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Chapter 4. Physical and Virtual Memory

Retrieving data is just as simple:

1. The address of the desired data is presented to the address connections.

2. The read/write connection is set to read mode.

3. The desired data is read from the data connections.

While these steps are simple, they take place at very high speeds, with the time spent at each step

measured in nanoseconds.

Nearly all RAM chips created today are sold as modules. Each module consists of a number of in 

dividual RAM chips attached to a small circuit board. The mechanical and electrical layout of the

module adheres to various industry standards, making it possible to purchase memory from a variety

of vendors.

Note

The main benefit to a system that uses industry standard RAM modules is that it tends to keep

the cost of RAM low, due to the ability to purchase the modules from more than just the system

manufacturer.

Although most computers use industry standard RAM modules, there are exceptions. Most notable

are laptops (and even here some standardization is starting to take hold) and high end servers.

However, even in these instances, it is likely that you will be able to find third party RAM modules,

assuming the system is relatively popular and is not a completely new design.

4.2.4. Hard Drives

All the technologies that have been discussed so far are volatile in nature. In other words, data con 

tained in volatile storage is lost when the power is turned off.

Hard drives, on the other hand, are non volatile   the data they contain remains there, even after the

power is removed. Because of this, hard drives occupy a special place in the storage spectrum. Their

non volatile nature makes them ideal for storing programs and data for longer term use. Another

unique aspect to hard drives is that, unlike RAM and cache memory, it is not possible to execute

programs directly when they are stored on hard drives; instead, they must first be read into RAM.

Also different from cache and RAM is the speed of data storage and retrieval; hard drives are at

least an order of magnitude slower than the all electronic technologies used for cache and RAM. The

difference in speed is due mainly to their electromechanical nature. Here are the four distinct phases

that take place during each data transfer to/from a hard drive. The times shown reflect how long it

would take a typical high performance drive, on average, to complete each phase:

Access arm movement (5.5 milliseconds)

Disk rotation (.1 milliseconds)

Heads reading/writing data (.00014 milliseconds)

Data transfer to/from the drive's electronics (.003 Milliseconds)

Of these, only the last phase is not dependent on any mechanical operation.