4.2.3. Main Memory -- RAM
RAM makes up the bulk of electronic storage on present-day computers. It is used as storage for both data and programs while those data and programs are in use. The speed of RAM in most systems today lies between the speed of cache memory and that of hard drives, and is much closer to the former than the latter.
The basic operation of RAM is actually quite straightforward. At the lowest level, there are the RAM chips -- integrated circuits that do the actual "remembering." These chips have four types of connections to the outside world:
- Power connections (to operate the circuitry within the chip)
- Data connections (to enable the transfer of data into or out of the chip)
- Read/Write connections (to control whether data is to be stored into or retrieved from the chip)
- Address connections (to determine where in the chip the data should be read/written)
Here are the steps required to store data in RAM:
- The data to be stored is presented to the data connections.
- The address at which the data is to be stored is presented to the address connections.
- The read/write connection is set to write mode.
Retrieving data is just as straightforward:
- The address of the desired data is presented to the address connections.
- The read/write connection is set to read mode.
- The desired data is read from the data connections.
While these steps seem simple, they take place at very high speeds, with the time spent on each step measured in nanoseconds.
Nearly all RAM chips created today are sold as modules. Each module consists of a number of individual 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.
The main benefit to a system using 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 third-party RAM modules are available, assuming the system is relatively popular and is not a completely new design.