More about RAM

Why is it random?

Random Access Memory (RAM) is the computer’s memory that stores all the data that a system requires for faster processing.

Why is it random?

Data stored within RAM can be randomly accessed if you know the physical address of the memory cell – hence the ‘R’ in RAM. (Think of the time that you looked up a street in a map; the back of the map will have an ‘address’ of the street such as J2 which you match to the front of the map)

Contrast this notion with Serial Access Memory (SAM), where each memory cell is accessed sequentially until the relevant information is found. (Think of the situation where you are fast-forwarding a cassette tape to find the song you are after. Following the analogy of the map, imagine you only have the front page of the map – you have to read each road’s name before you find the one you are after.)

As RAM only accesses one address while SAM accesses multiple addresses before reaching the desired data, random access is on average, faster than the serial access method.

Types of RAM: Static and Dynamic

Dynamic Random Access Memory (DRAM) is the more common type of the two. A memory cell of DRAM consists of a transistor and a capacitor that holds a charge to represent one bit of binary information (0 or 1).

The transistor performs two functions: 1) it reads the charge of the capacitor and 2) it changes the state of the capacitor (from 1 to 0 and vice versa).

When the capacitor is charged (more than 50%), it represents the binary ‘1’ in the memory cell. When the capacitor is discharged, it represents ‘0’.

DRAM gets the ‘dynamic’ in its name because it is refreshed thousands of times per second. The reason for this is that the capacitors within DRAM do not hold their charge (and thus the value of ‘1’) very well. It must be constantly recharged by a memory controller before the capacitor drops to below 50% of its charge (which will represent ‘0’).

As DRAM must always refresh itself, it is slower (but cheaper!) than static RAM where such a refresh operation is not needed. DRAM supports access times of 60 nanoseconds.

Static Random Access Memory (SRAM) uses a different architecture than DRAM to hold data and is more reliable and faster as a result. SRAM supports access times of just 10 nanoseconds.

The memory cell of SRAM consists of four or six transistors to place the cell in a state of true or false (akin to the binary ‘1’ and ‘0’). As a SRAM memory cell consists of more components than DRAM, this result in a higher price tag and more space consumption.

As there are no capacitors within a memory cell, power is required to always flow to the cell to maintain the data.

SRAM and DRAM are both volatile forms of memory as they require constant electrical power to maintain the data. Once you computer is switched off, the RAM is cleared of data.

Due to price discrepancy between SRAM and DRAM, the majority of the space in a computer’s RAM is made of DRAM while SRAM is used as a memory cache.

Useful Websites

How Stuff Works: How RAM works

PCGuide - Memory Technology Types