▁ 5 µ s ), ▁directly ▁address able ▁as ▁ 1 0 2 4 ▁full ▁or ▁ 2 0 4 8 ▁half - words . ▁This ▁was ▁complement ed ▁by ▁an ▁additional ▁ 8 1 9 2 ▁words ▁of ▁backing ▁store ▁on ▁magnetic ▁drum ▁( 3 0 0 0 ▁r pm ). ▁A ▁full ▁word ▁stored ▁ 4 0 - bit ▁numbers ▁in ▁two ' s - com plement ▁form , ▁or ▁two ▁ 2 0 - bit ▁instructions . ▁ ▁In ▁addition ▁to ▁two ▁accum ul ators , ▁the ▁D AS K ▁had ▁three ▁index ▁registers , ▁which ▁could ▁be ▁used ▁to ▁modify ▁the ▁address ▁of ▁most ▁instructions . ▁An ▁instruction ▁word ▁consisted ▁of ▁ 1 1 ▁bits ▁for ▁an ▁address , ▁two ▁bits ▁for ▁index ▁register ▁selection , ▁and ▁ 7 ▁bits ▁for ▁the ▁operation ▁code ▁and ▁its ▁mod ifiers . ▁ ▁Oper ations ▁included ▁addition ▁and ▁sub tra ction ▁( 5 6 ▁ µ s ), ▁multiplication ▁and ▁division ▁( 3 6 4 ▁ µ s ), ▁binary ▁shift ▁and ▁bit wise ▁conj unction . ▁ ▁Per i pher als ▁initially ▁included ▁ 5 - bit ▁paper ▁t ape ▁( 4 0 0 ▁c ps ▁read ▁time ) ▁and ▁te let ype writer ▁( 1 2 ▁c ps ); ▁magnetic ▁t ape ▁and ▁other ▁peri pher als ▁were ▁added ▁later ▁on . ▁ ▁See ▁also ▁List ▁of ▁vac u um ▁t ube ▁computers ▁ ▁References ▁ ▁External ▁links ▁Early ▁Comput ing ▁in ▁Den mark ▁H OP L ▁entry ▁for ▁D