On page 1C:

UnlockFlashFrom1C:
	di
	ld a, 1
	nop
	nop
	im 1
	di
	out (14h), a
	ret
LockFlashFrom1C:
	ld a, 0
	di
	nop
	nop
	im 1
	di
	out (14h), a
	ret

Elsewhere:

UnlockFlash:
	di
	ld a, 1Ch
	out (6), a
	call UnlockFlashFrom1C
	ei
	ret

Elsewhere again:

	call UnlockFlash
	ld a, 0FBh
	ld (0), a
	call LockFlash