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        Program Segment Prefix (PSP)

        Offset Size    Contens
        --------------------------------------------------------------------
        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 previos program (old INT 22)
        0E-11  DWord   Break address of previos program (old INT 23)
        12-15  DWord   Critical error address of previos program (old INT 24)
        16-17  Word    Parent psp segment (PSP of caller - usualy
                       command.com - internal)
        18-2B  Bytes   Handle entries (internal)
                       18: Handle 0
                       19: Handle 1
                       .
                       .
                       Contains the number of an internal DOS handle -
                       00=AUX, 01=CON, 02=PRN, 04=NUL, FF=not open
        2C-2D  Word    Enviroment 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 Standart FCB 1
        6C-7F          Unopened Standart 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)


        Note (max. handles): On pre DOS 3.30 systems, PSP:0032 and
        PSP:0034 can be used to set the maximum number of files that
        can be opened, by the program owning the PSP (create a new buffer,
        and copy the first 20 entries from PSP:0018 and fill the rest
        with FFh).

        The method descriped above should be avoidet (if possible),
        since DOS 3.30 provides a call to set the maximum number of handles
        (INT 21h, 67h).