Memory address

In computing, a memory address is a reference to a specific memory location in memory used by both software and hardware. These addresses are fixed-length sequences of digits, typically displayed and handled as unsigned integers. This numerical representation is based on the features of CPU (such as the instruction pointer and incremental address registers). Programming language constructs often treat the memory like an array.

A digital computer's main memory consists of many memory locations, each identified by a unique physical address (a specific code). The CPU or other devices can use these codes to access the corresponding memory locations. Generally, only system software (such as the BIOS, operating systems, and specialized utility programs like memory testers) directly addresses physical memory using machine code instructions or processor registers. These instructions tell the CPU to interact with a hardware component called the memory controller. The memory controller manages access to memory using the memory bus or a system bus, or through separate control, address, and data buses, to execute the program's commands. The bus managed by the memory controller consists of multiple parallel lines, each representing a binary digit (bit).

A computer program uses memory addresses to execute machine code, and to store and retrieve data. In early computers, logical addresses (used by programs) and physical addresses (actual locations in hardware memory) were the same. However, with the introduction of virtual memory, most application programs do not deal directly with physical addresses. Instead, they use logical or virtual addresses, which are translated to physical addresses by the computer's memory management unit (MMU) and the operating system's memory mapping mechanisms.

Most modern computers are byte-addressable. Each address identifies a single 8-bit byte (octet) of storage. Data larger than a single byte may be stored in a sequence of consecutive addresses. There exist word-addressable computers, where the minimal addressable storage unit is exactly the processor's word. For example, the Data General Nova minicomputer, and the Texas Instruments TMS9900 and National Semiconductor IMP-16 microcomputers, used 16-bit words, and there are many old mainframe computers that use 36-bit word addressing (such as the IBM 7090, with 15-bit word addresses, giving an address space of 2¹⁵ 36-bit words, approximately 128 kilobytes of storage, and the DEC PDP-6/PDP-10, with 18-bit word addresses, giving an address space of 2¹⁸ 36-bit words, approximately 1 megabyte of storage), not byte addressing. The range of addressing of memory depends on the bit size of the bus used for addresses – the more bits used, the more addresses are available to the computer. For example, an 8-bit-byte-addressable machine with a 20-bit address bus (e.g. Intel 8086) can address 2²⁰ (1,048,576) memory locations, or one MiB of memory, while a 32-bit bus (e.g. Intel 80386) addresses 2³² (4,294,967,296) locations, or a 4 GiB address space. In contrast, a 36-bit word-addressable machine with an 18-bit address bus addresses only 2¹⁸ (262,144) 36-bit locations (9,437,184 bits), equivalent to 1,179,648 8-bit bytes, or 1152 KiB, or 1.125 MiB — slightly more than the 8086.

A small number of older machines are bit-addressable. For example, a variable filed length (VFL) instruction on the IBM 7030 "Stretch" specifies a bit address, a byte size of 1 to 8 and a field length.

Some older computers (decimal computers) are decimal digit-addressable. For example, each address in the IBM 1620's magnetic-core memory identified a single six bit binary-coded decimal digit, consisting of a parity bit, flag bit and four numerical bits. The 1620 used 5-digit decimal addresses, so in theory the highest possible address was 99,999. In practice, the CPU supported 20,000 memory locations, and up to two optional external memory units could be added, each supporting 20,000 addresses, for a total of 60,000 (00000–59999).

Some older computers are character-addressable, with 6-bit BCD characters containing a 2-bit zone and a 4-bit digit; the characters in an address only have digit values representing 0–9. Typically some of the zone bits are part of the address and some are used for other purposes, e.g., index register, indirect address.

Some older computers are decimal-word addressable, typically with 4-digit addresses. In some machines the address fields also select index registers, restricting the range of possible address.