48-bit computing

In computer architecture, 48-bit integers can represent 281,474,976,710,656 (248 or 2.814749767×1014) discrete values. This allows an unsigned binary integer range of 0 through 281,474,976,710,655 (248 − 1) or a signed two's complement range of -140,737,488,355,328 (-247) through 140,737,488,355,327 (247 − 1). A 48-bit memory address can directly address every byte of 256 tebibytes of storage. 48-bit can refer to any other data unit that consumes 48 bits (6 octets) in width. Examples include 48-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size.

Computer architecture bit widths
Bit
Application
Binary floating-point precision
  • 16
  • 32
  • 40
  • 64
  • 80
  • 128
  • 256
Decimal floating-point precision

Word size

Computers with 48-bit words include the AN/FSQ-32, CDC 1604/upper-3000 series, BESM-6, Ferranti Atlas, and Burroughs large systems.[lower-alpha 1][lower-alpha 2]

Addressing

The IBM System/38 and the AS/400, in its CISC variants, are 48-bit addressing systems. The address size used in logical block addressing was increased to 48 bits with the introduction of ATA-6. The Ext4 file system physically limits the file block count to 48 bits.

The minimal implementation of the x86-64 architecture provides 48-bit addressing encoded into 64 bits; future versions of the architecture can expand this without breaking properly written applications.

The media access control address (MAC address) of a computer uses a 48-bit address space. This can be changed to 64-bit addressing.

Images

In digital images, 48 bits per pixel, or 16 bits per each color channel (red, green and blue), is used for accurate processing. For the human eye, it is almost impossible to see any difference between such an image and a 24-bit image, but the existence of more shades of each of the three primary colors (65,536 as opposed to 256) means that more operations can be performed on the image without risk of noticeable banding or posterization.

Notes
  1. The B5000, B5500 and B5700 took 3 bits in control words and numeric data for use as a tag; alphanumeric data and instruction syllables were stored in the full 48 bits and had no tags.
  2. The B5900-B8xxx additionally had a 3- or 4-bit type tag.
gollark: Thank you but probably not.
gollark: ddg! Ibniz
gollark: I have not.
gollark: Fine, I can unthingy your bad mather role.
gollark: The testbot instances connect over [REDACTED], so their traffic is secure, but the server to server links are entirely unsecured.
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