dict.org

The DICT Development Group


Search for:
Search type:
Database:

Database copyright information
Server information
Wiki: Resources, links, and other information


1 definition found
 for dynamic random-access memory
From The Free On-line Dictionary of Computing (18 March 2015) :

  dynamic random-access memory
  DRAM
  dynamic RAM
  
      (DRAM) A type of semiconductor memory in which the
     information is stored in capacitors on a MOS integrated
     circuit.  Typically each bit is stored as an amount of
     electrical charge in a storage cell consisting of a capacitor
     and a transistor.  Due to leakage the capacitor discharges
     gradually and the memory cell loses the information.
     Therefore, to preserve the information, the memory has to be
     refreshed periodically.  Despite this inconvenience, the DRAM
     is a very popular memory technology because of its high
     density and consequent low price.
  
     The first commercially available DRAM chip was the Intel
     1103, introduced in 1970.
  
     Early DRAM chips, containing up to a 16k x 1 (16384 locations
     of one bit each), needed 3 supply voltages (+5V, -5V and
     +12V).  Beginning with the 64 kilobit chips, charge pumps
     were included on-chip to create the necessary supply voltages
     out of a single +5V supply.  This was necessary to fit the
     device into a 16-pin DIL package, which was the preferred
     package at the time, and also made them easier to use.
  
     To reduce the pin count, thereby helping miniaturisation,
     DRAMs generally had a single data line which meant that a
     computer with an N bit wide data bus needed a "bank" of (at
     least) N DRAM chips.  In a bank, the address and control
     signals of all chips were common and the data line of each
     chip was connected to one of the data bus lines.
  
     Beginning with the 256 kilobit DRAM, a tendency toward
     surface mount packaging arose and DRAMs with more than one
     data line appeared (e.g. 64k x 4), reducing the number of
     chips per bank.  This trend has continued and DRAM chips with
     up to 36 data lines are available today.  Furthermore,
     together with surface mount packages, memory manufacturers
     began to offer memory modules, where a bank of memory chips
     was preassembled on a little printed circuit board (SIP =
     Single Inline Pin Module, SIMM = Single Inline Memory Module,
     DIMM = Dual Inline Memory Module).  Today, this is the
     preferred way to buy memory for workstations and personal
     computers.
  
     DRAM bit cells are arranged on a chip in a grid of rows and
     columns where the number of rows and columns are usually a
     power of two.  Often, but not always, the number of rows and
     columns is the same.  A one megabit device would then have
     1024 x 1024 memory cells.  A single memory cell can be
     selected by a 10-bit row address and a 10-bit column address.
  
     To access a memory cell, one entire row of cells is selected
     and its contents are transferred into an on-chip buffer.  This
     discharges the storage capacitors in the bit cells.  The
     desired bits are then read or written in the buffer.  The
     (possibly altered) information is finally written back into
     the selected row, thereby refreshing all bits (recharging the
     capacitors) in the row.
  
     To prevent data loss, all bit cells in the memory need to be
     refreshed periodically.  This can be done by reading all rows
     in regular intervals.  Most DRAMs since 1970 have been
     specified such that one of the rows needs to be refreshed at
     least every 15.625 microseconds.  For a device with 1024 rows,
     a complete refresh of all rows would then take up to 16 ms; in
     other words, each cell is guaranteed to hold the data for 16
     ms without refresh.  Devices with more rows have accordingly
     longer retention times.
  
     Many varieties of DRAM exist today.  They differ in the way
     they are interfaced to the system - the structure of the
     memory cell itself is essentially the same.
  
     "Traditional" DRAMs have multiplexed address lines and
     separate data inputs and outputs.  There are three control
     signals: RAS\ (row address strobe), CAS\ (column address
     strobe), and WE\ (write enable) (the backslash indicates an
     active low signal).  Memory access procedes as follows:
     1. The control signals initially all being inactive (high), a
     memory cycle is started with the row address applied to the
     address inputs and a falling edge of RAS\ .  This latches the
     row address and "opens" the row, transferring the data in the
     row to the buffer.  The row address can then be removed from
     the address inputs since it is latched on-chip.  2. With RAS\
     still active, the column address is applied to the address
     pins and CAS\ is made active as well.  This selects the
     desired bit or bits in the row which subsequently appear at
     the data output(s).  By additionally activating WE\ the data
     applied to the data inputs can be written into the selected
     location in the buffer.  3. Deactivating CAS\ disables the
     data input and output again.  4. Deactivating RAS\ causes the
     data in the buffer to be written back into the memory array.
  
