Int 21H, AH=31H

2 and higher

TERMINATE AND STAY RESIDENT

      AH = 31h
      AL = return code
      DX = number of paragraphs to keep resident

never returns

the value in DX only affects the memory block containing the PSP;
        additional memory allocated via AH=48h is not affected
      the minimum number of paragraphs which will remain resident is 11h
        for DOS 2.x and 06h for DOS 3.0+
      most TSRs can save some memory by releasing their environment block
        before terminating (see #01378 at AH=26h,AH=49h)
      any open files remain open, so one should close any files which will
        not be used before going resident; to access a file which is left
        open from the TSR, one must switch PSP segments first (see AH=50h)

AH=00h,AH=4Ch,AH=4Dh,INT 20,INT 22,INT 27

Text based on Ralf Brown Interrupt List Release 61