Internal Memory and Related Techniques

Registers

Memory within the CPU for data required to perform a specific task, such as the operands or result of a mathematical calculation, is stored in memory devices called registers. These registers are accessible by specific commands from the CPU. Other registers in the CPU are inaccessible for memory storage but are included as a part of the working system as a whole. Generally, registers hold the same number of bits, or binary digits, as the word length of the CPU. The number of registers in a CPU is variable, ranging from 1 to 16 or more.

Metal–Oxide–Semiconductor (MOS) RAM

This type of storage is semiconductor-based memory with wide-ranging applications in nearly all computer systems. RAM can be either dynamic or static. DRAM uses circuitry that must be periodically refreshed by rewriting the data in each block of memory. SRAM does not require the refreshing, but it is usually more expensive than DRAM. In general, SRAMs are faster than DRAMs. Both types of RAM are volatile; they lose their memory when power is turned off. Thus, RAM can be used only as temporary storage. To compensate, programs can be stored on magnetic disks or tapes (MRAM) or other forms of solid-state

memory or a battery can be used to supply the necessary power to maintain semiconductor memory when the CPU is not in use.

Often, it is useful to separate memory into two types: RAM and ROM. RAM is used in memory blocks from which the CPU must be able to ‘‘read’’ (access stored information) and ‘‘write’’ (store new data). Because information in RAM can be accessed, subsequently modified, and possibly erased, it does not provide the needed security for important programs. For those applications, such as system programs, function tables, and library subroutines, data are best stored in ROM. Because access is usually limited to retrieval, data are

not easily altered, and the integrity of key programs is ensured. Some ROMs can actually be erased and rewritten under certain conditions. These types are useful when a program needs periodic alterations but should be protected from general access and possible accidental erasure.

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Emory W. Zimmers, Jr. and Technical Staff

Enterprise Systems Center

Lehigh University

Bethlehem, Pennsylvania

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