Semiconductor Main memory • The two basic forms of semiconductor random access memory are : • Dynamic RAM (DRAM) and static RAM (SRAM). • SRAM is faster and more expensive than DRAM. • SRAM is used for cache memory. • DRAM is used for main memory.
Semiconductor Main memory • The two basic forms of semiconductor random access memory are : • Dynamic RAM (DRAM) and static RAM (SRAM). • SRAM is faster and more expensive than DRAM • SRAM is used for cache memory. • DRAM is used for main memory.
Semiconductor Memory Types Memory Type Category Erasure Random-access memory (RAM) Read-write memory Electrically, byte-level Read-only memory (ROM) Read-only Memory [Write can not possible] Not possible Programmable ROM (PROM) Erasable PROM (EPROM) Electrically Erasable PROM (EEPROM) Flash memory Read-mostly memory [Write can be possible] UV light, chip-level Electrically, byte-level Electrically, block-level Volatility Volatile Nonvolatile
DRAM & SRAM Architecture
DRAM • A dynamic RAM (DRAM) is made with such cells that store data as charge on capacitors. • The presence or absence of charge in a capacitor is interpreted as a binary 1 or 0. • Because capacitors have a natural tendency to discharge, dynamic RAMs require periodic refreshing to maintain data storage. • The term ‘dynamic’ refers to this tendency of the stored charge to leak away, even with power continuously applied. • The transistor acts as a switch that is closed (allowing current to flow) if a voltage is applied to the address line and open (no current flows) if no voltage is present on the address line.
DRAM • For the write operation, a voltage signal is applied to the bit line, allowing a charge to be transferred to the capacitor. • A high voltage represents 1, and a low voltage represents 0. • For the read operation, a voltage signal is applied to the address line, the transistor turns on and the charge stored on the capacitor
SRAM • In a SRAM, binary values are stored using traditional flip-flop logic-gate configurations • A static RAM will hold its data as long as power is supplied to it. • Four transistors(T 1, T 2, T 3 & T 4) are cross connected in an arrangement that produces a stable logic state. • In logic state 1, point C 1 is high and point C 2 is low; in this state, T 1 and T 4 are off and T 2 and T 3 are on. • In logic state 0, point C 1 is low and point C 2 is high; in this state, T 1 and T 4 are on and T 2 and T 3 are off. • Unlike the DRAM, no refreshing is needed to retain data.
ROM • Read Only Memory (ROM) contains a permanent pattern of data that cannot be changed. • A ROM is nonvolatile; that is, no power source is required to maintain the bit values in memory. • While it is possible to read a ROM, it is not possible to write new data into it.
Types of ROM[1/3] 1] PROM(Programmable ROM) : § The PROM is nonvolatile and may be written into only once. • For the PROM, the writing(programming) process is performed electrically and by the supplier itself. 2] Erasable Programmable read-only memory (EPROM) • In this read and written done electrically same as with PROM. • However, before a write operation, all the storage cells must be erased to the same initial state by exposure of chip to ultraviolet radiation. • EPROM is more expensive than PROM, but it has the advantage of the multiple update capability. • Major Drawback is selective erasing is not possible.
Types of ROM[2/3] 3] Electrically Erasable Programmable read-only memory (EEPROM/ E 2 PROM) : • This is a read-mostly memory that can be written into at any time without erasing prior contents • In this read and written done electrically same as with PROM. • Here, for erasing process instead ultraviolet radiation electrically erasing is used which reduces cost to some extent. • Major advantages are selective erasing is possible(byte) and before a write operation, no need to erase storage cells.
Types of ROM[3/3] 4] Flash memory: • Another form of semiconductor memory is flash memory (so named because of the speed with which it can be reprogrammed) • Flash memory uses an electrical erasing technology. An entire flash memory can be erased in one or a few seconds, which is much faster than EPROM • In addition, it is possible to erase just blocks of memory rather than an entire chip(Selective erasing is possible). • Flash memory is intermediate between EPROM and EEPROM in both cost and functionality.