Int 21/AX=3505h

AX = 3505h
BX = selector
ECX = desired limit

Return:
CF clear if successful
ECX = actual limit set
CF set on error (no more selectors available)

Note: The limit will be rounded down to nearest 4K boundary if the requested limit is greater than 1MB

See Also: AX=3503h - INT 31/AX=0008h