CS 325: CS Hardware and Software Organization and Architecture

1. CS 325: CS Hardware and SoftwareOrganization and Architecture Memory Organization

2. Computer Systems Structure

3. Storage Hierarchy & Characteristics

4. Storage/Memory Hierarchy

5. Principle of Memory Hierarchy • To optimize memory performance for a given cost: • A set of technologies are arranged in a hierarchy that contains a relatively small amount of fast memory and larger amounts of less expensive, but slower memory.

6. Memory Hierarchy Importance • 1980: no cache on CPU1995: 2-level cache on CPU • 1989: first Intel CPU with cache on chip

7. Memory Storage Characteristics • Location • Capacity • Unit of transfer • Access method • Performance • Physical type • Physical characteristics • Organization

8. Memory Storage - Location • CPU • Registers • L1, L2, L3, L4 Cache • Internal • Main Memory (System RAM) • BIOS (EEPROM) • External • Magnetic Disk (HDD) • Non Volatile Solid State (SSD) • Optical • Magnetic Tape

9. Memory Storage - Capacity • Word size • The natural unit of organization • Expected size of most data and instructions • Typically 32 bits or 64 bits • Past: 16 bits • Typical Storage • L1 Cache: 32 – 64 KB per core • L2 Cache: 128 – 512 KB per core • L3 Cache: 2 – 8 MB (shared) • L4 Cache: 0 – 128 MB (video memory) • Main Memory (RAM): 4 – 32 GB (Typical Desktop) • HDD Cache: 16 – 64 MB • SDD: 64 – 512 GB • HDD: 200 – 2000 GB (Inexpensive, but extremely slow) • Optical: • DVD: 4.7 – 17.08 GB • Blu-ray: 25 – 100 GB • Magnetic Tape: 10 – 35 TB per cartridge (uncompressed)

10. Memory Storage – Unit of Transfer • Internal • Usually governed by data bus width • External • Usually a block which is much larger than a word • Addressable unit • Smallest unit which can be uniquely addressed • Byte internally (typically)

11. Memory Storage – Access Methods • Sequential (tape): • Shared read/write mechanism • Start at the beginning and read through in order • Access time depends on location of data and previous location of magnetic tape • Direct (disk) • Shared read/write mechanism • Individual blocks have unique address • Access data by jumping to vicinity plus sequential search • Access time depends on location of data and previous location of read/write mechanism

12. Memory Storage – Access Methods • Random (RAM): • Individual addresses identify locations exactly • Access time is independent of location or previous access • Associative (Cache): • Data is located by a comparison with contents of a portion of the store • Access time is independent of location or previous access

13. Memory Storage – Performance • Latency/Access time • Time between presenting the address and getting the valid data • Memory Cycle time • Time may be required for the memory to “recover” before next access • Cycle time is latency + recovery • Transfer Rate • Rate at which data can be moved • # of bits * (1/cycle time)

14. Performance – Transfer Rate Example Problem • Assume we have a 32-Mbit SDRAM memory with 8 bits simultaneously read and a cycle time of 250 ns. • How fast can data be moved out of memory? 8b * (1/250ns) = 8b * (4x106/s) = 32 Mbps = 4 MBps

15. Memory Storage – Physical Types • Semiconductor • Cache • Main Memory (RAM) • SSD • Magnetic • HDD • Tape • Optical • CD • DVD • Blu-Ray • Others • Bubble • Hologram

16. Memory Storage – Physical Characteristics • Volatility • Erasable • Power consumption/Heat

17. Memory Storage – Organization • Physical arrangement of bits into words • Not always obvious • Striped across multiple disks

18. Memory Storage – RAID • RAID: Redundant Array of Inexpensive Disks • Combines multiple disks into a logical unit for the purposes of • Data redundancy • Performance Improvement • Or both • Can be implemented by software or hardware • Software: OS controlled • Hardware: Physical RAID controller

19. Memory Storage – RAID Levels • RAID 0 • Striped data without parity or mirroring. Performance increase. • RAID 1 • Mirrored data without parity or striping. Fault tolerance. • RAID 5 (most common) • Striped data with distributed parity. Requires at least three disks. Performance increase with Striped data and Fault tolerance. Can still operate with one failed drive.

20. Memory Storage – Bottom Line • How much? • Capacity • How fast? • Performance • How expensive?

21. Memory Storage – Hierarchy List • Registers • L1 Cache • L2 Cache • L3 Cache • Main Memory • Disk Cache • SSD • HDD • Optical • Tape

22. Memory Basics

23. Main Memory Basics • Memory: Where computer stores programs and data • Bit (binary digit): basic unit (8 bits = 1 byte) • Each memory cell (location) has an address numbered 0,…,n-1 (for n memory cells) • Possible address range limited by address size • m bits in address means 2m addresses • Memory cell size (typically 1 byte) grouped together into words (typically 32 or 64 bits) • 32 bit computer will typically have 32 bit registers and instructions for manipulating 32 bit words • 64 bit computer will be similar

24. Semiconductor Memory • Random Access Memory (RAM): • All semiconductor memory is random access • Directly accessed by address logic • Read/Write • Volatile • Requires constant power supply • Temporary storage • Static • Holds data • Dynamic • Periodically refreshes charge

25. Static RAM • Bits stored as on/off switches (transistors) • No charges to leak • Does not need refresh circuits • No refreshing needed when powered • Larger per bit • More expensive • Faster • Example: • Cache Memory:

26. SRAM Illustration • When write enable is high, output is the same as input. • Otherwise, output holds previous input value

27. Dynamic RAM • Bits stored as charge in capacitors (also uses transistors) • Charges leak from capacitors • Needs refreshing, even when powered • Needs refresh circuits • Smaller per bit • Less expensive • Slower • Asynchronous and Synchronous DRAMs • Example: • Main memory

28. DRAM Illustration • When write enable is high, output is the same as input. • Otherwise, output holds previous input value • Includes capacitor refresh circuits

29. Read Only Memory (ROM) • Permanent storage • Microprogramming • Library subroutines • Systems programs • Function tables

30. Measures of Memory Technology • Density • Latency and cycle time

31. Memory Density • Refers to memory cells per square area of silicon • Usually states as number of bits on standard chip size • Examples: • 1 mb chip • 4 mb chip • Memory cells typically structured in arrays • 1Mb x 1 chip • 256 Kb x 4 chips • Note: higher density chip generates more heat

32. Memory Packaging: Chips • 16 Mbit chip (4M x 4) • WE = Write Enable • OE = Output Enable • RAS = Row Address Select • CAS = Column Adress Select • A0 – A10 = 11 address bits • D1 – D4 = Data to be read/written • NC = No connect, for even # of pins • Vcc = Power Supply • Vss = Ground Pin

33. Read-Write Performance • In many memory technologies, the time required to fetch information from memory differs from the time required to store information in memory. • This difference can be dramatic. • Performance is then determined by Read and Write operations.