Compatibility: 2.0+
Expects: AH    31H
AL    exit code  to be made available to fn 4dH
DX    memory size to keep resident, in 16-byte paragraphs
Returns: none  (not applicable)
Info: Exits to the parent process, leaving code and data in memory and
setting the exit code in AL.  See TSR for related topics.

DOS sets the initial memory allocation (at your PSP segment
address) to the size specified by DX and returns control to the
parent process, keeping that much memory resident (bytes
remaining resident = DX * 16).

This function also performs the following actions:
  Flushes file buffers and closes all open files.
  Sets the INT 22H vector to the value at PSP:000AH.
  Sets the INT 23H vector to the value at PSP:000EH.
  Sets the INT 24H vector to the value at PSP:0012H (2.0+).

Notes:   This DOES NOT free up any other blocks allocated via Fn 48H.

It's wise to free your environment block beforehand (call
Fn 49H with ES= the WORD at PSP offset 2cH).

  This function supersedes INT 27H which does not return an exit
code and can't install resident programs larger than 64K.

See Also: TSR
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DOS Fn 31H: Terminate & Stay Resident