Memory types ROM – Read only memory RAM – Random Access Memory Word – a group of n bits processed as one word

ROM – Read only memory Holds 2n words of m bits n bits m bits

ROM – Read only memory 25x4 example Address Word 00000 1010 00001 1110 00010 1101 …

ROM – Read only memory 25x4 example A0 1 2 A1 A2 A3 A4 31 F3 F2 F1 F0

ROM – Read only memory A0 1 2 A1 An 2n מגשרים Fm F0

ROM Types ROM/PROM EPROM the bridges are permanent. “Programming” using other units (or in the factory). EPROM Can be reinitialized.

ROM with unusable combinations 12 bits Alpha numeric code (only 47 combinations) ROM 12 bits 6 bits

ROM with unusable combinations ROM - 12 entrances = 4096 words Using 47 words Wasting 4049 words

PLA – Programmable Logic Array Bridge n x k Bridge m OR k AND m outputs n entrances k x m Bridge n x k Bridge

PLA – Programmable Logic Array Typical PLA 16 entrances 48 and elements 8 outputs

RAM – Random Access memory Properties Can represent 2 bits Small dimensions Minimum Cost Reasonable access time

RAM - Random access memory m bits 0 …………………………1 Memory Address Register Read/Write n words Buffer Register Outgoing data Ingoing data

RAM – Random Access Memory Basic Cell Unit Write = 0 Read = 1

RAM – Random access memory (basic cell)

RAM – Random access memory (basic cell)