The main characteristic of RAM is that it can be written to and be read from.

Most RAM chips only retain their data as long as power remains applied to the chip.

There are also some types of RAM that retain their contents even when power is removed.

A system board supports different types of memory, all of which are used for storage.

Memory modules have usually been available as either nonparity or parity.

Nonparity memory is standard memory.

Parity memory works almost the same except that it adds an extra bit for every 8 bits of data.

Read Only Memory (ROM) is nonvolatile memory.

The contents of ROM memory are burned in so that the system can read from it but not write to it.

ROM is used to store small blocks of program code necessary for the system and the installed devices to operate.

PCs use Random Access Memory (RAM) for short-term storage of data and programs.

The central part of a RAM chip is called the memory cell array.

RAM can be broken down into two types: Dynamic RAM (DRAM) and Static RAM (SRAM).

DRAM chips must have electrical current supplied to maintain their electrical state. Access time is typically 60 to 80 ns.

SRAM access time 20 ns or less. SRAM is most commonly used as cache memory where speed is extremely important.

Although speed differs among the different types of RAM, data in RAM is accessed more quickly than data on a disk drive.

The term DRAM can be used generically to refer to most memory.

DRAM chips are rated according to access time. The rating number is printed on the chip (-15, -20, -10).

Speed is in tens of nanoseconds (ns).

FPM DRAM

One of the first attempts to overcome the slowness of DRAM was Fast Page Mode (FPM) DRAM. The major difference with FPM DRAM is the way that the memory controller accesses data.

EDO DRAM

Another advance in DRAM chip technology is Extended Data Output (EDO) memory.

On a computer that is designed to support EDO, the CPU can access memory 10% to 15% faster than similar FPM DRAM chips.

EDO DRAM is optimized to work in conjunction with a
66-MHz or faster Pentium (or equivalent) microprocessor.

SDRAM

Synchronous DRAM (SDRAM) is synchronized to the system clock that controls the computer’s microprocessor.

With SDRAM, the synchronization allows the memory controller to know on which clock cycles data requests will be available.

SRAM

Static RAM (SRAM) is used as cache memory. Because of the fast speeds of SRAM (around 20 ns), it is considerably more expensive than DRAM.

Cache memory

Cache memory normally refers to a small block of very high-speed memory used to cache (store) program instructions and data for the microprocessor, making for faster program execution.

The cache built into the processor is called internal cache. The cache closest to the processor is designated L1 and the next level of cache is designated L2.

SIMM

Single In-Line Memory Module (SIMM) chips are a common packaging for DRAM.

Most SIMM types are not interchangeable; the same configuration must be installed on the board.

They are available in 1-MB, 2-MB, 4-MB, 8-MB, 16-MB,
32-MB, and 64-MB configurations.

They are used with AT-compatible systems.

In case of failure, the entire SIMM board must be replaced.

There are two different types of SIMMs: 30- and
72-pin. Because of their ease of upgrading, 72-pin SIMMs are the most popular memory types.

SIMM packaging was first used with IBM PS/2 and Apple Macintosh systems.

Standards for DIMM modules include PC100, PC133, PC1600, and PC2100.

PC100 and PC133 are the industry standards for memory that is compatible with 100- and 133-MHz system bus speeds.

PC1600 and PC2100 are the industry standard memory types for DIMMs compatible with 200- and 266-MHz DDR bus speeds.

All four standard memory types use SDRAM.

RIMM

Rambus Inline Memory Modules (RIMMs) also contain several RAM chips mounted on a small circuit board.

RIMM chips are covered by a form of heat sink called a heat spreader.

RIMMs are available in three different speed grades:
PC600, PC700, and PC800. PC800 RIMM are capable of a theoretical peak data transfer rate of 1.6 GB per second.

VRAM

Video RAM (VRAM) was designed specifically for video adapters.

To provide the high speeds necessary, VRAM provides two separate data ports.

The first port is dedicated to the monitor for refreshing and updating the screen.

The second is dedicated for use by the microprocessor or video adapter for changing the data stored in VRAM.

WRAM

Window RAM (WRAM) is another type of memory that uses dual data ports. WRAM is seen on graphic-intensive systems.

SGRAM

Synchronous Graphics RAM (SGRAM) is an enhancement of SDRAM that contains graphic-specific features. SGRAM retrieves and modifies data in blocks instead of in the typical cell-by-cell manner.

Flash RAM

Flash RAM is nonvolatile memory.

One of the newer uses for Flash RAM is solid-state hard disk drives. Flash RAM hard disk drives have an access time hundreds of times faster than conventional hard drives.