Memory address - Citizendia

In computer science, a memory address is an identifier for a memory location, at which a computer program or a hardware device can store a piece of data for later retrieval. Computer science (or computing science) is the study and the Science of the theoretical foundations of Information and Computation and their Computer programs (also software programs, or just programs) are instructions for a Computer. By common example, this identifier could be a binary number from a finite monotonically ordered sequence. The binary numeral system, or base-2 number system, is a Numeral system that represents numeric values using two symbols usually 0 and 1. A number is an Abstract object, tokens of which are Symbols used in Counting and measuring.

In modern byte-addressable computers, each address identifies a single byte of storage; data too large to be stored in a single byte may reside in multiple bytes occupying a sequence of consecutive addresses. A byte (pronounced "bite" baɪt is the basic unit of measurement of information storage in Computer science. Some microprocessors were designed to be word-addressable, so that the addressable storage unit was larger than a byte. In Computing, " word " is a term for the natural unit of data used by a particular computer design Examples include the Texas Instruments TMS9900 and the National Semiconductor IMP-16, both of which used 16 bit words. Introduced in 1976 and based on the Texas Instruments 990 Minicomputer CPU, the TMS9900 was one of the first true 16-bit National Semiconductor ( is a Semiconductor manufacturer specializing in analog devices and subsystems headquartered in Santa Clara, California, The IMP-16, by National Semiconductor, was the first multi-chip 16-bit Microprocessor.

In computer program, an absolute address, also called explicit address or specific address, is a memory address that uniquely identifies a location in memory. Computer programs (also software programs, or just programs) are instructions for a Computer. This is opposed to a relative address, that is not unique and specifies a location only in relation to some other location (the base address). In Computing, a base address is an Address serving as a reference point ("base" for other addresses

1 Contents of a memory location
- 1.1 Word size versus address size
2 Virtual memory versus physical memory
3 See also

Contents of a memory location

Each memory location, in both ROM and RAM memory, holds a generic binary number of some sort. The binary numeral system, or base-2 number system, is a Numeral system that represents numeric values using two symbols usually 0 and 1. How it is interpreted, its type, and meaning, and usage, only depends on the context of the instructions which retrieve and manipulate it. In Computer science, an instruction is a single operation of a processor defined by an Instruction set architecture. Each such coded item has a unique physical position which is described by another unique binary number, the address of that single word, much like each house on a street has a unique number. A pointer is an address itself stored, as data, in some other memory location.

The interesting concept about items stored in memory: not only they can be interpreted as data—text data, binary numeric data, and so forth—but also as instructions themselves, in a uniform manner. Debt AIDS Trade in Africa (or DATA) is a Multinational non-government organization founded in January 2002 in London by U2 's This uniformity was introduced with von Neumann architecture and is prevalent in computers since the 1950s. The von Neumann architecture is a design model for a stored-program Digital computer that uses a processing unit and a single separate storage structure

Instructions in a storage address are contextually interpreted as command words to the system's main processing unit, and data is retrieved by such instructions placed in an internal and isolated memory structure called a storage register, where the subsequent instruction can manipulate it in conjunction with data retrieved into other internal memory locations (or internal addresses). Registers are the memory addresses within the part of the central processing unit known as the arithmetic logic unit (ALU), which responds to binary instructions (machine code) fetched into instruction registers selecting combinatorial logic determining which data registers should be added, subtracted, circulated (shifted), and so forth at the low machine language level of binary manipulation of data. In Computing, an arithmetic logic unit ( ALU) is a Digital circuit that performs Arithmetic and Logical operations Machine code or machine language is a system of instructions and data executed directly by a Computer 's Central processing unit.

Word size versus address size

A word size is characteristic to a given computer architecture. In Computer engineering, computer architecture is the conceptual design and fundamental operational structure of a Computer system It denotes the number of bits that a CPU can process at one time. Historically it has been sized in multiples of four and eight bits (nibbles and bytes, respectively), so sizes of 4, 8, 12, 16, 24, 32, 48, 64, and larger came into vogue with technological advances.

