AKT211 – CAO 07 – Computer Memory

1. AKT211 – CAO07 – Computer Memory Ghifar Parahyangan Catholic University Okt 24, 2011

2. Feedback Review • using B. Indonesia for text in the presentation slides • make slower and clearer the explanation of the course materials (x) (v)

3. Outline • Computer Memory System • Memory Characteristics • Memory Hierarchy • RAM Basic Technology • Semiconductor • SRAM vs DRAM • Advanced RAM Organization • SDRAM vs DDR-RAM

4. COMPUTER MEMORY SYSTEM

5. Characteristics

6. Location • CPU • Internal • External

7. Capacity • Word size • The natural unit of organization • Number of words • or Bytes

8. Unit of Transfer • Internal • Usually governed by data bus width • External • Usually a block which is much larger than a word • Addressable unit • Smallest location which can be uniquely addressed • Word internally • Cluster on M$ disks

9. Access Methods (1) • Sequential • Start at the beginning and read through in order • Access time depends on location of data and previous location • e.g. tape • Direct • Individual blocks have unique address • Access is by jumping to vicinity plus sequential search • Access time depends on location and previous location • e.g. disk

10. Access Methods (2) • Random • Individual addresses identify locations exactly • Access time is independent of location or previous access • e.g. RAM • Associative • Data is located by a comparison with contents of a portion of the store • Access time is independent of location or previous access • e.g. cache

11. Memory Hierarchy • Registers • In CPU • Internal or Main memory • May include one or more levels of cache • “RAM” • External memory • Backing store

12. Memory Hierarchy - Diagram

13. RAM BASIC TECHNOLOGY

14. Semiconductor Main Memory • universally used as RAM basic technology • The basic element : “memory cell” • Read/Write • Volatile • Temporary storage • Static or dynamic

15. Memory Cell Operation

16. Semiconductor Memory Types

17. Dynamic RAM • Bits stored as charge in capacitors • Charges leak • Need refreshing even when powered • Simpler construction • Smaller per bit • Less expensive • Need refresh circuits • Slower • Main memory • Essentially analogue • Level of charge determines value

18. Dynamic RAM Structure • Address line active when bit read or written • Transistor switch closed (current flows) • Write • Voltage to bit line • High for 1 low for 0 • Then signal address line • Transfers charge to capacitor • Read • Address line selected • transistor turns on • Charge from capacitor fed via bit line to sense amplifier • Compares with reference value to determine 0 or 1 • Capacitor charge must be restored

19. Static RAM • Bits stored as on/off switches • No charges to leak • No refreshing needed when powered • More complex construction • Larger per bit • More expensive • Does not need refresh circuits • Faster • Cache • Digital • Uses flip-flops

20. Static RAM Structure

21. SRAM v DRAM • Both volatile • Power needed to preserve data • Dynamic cell • Simpler to build, smaller • More dense • Less expensive • Needs refresh • Larger memory units • Static • Faster • Cache

22. ADVANCED RAM ORGANIZATION

23. Synchronous DRAM (SDRAM) • Access is synchronized with an external clock • Conventional DRAM is asynchronous,CPU must wait the access time delay • With SDRAM, CPU doesn’t have to wait, it can do something else • Takes advantage of the burst mode concept to greatly improve performance • staying on the row containing the requested bit and moving rapidly through the columns, reading each bits as it goes • Idea : most of the time the data needed by the CPU will be in sequence ! • Maximum transfer rate to L2 cache : 528 MBps

24. SDRAM - Diagram

25. SDRAM Read Timing

26. DDR-SDRAM • Double Data Rate Synchronous Dynamic RAM • Higher bandwidth => greater speed • Higher transfer rate • Uses double pumping  transferring data on both the rising and falling edges of the clock signal • Classes : DDR1, DDR2, DDR3 • Neither of which are either forward or backward compatible !

27. DDR SDRAM Read Timing DDR SDRAM Read Timing

28. Any Question ?

29. THANK YOU