Solid State Drive (SSD):

 

A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It is also sometimes called a solid-state device or a solid-state disk, even though SSDs lack the physical spinning disks and movable read–write heads used in hard disk drives (HDDs) and floppy disks.

Compared with electromechanical drives, SSDs are typically more resistant to physical shock, run silently, and have quicker access time and lower latency. SSDs store data in semiconductor cells. Hybrid drives or solid-state hybrid drives (SSHDs), such as Apple's Fusion Drive, combine features of SSDs and HDDs in the same unit using both flash memory and a HDD in order to improve the performance of frequently-accessed data.

SSDs based on NAND Flash will slowly leak charge over time if left for long periods without power. This causes worn-out drives (that have exceeded their endurance rating) to start losing data typically after one year (if stored at 30 °C) to two years (at 25 °C) in storage; for new drives it takes longer. Therefore, SSDs are not suitable for archival storage. 3D XPoint is a possible exception to this rule; it is a relatively new technology with unknown long-term data-retention characteristics.

SSDs can use traditional HDD interfaces and form factors, or newer interfaces and form factors that exploit specific advantages of the flash memory in SSDs. Traditional interfaces (e.g. SATA and SAS) and standard HDD form factors allow such SSDs to be used as drop-in replacements for HDDs in computers and other devices. SSDs have a limited lifetime number of writes, and also slow down as they reach their full storage capacity.

 

Comparison of NAND-based SSD and HDD
Attribute or characteristic	Solid-state drive (SSD)	Hard disk drive (HDD)
Price per capacity	SSDs generally are more expensive than HDDs.	They are cheaper than SSDs.
Storage capacity	In 2018, SSDs were available in sizes up to 100 TB, but less costly; 120 to 512 GB models were more common.	In 2018, HDDs of up to 16 TB were available.
Reliability – data retention	If left without power, worn out SSDs typically start to lose data after about one to two years in storage, depending on temperature. New drives are supposed to retain data for about ten years.	If kept in a dry environment at low temperatures, HDDs can retain their data for a very long period of time even without power. However, the mechanical parts tend to become clotted over time and the drive fails to spin up after a few years in storage.
Reliability – longevity	SSDs have no moving parts to fail mechanically so in theory, should be more reliable than HDDs. However, in practice this is unclear. SSDs have undergone many revisions that have made them more reliable and long lasting. New SSDs in the market today use power loss protection circuits, wear leveling techniques and thermal throttling to ensure longevity.	HDDs have moving parts, and are subject to potential mechanical failures from the resulting wear and tear so in theory, should be less reliable than SSDs. However, in practice this is unclear. When stored offline (unpowered on the shelf) in long term, the magnetic medium of HDD retains data significantly longer than flash memory used in SSDs.
Start-up time	Almost instantaneous; no mechanical components to prepare. May need a few milliseconds to come out of an automatic power-saving mode.	Drive spin-up may take several seconds.
Sequential access performance	In consumer products the maximum transfer rate typically ranges from about 200 MB/s to 3500 MB/s, depending on the drive. Enterprise SSDs can have multi-gigabyte per second throughput.	Once the head is positioned, when reading or writing a continuous track, a modern HDD can transfer data at about 200 MB/s. Data transfer rate depends also upon rotational speed, which can range from 3,600 to 15,000 rpm and also upon the track (reading from the outer tracks is faster). Data transfer speed can be up to 480 MB/s (experimental).
Random access performance	Random access time typically under 0.1 ms. As data can be retrieved directly from various locations of the flash memory; access time is usually not a big performance bottleneck.	Read latency time is much higher than SSDs. Random access time ranges from 2.9 (high end server drive) to 12 ms (laptop HDD) due to the need to move the heads and wait for the data to rotate under the magnetic head.
Noise (acoustic)	SSDs have no moving parts and therefore are silent, although, on some SSDs, high pitch noise from the high voltage generator (for erasing blocks) may occur.	HDDs have moving parts (heads, actuator, and spindle motor) and make characteristic sounds of whirring and clicking; noise levels vary depending on the RPM, but can be significant. Laptop hard drives are relatively quiet.
Susceptibility to environmental factors	No moving parts, very resistant to shock, vibration, movement, and contamination.	Heads flying above rapidly rotating platters are susceptible to shock, vibration, movement, and contamination which could damage the medium.
Installation and mounting	Not sensitive to orientation, vibration, or shock. Usually no exposed circuitry. Circuitry may be exposed in a card form device and it must not be short-circuited by conductive materials.	Circuitry may be exposed, and it must not be short-circuited by conductive materials (such as the metal chassis of a computer). Should be mounted to protect against vibration and shock. Some HDDs should not be installed in a tilted position.
Susceptibility to magnetic fields	Low impact on flash memory, but an electromagnetic pulse will damage any electrical system, especially integrated circuits.	In general, magnets or magnetic surges may result in data corruption or mechanical damage to the drive internals. Drive's metal case provides a low level of shielding to the magnetic platters.
Weight and size	SSDs, essentially semiconductor memory devices mounted on a circuit board, are small and lightweight. High performance models often have heat sinks attached to the device, or have bulky cases that serve as its heat sink, increasing its weight.	HDDs are generally heavier than SSDs, as the enclosures are made mostly of metal, and they contain heavy objects such as motors and large magnets. 3.5-inch drives typically weigh around 700 grams.
Read/write performance symmetry	Less expensive SSDs typically have write speeds significantly lower than their read speeds. Higher performing SSDs have similar read and write speeds.	HDDs generally have slightly longer (worse) seek times for writing than for reading.
Power consumption	High performance flash-based SSDs generally require half to a third of the power of HDDs. High-performance DRAM SSDs generally require as much power as HDDs, and must be connected to power even when the rest of the system is shut down. Emerging technologies like DevSlp can minimize power requirements of idle drives.	The lowest-power HDDs (1.8-inch size) can use as little as 0.35 watts when idle. 2.5-inch drives typically use 2 to 5 watts. The highest-performance 3.5-inch drives can use up to about 20 watts.
Maximum areal storage density (Terabits per square inch)	2.8	1.2

