PSP - DOS Program Segment Prefix Layout
Offset Size Description 00 word machine code INT 20 instruction (CDh 20h) 02 word top of memory in segment (paragraph) form 04 byte reserved for DOS, usually 0 05 5bytes machine code instruction long call to the DOS function dispatcher (obsolete CP/M) 06 word .COM programs bytes available in segment (CP/M) 0A dword INT 22 terminate address; DOS loader jumps to this address upon exit; the EXEC function forces a child process to return to the parent by setting this vector to code within the parent (IP,CS) 0E dword INT 23 Ctrl-Break exit address; the original INT 23 vector is NOT restored from this pointer (IP,CS) 12 dword INT 24 critical error exit address; the original INT 24 vector is NOT restored from this field (IP,CS) 16 word parent process segment addr (Undoc. DOS 2.x+) COMMAND.COM has a parent id of zero, or its own PSP 18 20bytes file handle array (Undocumented DOS 2.x+); if handle array element is FF then handle is available. Network redirectors often indicate remotes files by setting these to values between 80-FE. 2C word segment address of the environment, or zero (DOS 2.x+) 2E dword SS:SP on entry to last INT 21 function (Undoc. 2.x+) ╪ 32 word handle array size (Undocumented DOS 3.x+) 34 dword handle array pointer (Undocumented DOS 3.x+) 38 dword pointer to previous PSP (deflt FFFF:FFFF, Undoc 3.x+) ╪ 3C 20bytes unused in DOS before 4.01 ╪ 50 3bytes DOS function dispatcher CDh 21h CBh (Undoc. 3.x+) ╪ 53 9bytes unused 5C 36bytes default unopened FCB #1 (parts overlayed by FCB #2) 6C 20bytes default unopened FCB #2 (overlays part of FCB #1) 80 byte count of characters in command tail; all bytes following command name; also default DTA (128 bytes) 81 127bytes all characters entered after the program name followed by a CR byte - offset 5 contains a jump address which is 2 bytes too low for PSP's created by the DOS EXEC function in DOS 2.x+ ╪ - program name and complete path can be found after the environment in DOS versions after 3.0. See offset 2Ch. ╪ see Bibliography for reference to "Undocumented DOS"