Memory types

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

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

3. ROM – Read only memory 25x4 example

4. ROM – Read only memory 25x4 example 0 A0 1 2 A1 A2 A3 A4 31 F3 F2 F1 F0

5. ROM – Read only memory 0 A0 1 2 A1 An 2n מגשרים Fm F0

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

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

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

9. 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

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

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

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

13. RAM – Random Access MemoryBasic Cell Unit Write = 0 Read = 1

14. RAM – Random access memory (basic cell)

15. RAM – Random access memory (basic cell)