INT 21 - DOS 1+ - SET VERIFY FLAG
AH = 2Eh
DL = 00h (DOS 1.x/2.x only)
AL = new state of verify flag
00h off
01h on
Notes:  default state at system boot is OFF
when ON, all disk writes are verified provided the device driver
supports read-after-write verification

See also: 2154

212E - DOS 1+ - SET VERIFY FLAG