0 . ▁One ▁feature ▁of ▁ex F AT ▁( used ▁in ▁the ▁ex F AT ▁implementation ▁within ▁embedded ▁systems ) ▁provides ▁atomic ▁transactions ▁for ▁the ▁multiple ▁steps ▁of ▁updating ▁the ▁file ▁system ▁metadata . ▁The ▁feature , ▁called ▁Trans action ▁Sa fe ▁F AT , ▁or ▁Tex F AT , ▁was ▁granted ▁a ▁pat ent ▁by ▁the ▁US ▁pat ent ▁office ▁under ▁US 7 6 1 3 7 3 8 ▁on ▁November ▁ 3 , ▁ 2 0 0 9 . ▁ ▁Directory ▁file ▁set ▁ ▁ex F AT ▁and ▁the ▁rest ▁of ▁the ▁F AT ▁family ▁of ▁file ▁systems ▁does ▁not ▁use ▁indexes ▁for ▁file ▁names , ▁unlike ▁N TF S ▁which ▁uses ▁B - trees ▁for ▁file ▁searching . ▁When ▁a ▁file ▁is ▁accessed , ▁the ▁directory ▁must ▁be ▁sequ entially ▁searched ▁until ▁a ▁match ▁is ▁found . ▁For ▁file ▁names ▁shorter ▁than ▁ 1 6 ▁characters ▁in ▁length , ▁one ▁file ▁name ▁record ▁is ▁required ▁but ▁the ▁entire ▁file ▁is ▁represented ▁by ▁three ▁ 3 2 - byte ▁directory ▁records . ▁This ▁is ▁called ▁a ▁directory ▁file ▁set , ▁and ▁a ▁ 2 5 6 ▁Mi B ▁sub - directory ▁can ▁hold ▁up ▁to ▁ 2 , 7 9 6 , 2 0 2 ▁file ▁sets . ▁( If ▁files ▁have ▁longer ▁names , ▁this ▁number ▁will ▁decrease ▁but ▁this ▁is ▁the ▁maximum ▁based ▁on ▁the ▁minimum ▁three - record ▁file ▁set .) ▁To ▁help ▁improve ▁the ▁sequ ential ▁searching ▁of ▁the ▁directories ▁( including ▁the ▁root ) ▁a ▁hash