     Certain timing rules must be obeyed to guarantee reliable
     operation.  1. RAS\ must remain inactivate for a while before
     the next memory cycle is started to provide sufficient time
     for the storage capacitors to charge (Precharge Time).  2. It
     takes some time from the falling edge of the RAS\ or CAS\
     signals until the data appears at the data output.  This is
     specified as the Row Access Time and the Column Access Time.
     Current DRAM's have Row Access Times of 50-100 ns and Column
     Access Times of 15-40 ns.  Speed grades usually refer to the
     former, more important figure.
  
     Note that the Memory Cycle Time, which is the minimum time
     from the beginning of one access to the beginning of the next,
     is longer than the Row Access Time (because of the Precharge
     Time).
  
     Multiplexing the address pins saves pins on the chip, but
     usually requires additional logic in the system to properly
     generate the address and control signals, not to mention
     further logic for refresh.  Therefore, DRAM chips are usually
     preferred when (because of the required memory size) the
     additional cost for the control logic is outweighed by the
     lower price.
  
     Based on these principles, chip designers have developed many
     varieties to improve performance or ease system integration of
     DRAMs:
  
     PSRAMs (Pseudo Static Random Access Memory) are essentially
     DRAMs with a built-in address multiplexor and refresh
     controller.  This saves some system logic and makes the device
     look like a normal SRAM.  This has been popular as a lower
     cost alternative for SRAM in embedded systems.  It is not a
     complete SRAM substitute because it is sometimes busy when
     doing self-refresh, which can be tedious.
  
     Nibble Mode DRAM can supply four successive bits on one data
     line by clocking the CAS\ line.
  
     Page Mode DRAM is a standard DRAM where any number of
     accesses to the currently open row can be made while the RAS
     signal is kept active.
  
     Static Column DRAM is similar to Page Mode DRAM, but to access
     different bits in the open row, only the column address needs
     to be changed while the CAS\ signal stays active.  The row
     buffer essentially behaves like SRAM.
  
     Extended Data Out DRAM (EDO DRAM) can continue to output
     data from one address while setting up a new address, for use
     in pipelined systems.
  
     DRAM used for Video RAM ({VRAM) has an additional long
     shift register that can be loaded from the row buffer.  The
     shift register can be regarded as a second interface to the
     memory that can be operated in parallel to the normal
     interface.  This is especially useful in frame buffers for
     CRT displays.  These frame buffers generate a serial data
     stream that is sent to the CRT to modulate the electron beam.
     By using the shift register in the VRAM to generate this
     stream, the memory is available to the computer through the
     normal interface most of the time for updating the display
     data, thereby speeding up display data manipulations.
  
     SDRAM (Synchronous DRAM) adds a separate clock signal to the
     control signals.  It allows more complex state machines on
     the chip and high speed "burst" accesses that clock a series
     of successive bits out (similar to the nibble mode).
  
     CDRAM (Cached DRAM) adds a separate static RAM array used for
     caching.  It essentially combines main memory and cache
     memory in a single chip.  The cache memory controller needs to
     be added externally.
  
     RDRAM (Rambus DRAM) changes the system interface of DRAM
     completely.  A byte-wide bus is used for address, data and
     command transfers.  The bus operates at very high speed: 500
     million transfers per second.  The chip operates synchronously
     with a 250MHz clock.  Data is transferred at both rising and
     falling edges of the clock.  A system with signals at such
     frequencies must be very carefully designed, and the signals
     on the Rambus Channel use nonstandard signal levels, making it
     incompatible with standard system logic.  These disadvantages
     are compensated by a very fast data transfer, especially for
     burst accesses to a block of successive locations.
  
     A number of different refresh modes can be included in some of
     the above device varieties:
  
     RAS\ only refresh: a row is refreshed by an ordinary read
     access without asserting CAS\.  The data output remains
     disabled.
  
     CAS\ before RAS\ refresh: the device has a built-in counter
     for the refresh row address.  By activating CAS\ before
     activating RAS\, this counter is selected to supply the row
     address instead of the address inputs.
  
     Self-Refresh: The device is able to generate refresh cycles
     internally.  No external control signal transitions other than
     those for bringing the device into self-refresh mode are
     needed to maintain data integrity.
  
     (1996-07-11)
  

Questions or comments about this site? Contact webmaster@dict.org