PSP Structure

            Length
    Offset  (bytes)     Description

    00h        2        INT 20h instruction.
    02h        2        Top of memory. Segment address of the top of the
                        current program's memory  allocation block.
    04h        1        Reserved.
    05h        1        INT 21h instruction.
    06h        2        Available memory--the number of bytes available in
                        the segment.
    08h        2        Reserved.
    0Ah        4        Terminate address (IP,CS). Address of the
                        termination interrupt handler (INT 22h) inherited
                        by the current program. DOS uses this value to
                        restore the INT 22h vector when the program
                        terminates.
    0Eh        4        Ctrl-Break address (IP,CS). Address of the Ctrl-
                        Break interrupt handler (INT 23h) inherited by the
                        current program. DOS uses this value to restore the
                        INT 23h vector when the program terminates.
    12h        4        Critical error address (IP,CS). Address of the
                        Critical error interrupt handler (INT 24h)
                        inherited by the current program. DOS uses this
                        value to restore the INT 24h vector when the
                        program terminates.
    16h       22        Reserved.
    2Ch        2        Environment--the segment address of the DOS
                        environment.
    2Eh       46        Reserved.
    5Ch       16        Unopened standard FCB1, as parsed from the first
                        parameter in the command tail.
    6Ch       20        Unopened standard FCB2, as parsed from the second
                        parameter in the command tail.
    80h        1        Parameter length--the length, in bytes, of the
                        command tail.
    81h      127        Command tail. Also used as default DTA for FCB
                        functions.

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      Notes:    Programs should not alter any of the data below 5Ch in the
                PSP, since DOS uses this information when the program
                exits.

                The second FCB at 6Ch will be overwritten if the first FCB
                is used to open a file.  This happens because the only 16
                bytes are reserved for the first FCB, whereas an entire
                FCB is 37 bytes long.