Memory System Organization and Architecture: Data Compression, Data Integrity, Memory

1. CHAPTER 4 Memory System Organization and Architecture -DATA COMPRESSION, DATA INTEGRITY, MEMORY

2. Data compression • Reduction in size of data in order to save space or transmission time. • Transform a string of characters in some representation (such as ASCII) into a new string (of bits, for example) which contains the same information but whose length is as small as possible.

4. Reasons of Data Compression

5. Types of compression

6. Lossy compression • Greater compression ration but poorer image • Most commonly used to compress multimedia data • Standards: • JPEG – joint photographic experts group • MPEG – motion picture experts group

7. Example

8. Lossless compression • Required for text and data files, such as bank records, text articles, etc. • technique – identify repeating words and assign them a code. • During decompression, the code would be changed back to the actual word.

9. Examples

10. Lossyvs Lossless

11. Data integrity • The quality of:

12. Ways that can affects data integrity

13. Ways to keep data integrity

14. Memory • Memory / main memory / primary storage / working storage • Holds programs/instructions and data for access by the CPU • Made up of a large number of cells, each with its own address • Can be divided into two: • volatile • non-volatile.

15. Volatile memory • RAM (Random Access Memory) • Used to store program and user data. • Readable and rewritable (volatile). • Two types of RAM: • Static RAM (SRAM) • Dynamic RAM (DRAM) • DRAM(Dynamic RAM) • Less expensive, require less electrical power, and can be made smaller, with more bits of storage in a single integrated circuit. • Requires extra electronic circuitry that “refreshes” memory periodically.

16. SRAM (Static RAM) • Faster access compared to DRAM • Useful in very high speed computers and for small amounts of high speed memory. • Does not require refreshing. • More expensive and require more chips. • MRAM (Magnetoresistive RAM) • Stores data using magnetic charges • Greater storage, consumes less power and has faster access times compared to other types of RAM

17. Non - volatile Memory • ROM (Read Only Memory) • Used for programs that are permanent and unchanged • The program can only be read. • Example: operating system routine. • The programs on ROM were prewritten when it was manufactured. • EPROM(Erasable Programmable ROM) • Can be programmed by the user • Contents of EPROM can be erased by exposing it to ultra-violet light.

18. EEPROM(Electrically Erasable Programmable Read Only Memory) • Contents can be programmed and erased by the user • Non-volatile, writable memory • Rewriting can be done by erasing memory cells selectively, then writing new data into those cells. • Flash ROM • Similar to EEPROM (the way it is erased and written). • The only difference is that it is faster and more flexible than EEPROM. • Can erase and write data in blocks rather than one byte at a time. • Used in handphones, digital camera and MP3 players.

20. Memory capacity • The size of a memory is defined by the address scheme • If an address scheme has m bits, the maximum number of cells directly addressable is 2^m. • If a memory has n cells, the cells will have addresses 0 to n-1. • Eg: A memory with 32 cells would have addresses 0 to 31.

21. Eg: If the address scheme consists of 16 bits, the size of the memory would be 64K • 16 bit address = 2^16 => size of memory is 64K

22. Eg: Given a computer’s memory specification is 128MB RAM. Calculate the: • memory capacity in bytes • address size • largest address

23. Solution: • 1K = 1024 bytes = 2^10 • 1M = 1024 x 1024 = 2^10 x 2^10 = 2^20 • Memory capacity = 128 * 1024 * 1024 = 134217728 bytes

24. Address size 128 MB = 128 * 2^20 = 2^7 * 2^20 = 2^27 = 27 bits • Largest address = 134217728 –1 = 134217727