INT 21,5C - Lock/Unlock File Access (DOS 3.0+)

AH = 5C

AL = 00 lock file

= 01 unlock file

BX = file handle

CX = most significant word of region offset

DX = least significant word of region offset

SI = most significant word of region length

DI = least significant word of region length

on return:

AX = error code if CF set (see ~DOS ERROR CODES~)

- used for networking and multi-tasking system to preserve

data integrity

- duplicated handles inherit access to locked regions

- EXEC'd programs do not inherit access to locked regions