Memory Study Notes

RAM (Random Access Memory) [1970-Intel]
A group of Memory chips, typically of the dynamic RAM (DRAM) type, which functions as the
computer's primary workspace. The "random" in RAM means the contents of each byte can be
directly accessed without regard to the bytes before or after it. Also true of other Types of Memory chips, including ROMs (Read Only Memory) and PROMs(Programable ROM). However, unlike ROMs (Read Only Memory) and PROMs(Programable ROM), RAM chips require power to maintain their content, which is why you must save your data onto disk before you turn the computer off.

DRAM (Dynamic RAM) [1970-Intel] Burst Timing: 5-5-5-5
Dynamic random access Memory (DRAM) is the most common kind of random access Memory
(RAM) for personal computers and workstations. Memory is the network of electrically-charged
points in which a computer stores quickly accessible data in the form of 0s and 1s. Random access means the PC processor can access any part of the Memory or data storage space directly rather than having to proceed sequentially from some starting place. DRAM is dynamic in that, unlike static RAM (SRAM), it needs to have its storage cells refreshed or given a new electronic charge every few milliseconds. Static RAM does not need refreshing because it operates on the principle of moving current which is switched in one of two directions rather than a storage cell which holds a charge in place. Static RAM is generally used for cache Memory, which can be accessed more quickly than DRAM.

ROM (read-only Memory) [1971-Intel]
Semiconductor-based Memory which contains instructions or data which can be read but not
modified. (Generally, the term ROM often means any read-only device, as in CD-ROM for Compact Disk, Read Only Memory.) Once data has been written onto a ROM chip, it cannot be removed and can only be read. Unlike main Memory (RAM), ROM retains its contents even when the computer is turned off. ROM is referred to as being nonvolatile, whereas RAM is volatile. Most personal computers contain a small amount of ROM which stores critical programs such as the program which boots the computer. In addition, ROMs are used extensively in calculators and peripheral devices such as laser printers, whose fonts are often stored in ROMs.

Electrically Erasable Programmable Read-Only Memory (EEPROM)
Machines with flash BIOS capability use a special type of BIOS ROM called an EEPROM; which
stands for "Electrically Erasable Programmable Read-Only Memory". As you can probably tell by the name, is a ROM which can be erased and re-written using a special program. Procedure is called flashing the BIOS and a BIOS that can do this is called a flash BIOS. The advantages of this
capability are obvious; no need to open the case to pull the chip, and much lower cost. EEPROM is similar to flash mem. (sometimes called flash EEPROM). The principal difference is EEPROM requires data to be written or erased one byte at a time whereas flash mem. allows data to be written or erased in blocks. This makes flash mem. faster. Flash mem. works much faster than traditional EEPROMs because it writes data in chunks, usually 512 bytes in size, instead of a byte at a time.

SIMMs (Single In-line Memory Modules)
As the first mass-produced Memory packages, these were 30 pin modules ~3.50" X 0.75", and were used primarily in 386, early 486, and Apple® computers. Designed as Fast-Page Mode non-Parity (2 or 8 chips per SIMM), or Parity (3 or 9 chips per SIMM), these were in 1Mb, 4Mb and 16Mb denominations. Installation must be in either 1 or 2 "banks" of either 2 or 4 matching SIMMs.This design was soon replaced by 72 pin modules ~4.25" X 1.0", used primarily in later 486, 586 (Pentium®), and later Apple® models. Designed as Fast-Page Mode or EDO (explained later), these came as non-Parity or Parity with capacities of 4Mb, 8Mb, 16Mb, 32Mb, 64Mb and 128Mb. Most 486 and several Apple® machines only needed one SIMM per available socket, whereas Pentium® and PowerMacs® required matching pairs. Most machines required specific sizes and upgrade configurations.

DIMMs (Dual In-line Memory Modules)
As operating system Memory demands increased, larger Memory modules were required; yet the motherboard space was even more at a premium. To solve this problem the 168 pin DIMM module ~5.375" X 1" was developed.These are installed singly in later Pentium®s, Pentium® Pro's, and PowerMacs®, and are offered as non-Parity Fast-Page, EDO, ECC, or SDRAM modes, 3.3v or 5v. buffered or unbuffered, and 2-clock or 4-clock. Their capacities are 8Mb, 16Mb, 32Mb, 64Mb and 128Mb. Choosing the right module is very critical, as most machines require specific Types, sizes and upgrade configurations.The number of black components on a 184-pin DIMM may vary, but they always have 92 pins on the front and 92 pins on the back for a total of 184. 184-pin DIMMs are approximately 5.375" long and 1.375" high, though the heights may vary. While 184-pin DIMMs and 168-pin DIMMs are approximately the same size, 184-pin DIMMs have only one notch within the row of pins.

SODIMM (Small Outline DIMM Modules)
Many brands of notebook computers use proprietary mem. modules, but several manufacturers use RAM based on the small outline dual in-line mem. module (SODIMM) configuration. SODIMM cards are small, about 2 inches by 1 inch (5 centimeters by 2.5 centimeters), and have 144 pins. Capacity ranges from 16MB to 256MB per module. An interesting fact about the Apple iMac desktop computer is it uses SODIMMs instead of the traditional DIMMs.

Memory Cards
This style of Memory is primarily used in notebooks, and comes in two primary styles. "Credit cards" are proprietary designed modules which are often installed under the notebook keyboard. Most commonly, these are Non-Parity, however, choosing the right module is very critical, as most machines require specific Types, sizes and upgrade configurations. PCMCIA cards are a design standardized by industry OEMs. These come in three different Types, but Type I are used for Memory expansion.

PCMCIA
PCMCIA (Personal Computer Memory Card International Association) is an international standards body and trade association with over 300 member companies which was founded in 1989 to establish standards for Integrated Circuit cards and to promote interchangeability among mobile computers where ruggedness, low power, and small size were critical. As the needs of mobile computer users has changed, so has the PC Card Standard. By 1991, PCMCIA had defined an I/O interface for the same 68 pin connector initially used by Memory cards. At the same time, the Socket Services Specification was added and was soon followed by the Card Services Specifcation as developers realized common software would be needed to enhance compatibility.