Memory is the workspace for the computers processor. It is a temporary storage area where the programs and data being operated on by the processor must reside. Memory storage is considered temporary because the data and programs will remain there only as long as the computer has electrical power or is not reset. Before being shut down or reset, any data that has been changed should be saved to a more permanent storage device of some type (usually a hard disk) so it can be reloaded into memory again in the future. We often call memory RAM, for Random Access Memory. Main memory is called RAM because you can randomly (and quickly) access any location in memory. When we talk about a computers memory, we usually mean the RAM in the system, meaning primarily the memory chips or modules that make up the primary active program and data storage used by the processor. This is often confused with the term storage, which should be used when referring to things such as disk and tape drives (although some people do consider them a form of memory).
Types of Memory
To better understand physical memory in a system, it is necessary to see where and how it fits into the system. Three main types of physical memory used in modern PCs are • ROM: Read Only Memory
• DRAM: Dynamic Random Access Memory
• SRAM: Static RAM
Read Only Memory, or ROM, is a type of memory that can permanently or semipermanently hold data. It is called read-only because it is either impossible or difficult to write to. ROM is also often referred to as non-volatile memory because any data stored in ROM will remain, even if the power is turned off. As such, ROM is an ideal place to put the PCs startup instructionsthat is, the software that boots the system. Note that ROM and RAM are not opposites, as some people seem to believe. In fact, ROM is technically a subset of the systems RAM. In other words, a portion of the systems Random Access Memory address space is mapped into one or more ROM chips. This is necessary to contain the software that enables the PC to boot up; otherwise, the proces- sor would have no program in memory to execute when it was powered on.
For example, when a PC is turned on, the processor automatically jumps to address FFFF0h, expecting to find instructions to tell the processor what to do. This location is exactly 16 bytes from the end of the first megabyte of RAM space, and the end of the ROM. If this location was mapped into regular memory
chips, any data stored there would have disappeared when the power was turned off previously, and the processor would subsequently find no instructions to run the next time power was turned on. By placing a ROM chip at this address, a system startup program can be permanently loaded into the ROM and will be available every time the system is turned on. The motherboard ROM normally contains four main programs, including the following in most systems: • POST: Power-On Self Test. A series of test routines that ensure the system components are operating properly. • CMOS Setup: A menu-driven application that allows the user to set sys- tem configuration parameters, options, security settings, and preferences. • Bootstrap Loader: The routine that first scans the floppy drive and then the hard disk, looking for an operating system to load. • BIOS: Basic Input/Output System. A series of device driver programs designed to present a standard interface to the basic system hardware, especially hardware that must be active during the boot process. Four different types of ROM chips are
• ROM. Read Only Memory
• PROM. Programmable ROM
• EPROM. Erasable PROM
• EEPROM. Electrically Erasable PROM, also called a flash ROM No matter which type of ROM you use, the data stored in a ROM chip is non- volatile and will remain indefinitely unless intentionally erased or overwritten.
PROMs are a type of...
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