 

 

Rambus DRAM (RDRAM) : Specifications, Performance, and Uses

 

Rambus DRAM (RD-RAM):

 

Rambus DRAM (RDRAM), and its successors Concurrent Rambus DRAM (CRDRAM) and Direct Rambus DRAM (DRDRAM), are types of synchronous dynamic random-access memory (SDRAM) developed by Rambus from the 1990s through to the early-2000s. The third-generation of Rambus DRAM, DRDRAM was replaced by XDR DRAM. Rambus DRAM was developed for high-bandwidth applications, and was positioned by Rambus as replacement for various types of contemporary memories, such as SDRAM.

DRDRAM was initially expected to become the standard in PC memory, especially after Intel agreed to license the Rambus technology for use with its future chipsets. Further, DRDRAM was expected to become a standard for graphics memory. By around 2003, DRDRAM was no longer supported by any personal computer.

PC-800 RDRAM operated at 400 MHz and delivered 1600 MB/s of bandwidth over a 16-bit bus. It was packaged as a 184-pin RIMM (Rambus In-line Memory Module) form factor, similar to a DIMM (Dual In-line Memory Module). Data is transferred on both the rising and falling edges of the clock signal, a technique known as DDR. To emphasize the advantages of the DDR technique, this type of RAM was marketed at speeds twice the actual clock rate, i.e. the 400 MHz Rambus standard was named PC-800. This was significantly faster than the previous standard, PC-133 SDRAM, which operated at 133 MHz and delivered 1066 MB/s of bandwidth over a 64-bit bus using a 168-pin DIMM form factor.

Fig: Samsung RDRAM 6400 128 MB

Module specifications:

Designation	Bus width (bits)	Channels	Clock rate (MHz)	Bandwidth (MB/s)
PC600	16	Single	266	1066
PC700	16	Single	355	1420
PC800	16	Single	400	1600
PC1066 (RIMM 2100)	16	Single	533	2133
PC1200 (RIMM 2400)	16	Single	600	2400
RIMM 3200	32	Dual	400	3200
RIMM 4200	32	Dual	533	4200
RIMM 4800	32	Dual	600	4800
RIMM 6400	32	Dual	800	6400

 

Performance:

Compared to other contemporary standards, Rambus showed increase in latency, heat output, manufacturing complexity, and cost. Because of more complex interface circuitry and increased number of memory banks, RDRAM die size was larger than that of contemporary SDRAM chips, and results in a 10–20 percent price premium at 16 Mbit densities. Note that the most common RDRAM densities are 128 Mbit and 256 Mbit.

PC-800 RDRAM operated with a latency of 45 ns, more than that of other SDRAM varieties of the time. RDRAM memory chips also put out significantly more heat than SDRAM chips, necessitating heat spreaders on all RIMM devices. RDRAM includes additional circuitry (such as packet de-multiplexers) on each chip, increasing manufacturing complexity compared to SDRAM. RDRAM was also up to four times the price of PC-133 SDRAM due to a combination of higher manufacturing costs and high license fees. PC-2100 DDR SDRAM, introduced in 2000, operated with a clock rate of 133 MHz and delivered 2100 MB/s over a 64-bit bus using a 184-pin DIMM form factor.

To achieve RDRAM's 800 MHz clock rate, the memory module runs on a 16-bit bus instead of a 64-bit bus in contemporary SDRAM DIMM. At the time of the Intel 820 launch some RDRAM modules operated at rates less than 800 MHz.

Uses:

Video game consoles:

Rambus's RDRAM saw use in two video game consoles, beginning in 1996 with the Nintendo 64. The Sony PlayStation 2 was equipped with 32 MB of RDRAM, and implemented a dual-channel configuration resulting in 3200 MB/s available bandwidth.

Texas Instruments DLP:

RDRAM was used in Texas Instruments' Digital Light Processing (DLP) systems.

