Int 27H

1 and higher

TERMINATE AND STAY RESIDENT

DX = number of bytes to keep resident (max FFF0h)
CS = segment of PSP

 never
 

  this is an obsolete call
INT 22, INT 23, and INT 24 are restored from the PSP
does not close any open files
the minimum number of bytes which will remain resident is 110h for
  DOS 2.x and 60h for DOS 3.0+; there is no minimum for DOS 1.x, which
  implements this service in COMMAND.COM rather than the DOS kernel

INT 21/AH=31h

Text based on Ralf Brown Interrupt List Release 61