RAM IN DETAIL

What is RAM?
RAM is Random Access Memory. RAM is the area where your computer stores programs that you are currently running and data that you are currently working on.
RAM can be contrasted with disk storage. Disk storage holds all of your programs and all of your data -- whether you are working with them or not. When you turn off your computer, the contents of RAM instantly disappear, but the contents of your disk storage remain unharmed.
RAM is also sometimes contrasted with ROM. ROM (Read Only Memory) are memory chips which have had information stored on them which cannot be changed. Your motherboard may contain some ROM chips.
Types of RAM
The two main types of RAM are:
● Dynamic RAM (DRAM)
● Static RAM (SRAM)

What is DRAM?
DRAM is Dynamic Random Access Memory.
DRAM is the most common form of RAM.
When someone says that a computer has "one gigabyte of RAM", what they really mean is that the computer has one gigabyte of DRAM.
DRAM is called dynamic because it must constantly be refreshed or it will lose the data which it is supposed to be storing.
Refreshing DRAM consists of reading the contents from the DRAM and immediately writing them back to the DRAM.
DRAM is made up of large arrays of very small capacitors. Each of these capacitors is slowly leaking energy, and if the DRAM is not refreshed, eventually one or more of the capacitors will leak enough energy that a 1 will become a 0 and data corruption will occur.
DRAM is often contrasted with SRAM (Static RAM). SRAM is able to store data as long as power is applied to it, without needing to be refreshed. SRAM is also able to be faster than DRAM. The drawback, of course, is that SRAM is much more expensive than DRAM.
Both DRAM and SRAM lose their contents when the power to them is turned off.
What is SRAM?
SRAM is Static RAM.
SRAM is used in small amounts in computers where very fast RAM is required, such as in the L2 cache of many CPU's.
SRAM is often contrasted with DRAM (Dynamic RAM). Dynamic is much less expensive than SRAM, but is usually slower and must constantly be refreshed in order to preserve its contents.
Types of SRAM include:
● Asynchronous Static RAM
● Synchronous Burst Static RAM
● Pipeline Burst Static RAM

What is NVRAM?
NVRAM is Non-Volatile Random Access Memory.
NVRAM differs from DRAM and SRAM in that NVRAM retains the contents of its memory when the power is turned off.
NVRAM is typically implemented using flash memory, although pseudo-NVRAM designs consisting of battery backed-up Static RAM have also been utilized.

What is SDRAM?
SDRAM is Synchronous Dynamic RAM.
SDRAM is a variant of DRAM in which the memory speed is synchronized with the clock pulse from the CPU.
This synchronization enables the SDRAM to pipeline read and write requests. Pipelining enables the SDRAM to accept commands at the same time as it is processing other commands.
There are three variants of SDRAM:
● Single Data Rate SDRAM - SDR SDRAM
● Dual Data Rate SDRAM - DDR SDRAM
● Dual Data Rate 2 SDRAM - DDR2 SDRAM

What is SDR SDRAM?
SDR SDRAM is Single Data Rate SDRAM.
SDR SDRAM is the original SDRAM standard, which has since been replaced by DDR SDRAM.
DDR SDRAM doubles the bandwidth of SDR DRAM by transferring data twice per cycle on both edges of the clock signal, implementing burst mode data transfer.

Standard SDR SDRAM DIMMs
SDR SDRAM is normally packaged in DIMM modules.
SDRAM DIMM modules are sold according to clock speed (MHz), bus speed (megatransfers per second), and transfer rate (megabytes per second.
DIMM Module Chip Type Clock Speed Bus Speed Transfer Rate
PC66 10ns 66 66 533
PC100 8ns 100 100 800
PC133 7.5/7ns 133 133 1,066

DR SDRAM is Double Data Rate SDRAM.
DDR SDRAM is an improvement over regular SDRAM, also known as SDR SDRAM (Single Data Rate SDRAM).
DDR SDRAM doubles the bandwidth of SDR DRAM by transferring data twice per cycle on both edges of the clock signal, implementing burst mode data transfer.
DDR SDRAM is being supplanted by DDR2 SDRAM.
Standard DDR SDRAM DIMM's
DDR SDRAM is normally packaged in DIMM modules.
DIMM Module Chip Type Clock Speed Bus Speed Transfer Rate
PC1600 DDR200 100 200 1,600
PC2100 DDR266 133 266 2,133
PC2400 DDR300 150 300 2,400
PC2700 DDR333 166 333 2,667
PC3000 DDR366 183 366 2,933
PC3200 DDR400 200 400 3,200
PC3500 DDR433 216 433 3,466
PC3700 DDR466 233 466 3,733
PC4000 DDR500 250 500 4,000
PC4300 DDR533 266 533 4,266

Standards for DDR SDRAM
DDR SDRAM standards are still being developed and improved.
DDR SDRAM Standard Frequency (MHz) Voltage
DDR 400-533 2.5
DDR2
667-800 1.8
DDR3 1066 to ... 1.5
Higher frequencies enable higher rates of data transfer.
Lower voltages result in less heat radiation and longer batter life for portable computing devices. They also allow greater component density, which allows higher capacity in the same package size.
What is RDRAM?
RDRAM is Rambus DRAM.
RDRAM is a proprietary variant of DRAM which was developed by Rambus, Inc.
RDRAM incorporates technical advantages such as:
● Packet-based command protocol
● Command pipelining
● Data pipelining
● Low-voltage signaling
● Precise clocking
Manufacturers who wish to utilize RDRAM technology must pay royalties to Rambus Inc.

