Download Memory and Storage

Memory and Storage
Dr. Rebhi S. Baraka
[email protected]
Logic Design (CSCI 2301)
Department of Computer Science
Faculty of Information Technology
The Islamic University of Gaza
Outline
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Basics of Semiconductor Memory
Random-Access Memories (RAMs)
Read-Only Memories (ROMs)
Programmable ROMs (PROMS)
Flash Memories
Memory Expansion
Basics of Semiconductor Memory
• Units of binary data
– Bit
– Nibble (4 bits)
– Byte (8 bits)
– Word (1 or more bytes)
Memory Array
• A single binary bit is stored in a memory cell
• An organized group of cells is called a array
Memory Address and Capacity
• The location of a unit of data is called the address
• The capacity of a memory is the total number of
data units that can be stored in a memory.
Basic Memory Operations
• Write operation: puts data into a specified
address in the memory
Basic Memory Operations
• Read operation: takes data out of a specified
address in the memory
Random-Access Memories (RAMs)
• Data can be written into or read from any
selected address in any sequence
• When writing, old data is replaced by new
ones.
• When reading, data is not destroyed.
• It is typically used for short-term data sotrage
because it can’t retain data when power is
turned of.
The RAM Family
The RAM Family: Static RAM (SRAM)
• Flip-flops are used as storage cells
– Implemented in integrated circuits with several
MOS transistors, or
– Bipolar transistors.
The RAM Family: Static RAM (SRAM)
• The cell is selected by an active level on the
Select line and a data bit is written into the
cell by placing it on the Data in line.
• A Data bit is read by
taking it off the Data
out line.
The Basic SRAM Cell Array
The Basic SRAM Cell Array
• Organized in rows and columns
• Cells in a row share the same Row Select line.
• Each set of Data lines go to each cell in a given
column via a single line.
• This line serves as both input and output
(Data I/O)
Logic diagram for an asynchronous 32k x 8 SRAM.
•Operation is not
synchronized with a
system clock.
•In a read mode, 8 data
bits appear on output
lines.
•In a write mode, 8 bits are
applied to the data input
lines and are stored at a
selected address.
Tristate outputs and Buses
• Tristate buffers in a memory allow the data lines
to act as input or output lines and connect the
memory to the data bus in a computer.
• They have 3 output states:
– HIGH (1), LOW (0), and HIGH-Z (open)
– They are indicated by a small inverted triangle (See
the previous slide)
• A bus is a set of conductive paths that serve to
interconnect two or more functional components
of a system.
Memory Array
• SRAM can be organized in multiple bytes (16, 64,
32 bits, etc.)
• A typical 32K x 8 SRAM (see next slide) is
arranged in 256 rows and 128 x 8 columns.
• There are 215 =32,768 addresses and each
address contains 8 bits (32kbytes).
• Read
• Write
• Read/Write cycles
Basic organization of an asynchronous 32k x 8 SRAM.
Basic read and write cycle timing for the SRAM
tRC read cycle time
tAQ Adress access time
tEQ chip enable access time
tGQ output enable access time
tWC wrtie cycle time
ts(A) address setup time
tWD f time WE must remain low
th(D) data hold time
Basic Synchronous Burst SRAM Organization
• It is synchronized with the system clock
– For example in a computer system, it operates
with the same clock signal that operates the
processor.
• It uses clocked registers to synchronize all
inputs with the system clock.
• The address, the read/write input, the chip
enable, and the input data are all latched into
their respective register on an active clock
pulse edge.
basic block diagram of a synchronous SRAM with burst feature.
Basic Synchronous Burst SRAM Organization
• The Burst Feature
– It allows the memory to read or write at up to four
locations using a single address.
Address burst logic
Cache Memory
‫مخبأة‬/‫ذاكرة مخفية‬
• One of the major applications of SRAM.
• It is a relatively small, high speed memory that
stores the most recently used instructions or
data from the main memory which are
needed by the processor.
• It improves system performance
• Analogy: Home refrigerator and supermarket.
Cache Memory
Block diagram showing L1 and L2 cache memories in a computer system.
Dynamic RAMs (DRAMs)
• Use small capacitors.
• Simple, allowing very large memory arrays to
be constructed on a chip at a lower cost.
• Disadvantage: the capacitor can’t hold its
charge without being refreshed (recharged)
periodically.
• Refreshing requires additional circuitry.
MOSFET
Capacitor
A MOS DRAM cell consisting of a single MOSFET and
a capacitor
Basic Operation of a DRAM Cell
Transistor
acts as a
switch
•The R/W line, the row
line, the refresh line is
HIGH.
•The transistor turns on,
connecting the capacitor
to the bit line.
•The output buffer is
enabled, and the stored
data bit is applied to the
input of the refresh
buffer. HIGH
Read Only Memories (ROMs)
• A ROM contains permanently or
semipermanently stored data.
• A ROM stores data that are used repeatedly in
systems applications such as programmed
instructions for system initialization (BIOS).
• ROMs retain stored data when the power is
off.
The ROM family.
The ROM family.
• The Mask ROM – Known also as ROM, is
permanently programmed during the
manufacturing process to provide widely used
standard functions.
– Once it is programmed it can’t be changed.
ROMcells.
A representation of a 16 x 8-bit ROM array.
Representation of a ROM programmed as a binary-to-Gray
code converter.
A 1024-bit ROM with a 256 x 4 organization based on a 32
x 32 array.
Flash Memory
• Flash memory has a number of characteristics
that are not found in one of the other types of
memories.
– high-density (High storage capacity)
– Nonvolatility
– In-system read and write capability
– Fast operation
– Cost effectiveness
• A storage cell consists of a single floating-gate
MOS transistor.
The storage cell in a flash memory.
•The floating gate stores electrons (charge) as a result
of a sufficient voltage applied to the control gate.
•A 0 is stored when there is more charge
• A 1 is stored when there is less or no charge.
Programming operation
Storing a 0 or a 1 in a flash cell during the programming operation.
The read operation of a flash cell in an array.
Simplified illustration of removing charge from a cell during erase.
Basic flash memory array.
Basic flash memory array.
• When a cell in a given bit line turns on during
a read operation, there is current through the
bit line, which produces a voltage drop across
the active load .
• This voltage drop is compared to a reference
voltage with a comparator and an output
level indicating a 1 is produced.
• If a 0 is stored, then there is no current or
little current in the bit line and an opposite
level is produced on the comparator output.
Memory Expansion
• To increase the word length (no. of bits in
each address)
• To increase word capacity (no. of different
addresses)
• To increase both, word length and word
capacity
• This is accomplished by adding a no. of
memory chips to the address, data and control
buses.
Word Length Expansion
• The number of bits in the data bus must be increased
• Example: Expansion of two 65,536 x 4 ROMs into a 65,536 x 8
ROM to illustrate word-length expansion.
Example
• Expand the 65,536 x 4 ROM (64K x 4) to form a 64k x 8 ROM.
SIMMs and DIMMs
• Memories are supplied as:
– Single In-line Memory Modules (SIMMs)
– Dual In-line Memory Modules (DIMMs)
• The are small circuit boards on which memory
chips (ICs) are mounted with the inputs and
outputs connected to an edge connector on
the bottom of the board
• DIMMs are generally faster and used in newer
generations of machines.
30-pin and 72-pin SIMMs.
A SIMM/DIMM is inserted into a socket on a system board.
These slides are based on Digital Fundamentals 9th ed. By Thomas Floyd
End of the slides