Int 21H, AH=55H

2 and higher

CREATE CHILD PSP

AH = 55h
DX = segment at which to create new PSP
SI = (DOS 3.0+) value to place in memory size field at DX:[0002h]

 AL destroyed

creates a "child" PSP rather than making an exact copy of the current
  PSP; the new PSP's parent pointer is set to the current PSP and the
  reference count for each inherited file is incremented
(DOS 2.0+) sets current PSP to DX
(DOS 3.0+) marks "no inherit" file handles as closed in child PSP
this function is implemented using the same code as AH=26h, so unlike
  other DOS 2+ functions, it does not return status in CF, instead
  returning status in AL as DOS 1.x functions do (but it never puts an
  explicit return value in AL)
        

AH=26h,AH=50h

Text based on Ralf Brown Interrupt List Release 61