36-bit

Processors
1-bit	4-bit	8-bit	12-bit	16-bit	18-bit	24-bit	31-bit	32-bit	36-bit	48-bit	60-bit	64-bit	128-bit
Applications
		8-bit		16-bit				32-bit				64-bit
Data sizes
bit   nibble   octet   byte
halfword   word   dword   qword
IEEE floating-point standard
Single precision floating-point format (32-bit)  Double precision floating-point format (64-bit)  Quadruple precision floating-point format (128-bit)

In computer architecture, 36-bit integers, memory addresses, or other data units are those that are at most 36 bits (6 characters) wide. Also, 36-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size.

Many early computers aimed at the scientific market had a 36-bit word length. This word length was just long enough to represent positive and negative integers to an accuracy of ten decimal digits (35 bits would have been the minimum). It also allowed the storage of six alphanumeric characters encoded in a six-bit character encoding. Prior to the introduction of computers, the state of the art in precision scientific and engineering calculation was the ten-digit, electrically powered, mechanical calculator, such as those manufactured by Friden, Marchant and Monroe. These calculators had a column of keys for each digit and operators were trained to use all their fingers when entering numbers, so while some specialized calculators had more columns, ten was a practical limit. Computers, as the new competitor, had to match that accuracy. Decimal computers sold in that era, such as the IBM 650 and the IBM 7070, had a word length of ten digits, as did ENIAC, one of the earliest computers.

Computers with 36-bit words included the MIT Lincoln Laboratory TX-2, the IBM 701/704/709/7090/7094, the UNIVAC 1103/1103A/1105/1100/2200, the General Electric GE-600/Honeywell 6000, the Digital Equipment Corporation PDP-6/PDP-10 (as used in the DECsystem-10/DECSYSTEM-20), and the Symbolics 3600 series. Smaller machines like the PDP-1/PDP-9/PDP-15 used 18-bit words, so a double word would be 36 bits. EDSAC had a similar scheme.

These computers used 18-bit word addressing, not byte addressing, giving an address space of 2¹⁸ 36-bit words, approximately 1 megabyte of storage. Many of them were originally limited to a similar amount of physical memory as well. Architectures that survived evolved over time to support larger virtual address spaces using memory segmentation or other mechanisms.

The common character packings included

• six 5.32-bit DEC Radix-50 characters, plus four spare bits
• six 6-bit Fieldata or IBM BCD characters (ubiquitous in early usage)
• five 7-bit characters and 1 unused bit (the usual PDP-6/10 convention, called five-seven ASCII)
• four 8-bit characters (7-bit ASCII plus 1 spare bit, or 8-bit EBCDIC), plus four spare bits
• four 9-bit characters (the Multics convention).

Characters were extracted from words either using machine code shift and mask operations or with special-purpose hardware supporting 6-bit, 9-bit, or variable-length characters. The Univac 1100/2200 used the partial word designator of the instruction, the "J" field, to access characters. The GE-600 used special indirect words to access 6- and 9-bit characters; the PDP-6/10 had special instructions to access arbitrary-length byte fields.

The standard C programming language requires that the size of the character data type be at least 8 bits, and that that all data types other than bitfields have a size that is a multiple of the character size, so standard C implementations on 36-bit machines would typically use 9-bit characters, although 12-bit, 18-bit, or 36-bit would also satisfy the requirements of the standard.

By the time IBM introduced System/360, scientific calculations had shifted to floating point and mechanical calculators were no longer a competitor. The 360s also included instructions for variable length decimal arithmetic for commercial applications, so the practice of using word lengths that were a power of two quickly became commonplace, though some 36-bit computer systems are still sold to this day, e.g., the Unisys ClearPath series.

See also

• UTF-9 and UTF-18

External links

• 36bit.org