What is video RAM?
Video RAM is specialized RAM which is used on video cards.
Video RAM is dual-ported, which means it can be accessed by two different devices simultaneously. This enables data to be read from video RAM (i.e. sent to the computer monitor) at th same time data is written to video RAM.
What is flash memory?
Flash memory is memory which retains its contents even after power is removed.
Flash memory is a form of EAPROM (Electrically Alterable Programmable Read-Only Memory).
Each bit of data in a flash memory device is stored in a transistor called a floating gate. The floating gate can only be accessed though another transistor, the control gate.
The process the control gate uses to access the floating gate is a field emission phenomenon known as Fowler-Nordheim tunneling. Tunneling allows voltage to flow from the control gate to the floating gate through the dielectric layer of oxide which separates them.
Popular flash memory devices include:
● Sony's Memory Stick
● Compact Flash
● SDCard
● MultiMediaCard (MMC)








What is a Memory Stick?
A Memory Stick is an IC (Integrated Circuit) which is stored in a compact and rugged plastic enclosure. Memory Sticks are designed to store data and to enable the transfer of data between devices equipped with Memory Stick slots.
The Memory Stick standard was introduced by Sony in October of 1998.
Current Memory Stick capacities range up to 512MB.
A Memory Stick is 50mm long, 21.5mm wide, and 2.8mm thick.
An even more compact format, Memory Stick Duo, is 32mm long, 20mm wide, and 1.6mm thick.
The theoretical transfer speed of Memory Stick is 160Mbps.
What is Compact Flash?
A Compact Flash card is an IC (Integrated Circuit) which is stored in a compact and rugged plastic enclosure. Compact Flash cards are designed to store data and to enable the transfer of data between devices equipped with Compact Flash slots.
Current Compact Flash capacities range up to 4GB.
Compact Flash Type I cards are 43mm long, 36mm wide, and 3.3mm thick.
Compact Flash Type II cards are 43mm long, 36mm wide, and 5mm thick.
The theoretical transfer speed of Compact Flash 2.0 is 16MB/sec.
The Compact Flash standard was introduced by SanDisk Corporation in 1994.
Compact Flash Plus (CF+)
Compact Flash Plus (CF+) extends Compact Flash to provide functionality such as micro hard drives, modems, Ethernet cards, 802.11 Wi-Fi cards, serial cards, Bluetooth cards, and more.
This makes Compact Flash the most versatile of the flash media formats.
For more information on Compact Flash, refer to the Compact Flash Specification.

What is a SD Card?
A SD Card (Secure Digital Card) is an IC (Integrated Circuit) which is stored in a compact and rugged plastic enclosure. SD Cards are designed to store data and to enable the transfer of data between devices equipped with SD Card slots.
Current SD Card capacities range up to 1GB.
A SD Card is 32mm long, 24mm wide, and 2.1mm thick.
An even more compact format, the miniSD Card, is 20mm long, 21.5mm wide, and 1.4mm thick.
The theoretical transfer speed of a SD 1.0 Card is 12.5MB/s. SD 1.1 is expected to raise this to 50MB/s.
The SD Card standard was introduced by Toshiba, Matsushita Electric, and SanDisk in 1999.
SDIO
SDIO extends the SD Card standard to include 802.11b WiFi cards, Bluetooth cards, modems, GPS receivers, TV tuners, cameras, digital recorders, scanners, fingerprint scanners and more.
For more information on SD Card, visit the SD Card Association.

What is a MultiMediaCard (MMC)?
A MultiMediaCard (MMC) is an IC (Integrated Circuit) which is stored in a compact and rugged plastic enclosure. MultiMediaCard (MMC)s are designed to store data and to enable the transfer of data between devices equipped with MultiMediaCard (MMC) slots.
The MultiMediaCard (MMC) standard was introduced in November of 1997 by SanDisk and Siemens AG/Infeneon Technologies AG.
Current MultiMediaCard (MMC) capacities range up to 2GB.
A MultiMediaCard (MMC) is 32mm long, 24mm wide, and 1.4mm thick.
MultiMediaCards can be used in SD Card readers and writers.
The theoretical transfer speed of a MultiMediaCard is 2.5MB/s.