RAM

(Random Access Memory) A type of memory chip that is "byte addressable" and provides direct access to any location on the chip. The contents of any byte can be read or written without regard to the bytes before or after it. The most common RAM chip is the dynamic RAM (DRAM) used as the computer's main memory. Any chip that has RAM in its name implies this byte addressing flexibility, such as SRAM, SDRAM, PRAM, MRAM, NVRAM, NRAM and FeRAM.




Some Old Fashioned RAM
Not exactly random access, and hardly a chip, this magnetic drum unit was the memory in the IBM 650 computer, introduced in 1954. It held two thousand 10-digit words. That much memory today would fit inside the period at the end of this sentence.

MEMORY TYPES:

Following are the different types of memory technologies used in electronic devices. The two major categories are volatile and non-volatile. Volatile chips lose their content the instant power is removed, while non-volatile memories retain their content.

Volatile

Rewritable - Byte Addressable (DRAM and SRAM)
Dynamic RAM (DRAM) and static RAM (SRAM) chips are the "working storage" in every computer. DRAM is the main memory in a computer and SRAM is used for high-speed caches and buffers. Both types are "byte addressable," which means that data can be read and written one byte at a time. Their major drawback is that RAM chips require power to hold their content.

Future Memories
The Holy Grail for future computer memories is to create a byte addressable RAM chip with the speed of static RAM, the density of dynamic RAM and the non-volatility of flash.





Non-Volatile

Rewritable - Byte Addressable (EEPROM, FeRAM)
EEPROM and FeRAM chips offer the byte addressability of DRAM and SRAM chips, but are less dense and more costly.

Rewritable - Block Writes (flash memory)
Flash memory is the most widely used non-volatile memory chip in both computers and consumer electronics (CE) devices. Although reads are random access to the byte level, writing is done at the block level similar to writing a disk sector.

Rewritable When Removed (EPROM)
EPROM chips are initially written in an external "programmer" device and must be removed from the circuit board and placed back in the device for reprogramming.

Permanent (ROM and PROM)
Data and instructions in ROM and PROM chips can never be changed. ROMs are manufactured, while PROMs are programmed in an external device like EPROMs.

MEMORY MODULE:

A narrow printed circuit board that holds memory chips. The common memory module is the DIMM (dual in-line memory module). DIMMs with 184 pins are used for DDR SDRAM, while 240-pin DIMMs are used for DDR2 and DDR3 SDRAM. DDR, DDR2 and DDR3 DIMM modules are each keyed differently (notches in different locations) so that they cannot be inserted into the wrong slots. Because of space limitations, laptops use small outline DIMMs (SODIMMs).

PCs use either nine-bit memory (eight bits and parity) or eight-bit memory without parity. Macs use eight-bit memory without parity.

Singles or Pairs
Earlier SIMM and Rambus modules were installed in pairs, whereas a single DIMM can often be used. However, installing pairs of DIMMs in machines that support dual channel DDR SDRAM increases performance.

Upgrading Memory - Read the Manual (RTFM!)
To upgrade memory, read your motherboard or system manual to learn which combinations of modules can be plugged into memory slots. There may be restrictions.




Common Memory Modules
DIMMs are widely used in desktops and servers while the smaller SODIMMs are used in laptops. SIMMs were used in older PCs. For identification, look at the pin pattern and notches between the pins and on the sides. How the chips are laid out (horizontal or vertical) is up to the manufacturer.





Desktop Memory
To change memory on desktop computers, you have to open the cabinet and locate the slots. These three DIMM slots on this Macintosh motherboard are easy to find and reach.





Laptop Memory
To change memory on laptops, you have to unscrew a plate on the bottom of the machine to get to the SODIMM slots.





Two Sticks of Memory
Memory modules are often called \"sticks,\" because the chips are housed on long, thin printed circuit boards.

STATIC RAM:

A fast memory technology that requires power to hold its content. Static RAM (SRAM, S-RAM) is used for high-speed registers, caches and relatively small memory banks such as a frame buffer on a display adapter. In contrast, the main memory in a computer is typically dynamic RAM (DRAM, D-RAM). Static RAM chips have access times in the 10 to 30-nanosecond range, while dynamic RAM is usually above 30 ns. Bipolar and ECL memories are under 10 ns.

More Real Estate
Static RAM is fast because the six-transistor configuration of its pretzel-like flip-flop circuits keeps current flowing in one direction or the other (0 or 1). The 0 or 1 state can be written and read instantly without waiting for a capacitor to fill up or drain; however, the six transistors take more space than dynamic RAM cells made of one transistor and one capacitor.

Earlier asynchronous static RAM chips performed read and write operations sequentially. Newer synchronous static RAM chips overlap reads and writes. Contrast with dynamic RAM.




A Static RAM Cell
When opposite voltages are applied to the column wires, the flip-flop is oriented in one of two directions for a 0 or 1. At that point, the flip-flop (yellow center) becomes a self-perpetuating storage cell as long as a constant voltage is applied.

DYNAMIC RAM:

The most common type of computer memory. Dynamic RAM (DRAM, D-RAM) chips are very dense because they use only one transistor and one storage capacitor for each bit. Unlike non-volatile firmware chips (ROM, EEPROM, flash, etc.), both major types of RAM, dynamic RAM and static RAM, lose their content when the power is turned off.

