▁level ▁systems , ▁were ▁with dra wn ▁on ▁the ▁same ▁day , ▁October ▁ 7 , ▁ 1 9 7 7 . ▁ ▁Mod els ▁The ▁Model ▁ 2 5 ▁can ▁be ▁configured ▁with ▁ 1 6 K , ▁ 2 4 K , ▁ 3 2 K , ▁or ▁ 4 8 K ▁of ▁core ▁memory . ▁ ▁Character istics ▁The ▁base ▁Model ▁ 2 5 ▁implements ▁the ▁System / 3 6 0 ▁standard ▁and ▁commercial ▁instruction ▁sets . ▁The ▁scientific ▁or ▁universal ▁instruction ▁sets ▁are ▁optional ▁features . ▁ ▁The ▁Model ▁ 2 5 ▁logic ▁is ▁built ▁on ▁IBM ▁S LT ▁modules ▁mounted ▁on ▁pl ugg able ▁cards , ▁which ▁are ▁plug ged ▁into ▁printed ▁circuit ▁bo ards . ▁ ▁The ▁bo ards ▁are ▁mounted ▁on ▁h ing ed ▁metal ▁gates , ▁and ▁are ▁inter connected ▁by ▁flat ▁c ables . ▁ ▁The ▁Model ▁ 2 5 ▁is ▁the ▁first ▁IBM ▁system ▁to ▁store ▁its ▁micro code ▁in ▁a ▁re writ able ▁memory , ▁called ▁the ▁control ▁storage . ▁ ▁The ▁control ▁storage ▁uses ▁an ▁additional ▁ 1 6   K ▁( 1 6 , 3 8 4 ) ▁bytes ▁of ▁core ▁memory . ▁ ▁There ▁is ▁also ▁a ▁small ▁part ▁of ▁the ▁core ▁storage ▁that ▁is ▁used ▁to ▁store ▁the ▁contents ▁of ▁registers ▁accessible ▁by ▁software , ▁as ▁well ▁as ▁data ▁used ▁by ▁the ▁micro code . ▁ ▁The ▁Model ▁ 2 5 ▁also ▁has ▁a ▁ 6 4 - byte ▁high - speed ▁( 1 8 0   ns