ROM is used extensively to program operation of computers, as well as in devices like cameras, and controls for the fuel injectors in modern cars. However, ROM plays a limited role in the PC. Here, it holds the instructions for performing the POST routine and the BIOS information used to describe the system configuration. For more detailed information, refer to Tutorial 6, "Primary PC Components."
In most cases, a technician will need to be concerned with ROM only if it has failed and requires replacement, needs to be upgraded, or if it conflicts with other memory installed in the system. The actual code in ROM is not usually in the direct control of a repair person or technician.
RAM: Random Access Memory
RAM (random access memory) is what is most often referred to when PC memory is discussed. RAM is the form of volatile memory used to hold temporary instructions and data for manipulation while the system is running. The term "random" is applied because the CPU can access or place data to and from any addressable RAM on the system. If power to the system is lost, all RAM is lost as well.
Usually, when referring to RAM, we are speaking of some variation of DRAM (dynamic random access memory) or the newer SDRAM (synchronous DRAM). These are the most common forms of RAM used in the modern PC.
DRAM works by using a microscopic capacitor and a microscopic transistor to store each data bit. A charged capacitor represents a value of "1," and a discharged capacitor represents a value of "0." A capacitor works like a battery-it holds a charge and then releases it. Unlike a battery, which holds a charge for months, the tiny capacitors in DRAM hold their charges for only fractions of a second. Therefore, DRAM needs an entire set of circuitry just to keep the capacitors charged. The process of recharging these capacitors is called "refreshing." Without refreshing, the data would be lost. This is another reason why DRAM is called volatile memory.
All PC CPUs handle data in 8-bit blocks. Each block, known as a byte, denotes how many bits the CPU can move in and out of memory at one time. The number is an indication of how rapidly data can be manipulated and arranged in system memory. But don't confuse this byte with the amount of system memory, which is usually expressed in megabytes (MB). System memory is the total amount of active memory that is available to the CPU as a temporary work area.
Each transaction between the CPU and memory is called a bus cycle. The amount of memory that a CPU can address in a single bus cycle has a major effect on overall system performance and determines the design of memory that the system can use. The width of the system's memory bus must match the number of data bits per cycle of the CPU.