PSP: Program Segment Prefix
This structure is created for each program that DOS executes.
Offset Size Contents
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+0 2 wInt20 INT 20H instruction (cd 20) (old way to exit)
+2 2 wNextSeg Segment addr just beyond end of program image
+4 1 res1 (reserved)
+5 5 abDispatch FAR CALL to DOS function dispatcher (obs)
+0aH 4 pfTerminate terminate address. See INT 22H
+0eH 4 pfCtlBrk Ctrl-Break handler address INT 23H
+12H 4 pfCritErr Critical Error handler addr INT 24H
+16H 22 res2 DOS reserved area
2 wParentPsp ◄undoc► segment of parent's PSP
+2cH 2 wEnvSeg segment address of DOS environment
+2eH 46 res3 DOS reserved area (handle table, et al.)
+5cH 16 rFCB_1 an unopened FCB for 1st cmd parameter
+6cH 20 rFCB_2 an unopened FCB for 2nd cmd parameter
+80H 1 bCmdTailLen count of characters in command tail at 81H (also
default setting for the DTA)
+81H 127 abCmdTail characters from DOS command line
256 size of a PspRec structure
wInt20 An obsolete way to terminate a program is to JMP here.
wNextSeg Segment address of the paragraph just above the end of the
program. This can be used to calculate the size of the load
block (by subtracting the segment of the PSP itself). Memory
at this address is NOT free for use.
abDispatch An obsolete way to call DOS fns was to JMP here.
pfTerminate On exit, DOS copies this to INT 22H vector
pfCtrlBrk On exit, DOS copies this to INT 23H vector
pfCritErr On exit, DOS copies this to INT 24H vector
wParentPsp This undocumented 2-byte field at (offset 16H) contains the
segment of the parent process's PSP (usually COMMAND.COM). By
tracing backward along this chain, you can locate a wParentPsp
which is the same as the PSP in which it resides. The wEnvSeg
in that PSP points to the master environment, and careful
manipulation of that data area will make changes to the
DOS Environment that are remembered after your program exits
(just changing your own copy of the environment will only
affect you and your own spawns).
wEnvSeg At program startup, DOS creates a copy of the DOS Environment
area and places its segment address in this field.
rFCB_1 The first filename in the command tail is formatted into an
unopened FCB and placed here.
rFCB_1 The second filename (note: no path info, so rarely used)
bCmdTailLen The length of the command-line text tail.
Also: The default address of the DTA is here (PSP:0080)
abCmdTail The text of command-line parameters, not including the command
name (except I/O redirection info has been removed). A CR
(0dH) is appended, but not included in aCmdTailLen.
Notes: ■ When a program starts running, DS and ES point to the start of
that program's PSP. You may also obtain the PSP address via
■ The PSP segment address is used as the "process ID" in fns 50H,
51H, and 62H; it is used as the wOwnerID in an MemBlockRec.
■ Use the PSP information to parse filenames and options from the
command line tail, learn how much RAM is available, find the
environment etc. See Program Startup & Exit.
See Also: Program Startup & Exit
Process Control Functions