Computer Memory

By Jayaram V

A computer uses two types of storage devices called primary and secondary storage devices. The difference between the two is that the primary storage devices hold are essentially temporary storage devices where information lasts for a short period where are secondary storage devices allow the storage of information for much longer period, till either it is removed or modified by some external means.

The Central Processor in the computer (CPU) can access the primary storage directly but cannot interact with the secondary storage directly. To do information has to be first loaded into the primary storage devices where it would be processed before being sent back or directed elsewhere.

Primary storage devices are more popularly called memory chips or RAM (Random Access Memory) chips. The chips hold both instructions and data temporarily to enable the central processing unit carry out different tasks and keep the computer running. The RAM is basically of two types: dynamic RAM also called DRAM and static RAM also called SRAM. DRAM holds information for a very short time, while SRAM holds it as long as the power is not turned off. Both hold information temporarily but SRAM holds a bit longer.

In older computer RAM chips used to exist as individual chips embedded in the system board in rows of nine chips each. Each of the eights chips held one byte of memory while the ninth one, called a parity chip, held a parity bit. The parity chip played a key role by keeping the computer from crashing by maintaining the integrity of the bits stored in the other eight chips.

In later day computer models RAM chips got organized into SIMMs or single inline memory modules. A SIMM consisted of a single mini board with several chips and 30 or 72 pins on its edge. A SIMM can hold from 8 MB to 64 MB of RAM on a single board. Since RAM is now located on detachable mini boards, it is easier to upgrade RAM by adding more SIMMs.

The most popular form memory module used in computers today is DIMM or Dual inline memory module. While a SIMM has a data path of 32 bits, a DIMM has a data path of 64 bits. DIMMs are generally available in 8 MB, 16 MB, 32 MB and 64 MB sizes, but it is not uncommon to find DIMMs in Gigabytes of memory. DIMMs are available with varying number of pins from 72-pin-DIMMs, used for SO-DIMM to 240-pin-DIMMs, used for DDR2 SDRAM.