Int 21/AH=64h/DX=0000h

AH = 64h
DX = 0000h (function number)
CX = 636Ch (magic value, "cl")
BX = 0000h (indicates special request)

Note: If CX is not 636Ch on entry, INT 21/AH=6Ch is invoked, because a bug in OS/2 1.x FAPI erroneously called AH=64h instead of AH=6Ch

See Also: AH=64h/DX=0001h - INT 21/AH=4Bh