Very often, when referring to the word size of a modern computer, one is also describing the size of address space on that computer. For instance, a computer said to be "32-bit" also usually allows 32-bit memory addresses; a byte-addressable 32-bit computer can address $232 = 4,294,967,296$ bytes of memory, or 4 gibibytes (GiB). The range of Integer values that can be stored in 32 bits is 0 through 4294967295 or −2147483648 through 2147483647 using Two's complement encoding A gibibyte (a contraction of gi ga bi nary byte) is a unit of Information or Computer storage, abbreviated GiB. This seems logical and useful, as it allows one address to be efficiently stored in one word.

However, this is not always the case. Computers often have memory addresses larger or smaller than their word size. For instance, almost all 8-bit processors, such as 6502, supported 16-bit addresses, or else they would have been limited to a mere 256 byte capacity. Eight-bit CPUs normally use an 8-bit data bus and a 16-bit address bus which means that their Address space is limited to 64 KBs This is not a "natural The MOS Technology 6502 is an 8-bit Microprocessor that was designed by Chuck Peddle for MOS Technology in 1975 A byte (pronounced "bite" baɪt is the basic unit of measurement of information storage in Computer science. Similarly, the 16-bit Intel 8086 supported 20-bit addressing, allowing it to access 1 MiB rather than 64 KiBs of memory. The 8086 is a 16-bit Microprocessor chip designed by Intel and introduced on the market in 1978 which gave rise to the X86 architecture A mebibyte (a contraction of me ga bi nary byte) is a unit of Information or Computer storage, abbreviated MiB. A kibibyte (a contraction of ki lo bi nary byte) is a unit of Information or Computer storage, established by the International Also popular Pentium processors since introduction of Physical Address Extensions (PAE) support 36-bit physical addresses, while generally having only a 32-bit word. The Pentium brand refers to Intel 's single-core x86 Microprocessor based on the P5 fifth-generation Microarchitecture. In Computing, Physical Address Extension ( PAE) refers to a feature of X86 and X86-64 processors that allows more than 4 Gigabytes (

A modern byte-addressable 64-bit computer—with proper OS support—has the capability of addressing $264$ bytes (or 16 exbibytes) which as of 2007 is considered practically unlimited, being far more than the total amount of RAM ever manufactured. '64-bit' CPUs have existed in Supercomputers since the 1960s and in RISC -based workstations and servers since the early 1990s. An exbibyte (a contraction of ex a bi nary byte) is a unit of Information or Computer storage, abbreviated EiB. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century.

Virtual memory versus physical memory

Main article: Virtual memory

Virtual memory is a mapping of real memory to page tables. Virtual memory is a Computer system technique which gives an application program the impression that it has contiguous working memory while in fact it may be physically A page table is the data structure used by a Virtual memory system in a Computer Operating system to store the mapping between Virtual addresses The purpose of virtual memory is to abstract memory allocation, allowing the physical space to be allocated as is best for the hardware (that is, usually in non-contiguous blocks), and to still be seen as contiguous from a program perspective. Virtual memory is supported by some operating systems (for example, Windows but not DOS) in conjunction with the hardware. It is possible to think of virtual memory as a filter, or an alternate set of memory addresses (that are mapped to real address) that allow programs (and by extension, programmers) to read from memory as quickly as possible without requiring that memory to be specifically ordered. Programs use these contiguous virtual addresses, rather than real, and often fragmented, physical addresses, to store instructions and data. In Computer terminology a virtual address is an address identifying a virtual (non-physical entity In Computing, a physical address, also real address, or binary address, is the Memory address that is electronically (in the form of binary When the program is actually executed, the virtual addresses are translated on the fly into real memory addresses. Logical address is a synonym of virtual address. In Data Networks In computer networks a logical address refers to a Network layer address such as an IP 192

Virtual memory also allows enlarging the address space, the set of addresses a program can utilize and thus allows computers to make use of secondary storage that looks, to programs, like main memory. In Computing, an address space defines a range of discrete addresses each of which may correspond to a physical or virtual Memory register, a network host For example, virtual address space might contain twice as many addresses as main memory with the extra addresses mapped to hard disk space in the form of a swap file (also known as page file). In Computer Operating systems that have their Main memory divided into pages, paging (sometimes called swapping) is a transfer In Computer Operating systems that have their Main memory divided into pages, paging (sometimes called swapping) is a transfer It copies them back (called swapping) into main memory as soon as they are needed. These movements are performed in the background and in a way invisible for programs.

Contents

Contents of a memory location

Word size versus address size

Virtual memory versus physical memory

See also