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.