Asynchronous DRAM (ADRAM) is widely used due to its internal architecture and interface to the processor's memory bus. However, ADRAM has slow access times which degrade system performance. Synchronous DRAM (SDRAM) was developed to exchange data with the processor synchronized by an external clock, allowing full processor speed without wait states. Later, Double Data Rate SDRAM and Rambus DRAM were introduced to increase data transfer rates.

1. DRAM
Ankush Srivastava
Email:- anksri000@gmail.com

2. Asynchronous DRAM
Asynchronous DRAM(ADRAM) chip is widely
used by following reason:
•Its internal architecture
•Its interface to the processor’s memory bus.

3.  In ADRAM, the processor gives addresses
and control levels to the memory, indicating
that the set of data at particular location
should be either read from or written into the
DRAM.
 After the access time, ADRAM either writes
or reads the data.

4.  During the access time, the ADRAM performs
the various internal functions, such as
activating the high capacitance of the row
and column lines, sensing data and routing
the data out through the output buffer.
 The processor must simply wait through this
delay, as the result the processing speed of
the whole system is degraded.

5.  But the problem with ADRAM that it is need
to insert one or more levels of SRAM cache
between the ADRAM main memory and the
processor.
 And SRAM is much costly compare to
DRAM.

6.  In recent year, a number of changes in
ADRAM architecture have been explored,
and some of these now in the market. These
are follows:
 SDRAM
 DDR DRAM
 RDRAM

7. Synchronous DRAM
 One of the most widely used form of DRAM is
the synchronous DRAM(SDRAM).
 The SDRAM exchange data with the
processor synchronized through an external
clock signal and running at the full speed of
the processor without any wait state.
 In SDRAM, the data moves in and out under
control of the system clock.

8.  The processor issue the instruction and
address information to SDRAM. The SDRAM
then responds after a set of number of clock
cycle. Meanwhile, the processor can safely
do other tasks while the SDRAM is
processing the request.
 As the result the processing speed of the
whole system is increase.

9. Rambus DRAM
 RDRAM, developed by Rambus, has been
adopted by Intel for its Pentium processor.
 It has become the main competitor to
SDRAM.
 RDRAM chips are vertical packages with all
pins one side. The chip exchanges data with
the processor over 28 wires no more than 12
cm long.
 The bus can address up to 320 RDRAM
chips and is rated at 1.6 GBps.

10. DDR DRAM
 SDRAM is limited by the fact that it can only
send data to the processor once per bus
clock cycle.
 A new version of SDRAM, referred to as
double data rate SDRAM can send data twice
per clock cycle.
 Once on the rising edge of the clock pulse
and once on the falling edge.

11. Cache DRAM
 Cache DRAM(CDRAM) integrates a small
SRAM cache(16 kb) on to a generic DRAM
chip.
 The SRAM on the CDRAM used in two ways:
 It can used as a true cache, consisting of 64-
bit line.
 Used as a buffer to support the serial access
of a block of data.

12. Clock Transfer Access
Frequency Rate Time
(MHz) (GB/s) (ns)
SDRAM 166 1.3 18
DDR 200 3.2 12.5
RDDRAM 600 4.8 12