ROM and RAM

ROM and RAM provide memory for a vast amount of computer equipment. They come in different memory sizes and module sizes and have different features. ROM contains basic instructions for computers such as booting and loading the operating system. RAM chips are stored on memory modules that can easily be installed and removed to expedite upgrades. Some RAM uses parity to increase data integrity and reliability. Another kind of specialized memory is called cache. It stores frequently used data and commands. The following sections cover ROM, RAM, modules, parity, and cache in greater detail.

ROM

Memory chips store data in the form of bytes. Bytes represent information such as letters, numbers, and symbols. A byte is a grouping of 8 bits. Each bit is stored as either 0 or 1 in the memory chip. ROM chips are located on the motherboard and other circuit boards. ROM chips contain instructions that can be directly accessed by a CPU. Basic instructions for operation, such as booting the computer and loading the operating system, are stored in ROM. ROM chips retain their contents even when the computer is powered down. The contents cannot be erased or changed by normal means. ROM types include the following:

•  Read-only memory (ROM): Information is written to a ROM chip when it is manufactured. A ROM chip cannot be erased or rewritten. This type of ROM is obsolete.
• Programmable read-only memory (PROM): Information is written to a PROM chip after it is manufactured. A PROM chip cannot be erased or rewritten. 
•  Erasable programmable read-only memory (EPROM): Information is written to an EPROM chip after it is manufactured. An EPROM chip can be erased with exposure to UV light. Special equipment is required. 
• Electrically erasable programmable read-only memory (EEPROM): Information is written to an EEPROM chip after it is manufactured. EEPROM chips are also called flash ROMs. An EEPROM chip can be erased and rewritten without having to remove the chip from the computer. 

RAM

Random-access memory (RAM) is the temporary storage for data and programs that are being accessed by the CPU. RAM is volatile memory, which means that the contents are erased when the computer is powered off. The more RAM in a computer, the more capacity the computer has to hold and process large programs and files. More RAM also enhances system performance. The maximum amount of RAM that can be installed is limited by the motherboard and the operating system. The different types of RAM are as follows:

• Dynamic RAM (DRAM) is a memory chip that is used as main memory. DRAM must be constantly refreshed with pulses of electricity to maintain the data stored within the chip. 
• Static RAM (SRAM) is a memory chip that is used as cache memory. SRAM is much faster than DRAM and does not have to be refreshed as often. SRAM is much more expensive than DRAM. 
•  Fast Page Mode (FPM) DRAM is memory that supports paging. Paging enables faster access to the data than regular DRAM. FPM memory was used in Intel 486 and Pentium systems. 
•  Extended Data Out (EDO) RAM is memory that overlaps consecutive data accesses. This speeds up the access time to retrieve data from memory because the CPU does not have to wait for one data access cycle to end before another data access cycle begins. 
• Synchronous DRAM (SDRAM) is DRAM that operates in synchronization with the memory bus. The memory bus is the data path between the CPU and the main memory. Control signals are used to coordinate the exchange of data between SDRAM and the CPU. 
•  Double data rate SDRAM (DDR SDRAM) is memory that transfers data twice as fast as SDRAM. DDR SDRAM increases performance by transferring data twice per clock cycle. 
• Double data rate 2 SDRAM (DDR2 SDRAM) is a faster than DDR-SDRAM memory. DDR2 SDRAM improves performance over DDR SDRAM by decreasing noise and crosstalk between the signal wires. 
• Double data rate 3 SDRAM (DDR3 SDRAM) expands memory bandwidth by doubling the clock rate of DDR2 SDRAM. DDR3 SDRAM consumes less power and generates less heat than DDR2 SDRAM. 
• Rambus DRAM (RDRAM) is a memory chip that was developed to communicate at very high rates of speed. RDRAM chips are not commonly used.