Video cards:

Cirrus Logic implemented RDRAM support in their Laguna graphics chip, with two members of the family; the 2D-only 5462 and the 5464, a 2D chip with 3D acceleration. 

Synchronous Dynamic Random Access Memory (SD-RAM) : Development, Basics, and Types

 

Synchronous Dynamic Random Access Memory (SD-RAM):

 

SDRAM or Synchronous Dynamic Random Access Memory is a form of DRAM semiconductor memory that can run at faster speeds than conventional DRAM.

SDRAM memory is widely used in computers and other computing related technology. After SDRAM was introduced, further generations of double data rate RAM have entered the mass market – DDR which is also known as DDR1, DDR2, DDR3 and DDR4.

The use of SDRAM was so effective that it only took about four years after its introduction in 1996/7 before its use had exceeded that of DRAM in PCs because of its greater speed of operation. Nowadays SDRAM based memory is the major type of dynamic RAM used across the computing spectrum.

SDRAM Development:

The basic idea behind SDRAM has been in existence for many years. The first ideas appeared as early as the 1970s. The SDRAM concept was also used in some early Intel processors.

One of the first commercial SDRAM offerings was the KM48SL2000 which was introduced by Samsung in 1993. Although this did not gain universal acceptance immediately, the uptake was relatively quick. The improved speed of SDRAM meant that by about the turn of the century, i.e. 2000 SDRAM had virtually replaced the standard DRAM technology in most computer applications.

In order to ensure that SDRAM technology is interchangeable, JEDEC, the industry body for semiconductor standards, adopted its first SDRAM standard in 1993. This facilitated an open common standard for developing SDRAM. It also enabled developers to be able to have the facility of utilising product from more than one manufacturer and having a viable second source option.

With the basic SDRAM established, further develops took place. A form of SDRAM known as double data rate, DDR SDRAM appeared in 2000 with JEDEC Release 1 of their standard 79C which was updated to Release 2 in May 2002 and then Release C in March 2003.

DDR SDRAM was followed by the next version named DDR2 SDRAM. It was first introduced in mid 2003 when two clock rates were available: 200 MHz (referred to as PC2-3200) and 266 MHz (PC2-4200). The first offerings of DDR2 SDRAM were inferior to the previous DDR SDRAM, but by the end of 2004 its performance had been improved making its performance exceed that of DDR formats.

Later, the next version of SDRAM was launched. Known as DDR3 SDRAM, the first prototypes were announced in early 2005. However it took until mid-2007 before the first computer motherboards using DDR3 became available. Further developments include the next phase of SDRAM which will be DDR4 SDRAM and the most recent is DDR5 SDRAM.

SDRAM Basics:

Traditional forms of memory including DRAM operate in an asynchronous manner. They react to changes as the control inputs change, and also they are only able to operate as the requests are presented to them, dealing with one at a time.

SDRAM is able to operate more efficiently. It is synchronised to the clock of the processor and hence to the bus. With SDRAM having a synchronous interface, it has an internal finite state machine that pipelines incoming instructions. This enables the SDRAM to operate in a more complex fashion than an asynchronous DRAM. This enables it to operate at much higher speeds.

As a result of this SDRAM is capable of keeping two sets of memory addresses open simultaneously. By transferring data alternately from one set of addresses, and then the other, SDRAM cuts down on the delays associated with asynchronous RAM, which must close one address bank before opening the next.

The term pipelining is used to describe the process whereby the SDRAM can accept a new instruction before it has finished processing the previous one. In other words, it can effectively process two instructions at once.

For writing, one write command can be immediately followed by another without waiting for the original data to be stored within the SDRAM memory itself. For reading, the requested data appears a fixed number of clock pulses after the read instruction was presented. It is possible to send additional instructions during the delay period which is termed the latency of the SDRAM.

SDRAM Types:

SDRAM technology underwent a huge amount of development. As a result several successive families of the memory were introduced, each with improved performance over the previous generation.

·         SDR SDRAM: This is the basic type of SDRAM that was first introduced. It has now been superseded by the other types below. It is referred to as Single Data Rate SDRAM, or just SDRAM.

·         DDR SDRAM: DDR SDRAM, also known as DDR1 SDRAM gains its name from the fact that it is Double Data Rate SDRAM. This type of SDRAM provides data transfer at twice the speed of the traditional type of SDRAM memory. This is achieved by transferring data twice per cycle.

·         DDR2 SDRAM: DDR2 SDRAM can operate the external bus twice as fast as its predecessor and it was first introduced in 2003.

·         DDR3 SDRAM: DDR3 SDRAM is a further development of the double data rate type of SDRAM. It provides further improvements in overall performance and speed.

·         DDR4 SDRAM: DDR4 SDRAM was the next generation of DDR SDRAM. It provided enhanced performance to meet the demands of the day. It was introduced in the latter half of 2014.

·         DDR5 SDRAM: Development of SDRAM technology is moving forwards and the next generation of SDRAM, labelled DDR5 is currently under development. The specification was launched in 2016 with expected first production in 2020. DDR5 will reduce power consumption while doubling bandwidth and capacity.