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 2 n words of m bits. n bits. m bits. ROM – Read only memory 2 5 x4 example. ROM – Read only memory 2 5 x4 example. 0. A0. 1. 2.
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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 0 A0 1 2 A1 A2 A3 A4 31 F3 F2 F1 F0
ROM – Read only memory 0 A0 1 2 A1 An 2n מגשרים Fm F0
ROM Types • ROM/PROM • 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 m 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 Read/Write Address Register n words Buffer Register Outgoing data Ingoing data
RAM – Random Access MemoryBasic Cell Unit Write = 0 Read = 1