A Checkerboard of Bytes
Each checkerboard square of memory holds one byte. The contents of any single byte or group of bytes can be calculated, compared and copied independently. That is how fields are put together to form records and broken apart when read back in. On a disk, data are stored in sectors, typically 512 bytes long, that are the smallest unit that can be read or written by the drive.
Memory Does Not Remember Oddly enough, the computer's memory does not "remember" anything when the power is turned off. So why do they call it memory? Because the first memory did "remember," but today's RAM chips do not, which is why files have to be saved before the application is ended. Although there are memory chips that do hold their content permanently (ROMs, PROMs, EPROMs, Flash, etc.), they are used for internal control purposes and not for the user's data. Just to bewilder you more, it is quite possible that in the future, memory will again "remember" (see future memory chips). "Remembering" memory in a computer system is its disks and tapes, and although they are sometimes called "memory devices," many prefer to call them "storage devices" (as we do) in order to differentiate them from internal memory. Memory Can Get Clobbered! Memory is an important resource that cannot be wasted. It must be allocated by the operating system as well as by applications and then released when not needed. Errant programs can grab memory and not let go of it, which results in less and less memory available as you load and use more programs. Restarting the computer gives memory a clean slate, which is why rebooting the computer clears up so many problems with applications. In addition, if the operating system has bugs, a malfunctioning application can write into the same memory used by another program, causing all kinds of unspecified behavior. You discover it when the system freezes or something weird happens all of a sudden. If you were to be able to look into memory and watch how fast data and instructions are written into and out of it in the course of even 10 minutes, you would know it is truly a miracle that it works at all. Other terms for the computer's main memory are RAM, primary storage and read/write memory. Earlier terms were core and core storage. See dynamic RAM, static RAM and memory module.
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