Capacitors were used in the 1960s for computer memory, but dynamic RAM was patented by IBM in 1968. The first commercial chips came from Intel and Mostek in the early 1970s.

Dynamic RAM Is a Total Loser
The capacitors in a dynamic RAM chip are electrical storage tanks that do a poor job of holding a charge. They constantly leak, and the chip would lose its content even when the power is on if it were not for the refresh circuitry that continuously re-energizes the capacitors hundreds of times per second. However, even with the refresh circuitry, dynamic RAM cells take up significantly less space than static RAM cells. Contrast with static RAM.




A Dynamic RAM Cell
DRAM cells are very simple. The combination of voltage on the row and column lines charges a capacitor. The only problem is that the capacitors keep losing their charges, and the bits must be read and re-written hundreds of times per second.

Many companies are working on non-volatile memory technologies with byte addressable, random access capability. They are expected to initially provide an alternative to flash memory and compete with DRAM chips in certain applications.

The flash memory in common use is non-volatile, but it must be written in blocks somewhat like disk sectors, which makes it slower. DRAM memory can be written one byte at a time, but loses its content without power. Both have drawbacks the industry would love to eliminate. New memory technologies are in various stages of development or implementation.

A DRAM Replacement?
Any RAM technology that replaces flash memory will be a huge success. However, if a non-volatile technology eventually succeeds in replacing DRAM chips, the main memory used in computers, it will dramatically change the way software is written. All operating systems and applications are designed to continuously save data to the disk. When main memory finally "remembers," many data elements would reside in memory at all times. In addition, computers would always be "instant-on" and would not waste AC or battery power when idle.

MRAM
(Magnetic RAM) A non-volatile, random access memory technology that is designed to initially replace flash memory and, potentially, DRAM memory. MRAM uses magnetic, thin film elements on a silicon substrate that can be built on the same chip with the logic circuits. DRAM, SRAM and flash memories cannot all be embedded on the CPU chip.

Although many large companies, such as IBM and Intel, are working on MRAM, NVE Corporation, Eden Prairie, MN is a small company that is a leader in this field with more than 30 patents.

Similar and Different
Writing bits in MRAM is similar to magnetic disks and early magnetic core storage. The 0s and 1s are created by different polarizations of the electrons within a ferromagnetic material. The major difference between MRAM and other magnetic technologies is in the reading. MRAM uses a tunnel junction, and the bit is read as the resistance in that junction.




A Magnetoresisive RAM Bit
This diagram shows one magnetic bit in the Magnetoresistive RAM technology from NVE Corporation. The data state (0 or 1) is determined by the polarization of the tunnel junction, and the bit is read as the resistance in that junction.

NRAM
(NanoRAM) A non-volatile, random access memory technology from Nantero, Inc., Woburn, MA that is designed to initially replace flash memory and, potentially, DRAM memory. NRAM uses carbon nanotube ribbons for the bits. The ribbons physically move within a 13 nm space, which determines their data state (0 or 1). Using standard CMOS fabrication facilities, in late 2006, Nantero stated that it developed all the solutions required to use carbon nanotubes in mass production.




Carbon Nanotube Bits
The data state (0 or 1) of an NRAM bit is determined by the physical arrangement of the carbon nanotube ribbons.

PHASE CHANGE MEMORY
A non-volatile, random access memory technology that is designed to initially replace flash memory and, potentially, DRAM memory. It employs the same phase change principle used in rewritable optical discs (CD-RWs, DVD-RWs, etc.). Also called "phase change RAM" (PRAM and PCRAM), chalcogenide RAM (C-RAM) and Ovonyx Unified Memory (OUM), Ovonyx, Inc., Rochester Hills, MI is the pioneer in this field with patents that date back to the 1960s. Via licensing agreements with Ovonyx, Samsung introduced a prototype and BAE Systems introduced its C-RAM chip in 2006.

Memory Vs. Disk
In phase change technology, the bit is switched between amorphous (unstructured) and crystalline (highly structured) states. However, in phase change memory, the bits are altered by electricity rather than by laser as in optical discs. Phase change memory bits are also denser than the bits in optical discs.

MEMRITOR
(MEMory ResISTOR) A non-volatile memory technology that can change its resistance in varying levels. It comprises a cell made of two layers of titanium dioxide, one of which is conductive because it is missing a few oxygen atoms. When a positive charge is applied to the layer with the missing atoms, the vacancies are pushed into the other layer, making it conductive as well and changing its resistance. The more the cell is charged, the lower its resistance. It can offer resistance in two states for a digital 0 or 1 or to levels in between to go beyond a binary system. Negative charges are used to reverse the effect.

Very Fast and High Density
Two distinct advantages of memristors are that moving the vacancies between the adjacent levels can be done much faster than other known switching methods, and the cell density approaches that of hard disks. Theorized by Leon Chua in the early 1970s, the first memristors were demonstrated by HP in 2008. If commercially viable, memristors may replace flash memory and dynamic RAM (DRAM) in the future.

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