Memory and Storage - Sheetal Gosrani

Overview Memory Hierarchy RAM Memory Chip Organization ROM Flash Memory

Memory Hierarchy Slow and inexpensive Increasing performance and increasing cost

Memory Hierarchy (contd)‏ Registers top in the hierarchy matched in speed to the CPU but are large and consume significant power number of registers in a processor are small Secondary and off-line storage devices at the bottom of the hierarchy cost per stored bit small in terms of money and power but access time is very long compared to registers

Random Access Memory(RAM)‏ The term “random” means any memory location can be accessed in the same amount of time regardless of its position in the memory. Volatile memory Types of RAM:  SRAM  DRAM

Static RAM Static RAM (SRAM) based on flip flops Contents of each location persist as long as power is applied. Fast but relatively large Consume a lot of power Used for memory applications that are small but fast

Dynamic RAM Dynamic RAM (DRAM) employ capacitors Capacitor stores electric charge whose level represents a 1 or 0 Capacitors dissipate with time and hence the charge must be restored frequently DRAMs smaller, slower than SRAMs support low cost, low power and high density and hence used in main memory

Memory Chip organization

RAM chip A0 – Am-1 : Address lines from 0 to m-1 CS : Chip Select (CS = 0, chip selected)‏ WR : ReadWrite (WR = 0, write operation)‏

RAM Grid

During read operation:  Entire row is selected  It is fed into the column MUX  MUX selects a single bit for output During write operation:  Single bit to be written is distributed by the DEMUX to the target column  Row decoder selects the proper column to be written

Read Only Memory(ROM)‏ Data stored in these chips is nonvolatile -- it is not lost when power is turned off. On turn on, the computer loads BIOS from ROM Data stored in these chips is either unchangeable or requires a special operation to change Applications: Videogames, calculators, microwave oven, etc

Read Only Memory(ROM) (contd)‏ Lets see the circuit for a ROM that stores four four-bit words(0101, 1011, 1110, 0000)‏ PROM burner allows contents of the ROM to be overwritten only once EPROMs – could be written several times EEPROMs – allow the content to be rewritten electrically

Flash Memory A section of memory cells can be erased in a single step, or 'flash', thus the name Nonvolatile memory Flash memory is used for easy and fast information storage in such devices as digital cameras, mobile, video game consoles

References Textbook: Computer Organization and Architecture Wiki pages   s_memory memory1.htm

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