Computer Main Memory Types: RAM and ROM Details

1.3 Main Memory Fundamentals

The main memory can be defined as a device capable of storing information. This information is made up of bits, which are the basic elements of memory. A bit is the basic unit of information and can only take two values: (0 or 1). The quantity of bits of information that can be stored in a given memory defines its capacity. Main memory can be distinguished into two main groups: RAM and ROM memory.

RAM (Random Access Memory)

RAM is characterized as being very fast for reading and writing and is volatile (loses data when power is off).

RAM Divisions

• SRAM (Static RAM): In static memory cells, the storage of information resides in a bistable state; each bit is formed by a continuous bistable order so that the information is not lost. SRAM retains its data with only the initial electrical power contribution. These memories are characterized by a huge access speed and are typically used to increase cache size.
• DRAM (Dynamic RAM): This type of information is stored on tiny capacitors. Each bit is represented by the charge stored in a capacitor. The main drawback of DRAM is that information is not maintained simply by keeping power on because the capacitors discharge slowly due to an effect called leakage current. This leakage produces a charge state below a certain level, which would be considered as 'no charge'. To avoid such problems, a refresh operation is necessary—a periodic restoration of the charge on the capacitors. Dynamic RAM has a great advantage over static RAM: for the same storage capacity, it takes up much less physical space than static storage.

ROM (Read-Only Memory)

ROM memory is implemented by the manufacturer for computer use, where all data is permanently recorded during the manufacturing process. Once recorded, the data cannot be changed, so the information stored in ROM is permanent.

ROM Variations

The term ROM actually brings together several memories with slight variations, although they resemble each other in internal construction and recording method. These variations include:

• PROM (Programmable ROM): This is a blank chip that can be recorded once after leaving the factory.
• EPROM (Erasable Programmable ROM): This has similar characteristics to PROM but can be rewritten several times. These memories allow for several cycles of erasing and recording.
• EEPROM (Electrically Erasable Programmable ROM): This is similar to EPROM, with the difference that the erase process is performed via software control.
• EAROM (Electrically Alterable ROM): In this memory, data can be altered without a prior erase cycle. Intel's flash memory is an example of this type.