Chapter 5 Internal Memory

1. Chapter 5 Internal Memory 5.1 Semiconductor Main Memory 5.2 Error Correction 5.3 Advanced DRAM Organization

2. 5.1 Semiconductor Main Memory The basic element of a semiconductor is the memory cell • Properties of memory cells: • Exhibits two stable states (binary 1 and 0) • Capable of being written into –to set the state • Capable of being read to sense the state

3. 5.1 Semiconductor Main Memory

4. 5.1 Semiconductor Main Memory Random-access memory (RAM) • Characteristics: • Reads data from memory (fast) • Writes new data into memory (fast) • Requires constant power supply No Power, No data!!! THUS, RAM can be used only as temporary storage

5. 5.1 Semiconductor Main Memory Two types of RAM: DRAM SRAM • Dynamic • Memory cell is simpler • Requires data to be refreshed periodically in order to retain the data • Static • No need to be refreshed since transistors inside hold the data as long as the power supply is not cut off. 1. SRAM is static while DRAM is dynamic2. SRAM is faster compared to DRAM3. SRAM consumes less power than DRAM4. SRAM uses more transistors per bit of memory compared to DRAM5. SRAM is more expensive than DRAM6. Cheaper DRAM is used in main memory while SRAM is commonly used in cache memory

6. 5.1 Semiconductor Main Memory Read-only memory (ROM) • Characteristics: • No source of power is required to • maintain the data in memory • Reads data, but can’t write data • Applications used in ROMs : • Library subroutines • System programs • Function tables

7. 5.1 Semiconductor Main Memory Three common forms: EPROM (Erasable Programmable Read-Only Memory) – can be erased by exposure to strong ultraviolet light EEPROM (Electrically Erasable Programmable Read-Only Memory) -  is based on a similar semiconductor structure to EPROM, but allows its entire contents (or selected banks) to be electrically erased, then rewritten electrically, so that they need not be removed from the computer (or camera, MP3 player, etc.). Flash Memory an be erased and rewritten faster than ordinary EEPROM, and newer designs feature very high endurance

8. 5.2 Error Correction Error correction is the detection of errors and reconstruction of the original, error-free data. Semiconductor memory system is subject to errors

9. 5.2 Error Correction Two types of categories: Hard failures and soft errors Hard failure – permanent physical defects so that the memory cells affected cannot reliably store data, but become stuck at 0 or 1 or switch erratically between 0 and 1. Soft error– random, nondestructive event that alters the contents of memory cells without damaging the memory.

10. 5.2 Error Correction Most modern main memory systems include logic for both detecting and correcting errors

11. 5.2 Error Correction • No communication channel or storage device is completely error-free • As the number of bits per area or the transmission rate increases, more errors occur. • Impossible to detect or correct 100% of the errors • There are several Error Detection/Correction Methods

12. 5.2 Error Correction Hamming Code • One of the most effective codes for error-recovery • Used in situations where random errors are likely to occur • Single parity bit can only detect error, not correct it • Error-correcting codes require more than a single parity bit • Hamming codes work well when we can reasonably expect errors to be rare events. (ex: hard drives) • Hamming codes are useless when multiple adjacent errors are likely to occur.

13. 5.2 Error Correction Hamming Code

14. 5.3 Advanced DRAM Organization To compensate for the relatively slow speed of DRAM, new technologies have been introduced • Two most common: • Synchronous DRAM • RamBus DRAM Involve using the system Clock to provide the transfer of blocks of data