /C: A0C6h) and later “full” rebuilds fix this problem (see INT 21/AX=4452h).

Format of Program Segment Prefix (PSP):

Notes: in DOS v3+, the limit on simultaneously open files may be increased by allocating memory for a new open file table, filling it with FFh, copying the first 20 bytes from the default table, and adjusting the pointer and count at 34h and 32h. However, DOS will only copy the first 20 file handles into a child PSP (including the one created on EXEC).

in an OS/2 DOS box, values of D0h-FEh in the open file table indicate device drivers

network redirectors based on the original MS-Net implementation use values of 80h-FEh in the open file table to indicate remote files; Novell NetWare also uses values from FEh down to 80h or one more than FILES= (whichever is greater) to indicate remote files (except on OS/2, where is uses CFh down to 80h)

MS-DOS 5.00 incorrectly fills the FCB fields when loading a program high; the first FCB is empty and the second contains the first parameter

some DOS extenders place protected-mode values in various PSP fields such as the “parent” field, which can confuse PSP walkers. Always check either for the CDh 20h signature or that the suspected PSP is at the beginning of a memory block which owns itself (the preceding paragraph should be a valid MCB with “owner” the same as the suspected PSP).

Novell NetWare updates the fields at offsets 3Eh and 3Fh without checking that a legal PSP segment is current; see AH=50h for further discussion

for 4DOS and Windows95, the command tail may be more than 126 characters; in that case, the length byte will be set to 7Fh (with an 0Dh in the 127th position at offset FFh), and the first 126 characters will be stored in the PSP, with the entire command line in the environment variable CMDLINE; under at least some versions of 4DOS, the byte at offset FFh is *not* set to 0Dh, so there is no terminating carriage return in the PSP's command tail.

BUG: When shelling out from the Borland Pascal 7.00 IDE, overly-long command lines will not be delimited by a 0Dh character, and the length byte is set to 80h! A workaround is to always patch in a 0Dh at the last position of the command line buffer before scanning the command line.

Format of environment block:

AH=4Bh,AH=50h,AH=51h,AH=55h,AH=62h,AH=67h

Text based on Ralf Brown Interrupt List Release 61