 The ZX81 came with 1 KB of on @-@ board memory that could officially be expanded externally to 16 KB . Its single circuit board was housed inside a wedge @-@ shaped plastic case measuring 167 millimetres ( 6 @.@ 6 in ) deep by 40 millimetres ( 1 @.@ 6 in ) high . The memory was provided by either a single <unk> ( 1024 bit × 8 ) or two 2114 ( 1024 bit × 4 ) RAM chips . There were only three other chips on board : a 3 @.@ 5 MHz <unk> 8 @-@ bit microprocessor from NEC , an uncommitted logic array ( ULA ) chip from Ferranti and an 8 KB ROM providing a simple BASIC interpreter . The entire machine weighed just 350 grams ( 12 oz ) . Early versions of the external RAM cartridge contained 15 KB of memory using an assortment of memory chips , while later versions contained 16 KB of chips , but the lowest addressed kilobyte was disabled . 
