Page 570 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

Dynamic vs. Static RAM

There are two main types of RAM: dynamic RAM and static RAM.
Most computers contain a combination of both types and use them
for different purposes.

To store data, dynamic RAM uses a series of capacitors, tiny
electrical devices that hold a charge. These capacitors either hold a

charge (representing a 1 bit in memory) or do not hold a charge
(representing a 0 bit). However, because capacitors naturally lose
their charges over time, the CPU must spend time refreshing the
contents of dynamic RAM to ensure that 1 bits don’t
unintentionally change to 0 bits, thereby altering memory
contents.

Static RAM uses more sophisticated technology—a logical device

known as a flip-flop, which to all intents and purposes is simply an
on/off switch that must be moved from one position to another to
change a 0 to 1 or vice versa. More important, static memory
maintains its contents unaltered as long as power is supplied and
imposes no CPU overhead for periodic refresh operations.

Dynamic RAM is cheaper than static RAM because capacitors are
cheaper than flip-flops. However, static RAM runs much faster

than dynamic RAM. This creates a trade-off for system designers,
who combine static and dynamic RAM modules to strike the right
balance of cost versus performance.

Registers

The CPU also includes a limited amount of onboard memory, known
as registers, that provide it with directly accessible memory locations
that the brain of the CPU, the arithmetic-logical unit (ALU), uses
when performing calculations or processing instructions. In fact, any
data that the ALU is to manipulate must be loaded into a register

unless it is directly supplied as part of the instruction. The main
advantage of this type of memory is that it is part of the ALU itself and,
therefore, operates in lockstep with the CPU at typical CPU speeds.