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Addressability ○｜Definition｜1st｜20251119205401-00-⌔

Memory address - Wikipedia#Unit_of_address_resolution

Unit of address resolution

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.1 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 2 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 2 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 2 (1,048,576) memory locations, or one MiB of memory, while a 32-bit bus (e.g. Intel 80386) addresses 2 (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 2 (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.2 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.3

Some older computers are decimal-word addressable, typically with 4-digit addresses.4 In some machines the address fields also select index registers, restricting the range of possible address.5

Printed 2026-06-28.

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Footnotes

  1. Some word-addressable computers have byte-handling instructions, typically supporing byte sizes up to a word but not supporting bytes that straddled words. For example, the DEC PDP-6 and PDP-10 have Deposit Byte (DPB) Increment Byte Pointer (IBP) Increment and Deposit Byte (IDPB) Increment and Load Byte (ILDB) Load Byte (LDB) ↩

  2. IBM 1620 Central Processing Unit, Model 1 (PDF). Systems Reference Library. IBM. A26-5706-3. Retrieved March 7, 2025. ↩

  3. System Reference Manual - RCA 3301 (PDF). RCA EDP. September 1967. 94-16-000. Retrieved March 7, 2025. ↩

  4. IBM 7070-7074 Principles of Operation (PDF). Systems Reference Library. IBM. GA22-7003-6. Retrieved March 7, 2025. ↩

  5. 650 magnetic drum data-processing machine - manual of operation (PDF). IBM22-6060-2. June 1955. 22-6060-2. Retrieved March 7, 2025. ↩

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