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<TITLE>Int 21/AH=26h
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Int 21/AH=26h - DOS 1+ - CREATE NEW PROGRAM SEGMENT PREFIX -
AH = 26h
DX = segment at which to create PSP (see #01378)
Return: AL destroyed
Notes: 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
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<h2>Int 21/AH=26h
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<h3><font color=#C00040>DOS 1+ - CREATE NEW PROGRAM SEGMENT PREFIX</font></h3>
<PRE>
AH = 26h
DX = segment at which to create PSP <a href="rb-2682.htm#Table1378" tppabs="http://www.ctyme.com/intr/rb-2682.htm#Table1378">(see #01378)</a><br>
<p><b>Return:</b><br>AL destroyed
</PRE>
<p><b>Notes:</b>
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
<p><b>BUG:</b>
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
<PRE>
<p><b>DR DOS 6.0 BDOS patch "PAT312" English (1992/01/07, XDIR /C:</b><br>A0C6h)
and later "full" rebuilds fix this problem (see INT 21/AX=4452h).
</PRE>
<p><b>See Also:</b>
<a href="rb-2939.htm" tppabs="http://www.ctyme.com/intr/rb-2939.htm">AH=4Bh</a> - <a href="rb-2980.htm" tppabs="http://www.ctyme.com/intr/rb-2980.htm">AH=50h</a> - <a href="rb-2982.htm" tppabs="http://www.ctyme.com/intr/rb-2982.htm">AH=51h</a> - <a href="rb-2989.htm" tppabs="http://www.ctyme.com/intr/rb-2989.htm">AH=55h</a> - <a href
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<p>
Format of Program Segment Prefix (PSP):
<a name=table1378></a>
Offset Size Description (Table 01378)
00h 2 BYTEs INT 20 instruction for CP/M CALL 0 program termination
the CDh 20h here is often used as a signature for a valid PSP
02h WORD segment of first byte beyond memory allocated to program
04h BYTE (DOS) unused filler
(OS/2) count of fake DOS version returns
05h BYTE CP/M CALL 5 service request (FAR CALL to absolute 000C0h)
</PRE>
<p><b>BUG:</b>
(DOS 2+ DEBUG) PSPs created by DEBUG point at 000BEh
06h WORD CP/M compatibility--size of first segment for .COM files
08h 2 BYTEs remainder of FAR JMP at 05h
0Ah DWORD stored INT 22 termination address
0Eh DWORD stored INT 23 control-Break handler address
12h DWORD DOS 1.1+ stored INT 24 critical error handler address
16h WORD segment of parent PSP
18h 20 BYTEs DOS 2+ Job File Table, one byte per file handle, FFh = closed
2Ch WORD DOS 2+ segment of environment for process <a href="rb-2682.htm#Table1379" tppabs="http://www.ctyme.com/intr/rb-2682.htm#Table1379">(see #01379)</a>
2Eh DWORD DOS 2+ process's SS:SP on entry to last INT 21 call
32h WORD DOS 3+ number of entries in JFT (default 20)
34h DWORD DOS 3+ pointer to JFT (default PSP:0018h)
38h DWORD DOS 3+ pointer to previous PSP (default FFFFFFFFh in 3.x)
used by SHARE in DOS 3.3
3Ch BYTE DOS 4+ (DBCS) interim console flag (see AX=6301h)
Novell DOS 7 DBCS interim flag as set with AX=6301h
(possibly also used by Far East MS-DOS 3.2-3.3)
3Dh BYTE (APPEND) TrueName flag (see INT 2F/AX=B711h)
<PRE>
<p><b>3Eh BYTE (Novell NetWare) flag:</b><br>Next byte initialized if CEh
(OS/2) capabilities flag
3Fh BYTE (Novell NetWare) Novell task number if previous byte is CEh
40h 2 BYTEs DOS 5+ version to return on INT 21/AH=30h
42h WORD (MSWindows3) selector of next PSP (PDB) in linked list
Windows keeps a linked list of Windows programs only
44h WORD (MSWindows3) "PDB_Partition"
46h WORD (MSWindows3) "PDB_NextPDB"
48h BYTE (MSWindows3) bit 0 set if non-Windows application (WINOLDAP)
49h BYTE unused by DOS versions <= 6.00
4Ch WORD (MSWindows3) "PDB_EntryStack"
4Eh 2 BYTEs unused by DOS versions <= 6.00
50h 3 BYTEs DOS 2+ service request (INT 21/RETF instructions)
53h 2 BYTEs unused in DOS versions <= 6.00
55h 7 BYTEs unused in DOS versions <= 6.00; can be used to make first FCB
into an extended FCB
5Ch 16 BYTEs first default FCB, filled in from first commandline argument
overwrites second FCB if opened
6Ch 16 BYTEs second default FCB, filled in from second commandline argument
overwrites beginning of commandline if opened
7Ch 4 BYTEs unused
80h 128 BYTEs commandline / default DTA
command tail is BYTE for length of tail, N BYTEs for the tail,
followed by a BYTE containing 0Dh
</PRE>
<p><b>Notes:</b>
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.
<p><b>BUG:</b>
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.
<PRE>
<p>
Format of environment block:
<a name=table1379></a>
Offset Size Description (Table 01379)
00h N BYTEs first environment variable, ASCIZ string of form "var=value"
N BYTEs second environment variable, ASCIZ string.
...
N BYTEs last environment variable, ASCIZ string of form "var=value".
BYTE 00h
---DOS 3.0+ ---.
WORD number of strings following environment (normally 1)
N BYTEs ASCIZ full pathname of program owning this environment
other strings may follow
</PRE>
<p>
<b>Category: <a href="cat-010.htm" tppabs="http://www.ctyme.com/intr/cat-010.htm">
DOS Kernel</a>
- <a href="int-21.htm" tppabs="http://www.ctyme.com/intr/int-21.htm">
Int 21h</a>
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