The Program Segment Prefix (PSP) is a data structure used in DOS systems to store the state of a program. It has the following structure:
Offset Size Contents
--------------------------------------------------------------------
00-01 Code CP/M exit (always contain INT 20)
02-03 Word Memory size in paragraphs
04 Reserved
05-09 Code Far call to DOS (internal)
0A-0D DWord Terminate address of previous program (old INT 22)
0E-11 DWord Break address of previous program (old INT 23)
12-15 DWord Critical error address of previous program (old INT 24)
16-17 Word Parent psp segment (PSP of caller - usually
command.com - internal)
18-2B Bytes Job File Table(JFT) (internal)
2C-2D Word Environment segment
2E-31 DWord SS:SP on entry to last INT 21 call (Internal)
32-33 Word Max open files (Internal - see below)
34-37 DWord Handle-entries address (Internal - see below)
38-4F Reserved
50-52 Code CP/M function call (always contain INT 21 + RETF)
53-5B Reserved
5C-6B Unopened Standard FCB 1
6C-7F Unopened Standard FCB 2 (overwritten if FCB 1
is opened)
80 Byte Number of characters on command-line (excl. filename
and trailing blanks but including leading blanks)
81-FF Bytes Command-line (terminated by a 0Dh)