12-bit computing

Before the widespread adoption of ASCII in the late 1960s, six-bit character codes were common and a 12-bit word, which could hold two characters, was a convenient size. This also made it useful for storing a single decimal digit along with a sign. Possibly the best-known 12-bit CPUs are the PDP-8 and its descendants (such as the Intersil 6100 microprocessor), which were produced in various forms from August 1963 to mid-1990. Many analog to digital converters (ADCs) have a 12-bit resolution. Some PIC microcontrollers use a 12-bit instruction word but handle only 8-bit data.

12 binary digits, or 3 nibbles (a 'tribble'), have 4096 (10000 octal, 1000 hexadecimal) distinct combinations. Hence, a microprocessor with 12-bit memory addresses can directly access 4096 words (4 kW) of word-addressable memory. IBM System/360 instruction formats use a 12-bit displacement field which, added to the contents of a base register, can address 4096 bytes of memory in a region that begins at the address in the base register.

List of 12-bit computer systems

• Digital Equipment Corporation
  • PDP-5
  • PDP-8
    • DECmate, a personal computer based on the Intersil 6100
  • PDP-12
  • PDP-14
• Ford EEC I automotive engine control unit
  • Toshiba TLCS-12 microprocessor
• Intersil IM6100 microprocessor (PDP-8-compatible)
• Control Data Corporation
  • CDC 160 series computers
  • CDC 6600 - Peripheral Processor (PP)
• National Cash Register NCR 315
• Scientific Data Systems SDS 92
• Nuclear Data, Inc. ND812
• PC12 minicomputer
• Ferranti Argus
• LINC, later commercialized by DEC as the LINC-8