View source for 16-bit

{{N-bit|16|(2 octets)}} 16-bit [[Microcomputer|microcomputer]]s are computers in which 16-bit [[Engineering:Microprocessor|microprocessor]]s were the norm.

A 16-bit register can store 2<sup>16</sup> different values. The [[Signedness|signed]] range of [[Integer|integer]] values that can be stored in 16 bits is −32,768 (−1 × 2<sup>15</sup>) through 32,767 (2<sup>15</sup> − 1); the [[Signedness|unsigned]] range is 0 through 65,535 (2<sup>16</sup> − 1).  Since 2<sup>16</sup> is 65,536, a processor with 16-bit [[Memory address|memory address]]es can directly access 64 [[Kilobyte|KB]] (65,536 bytes) of [[Byte addressing|byte-addressable]] memory.  If a system uses [[Memory segmentation|segmentation]] with 16-bit segment offsets, more can be accessed.

==16-bit architecture==
The [[Engineering:Whirlwind I|MIT Whirlwind]] ({{Circa}} 1951)<ref>Computer History Museum, [http://www.computerhistory.org/timeline/?year=1951 Year 1951] (see also [http://www.computerhistory.org/timeline/?year=1943 Year 1943]).</ref><ref>Digital Press, [http://www.computerhistory.org/collections/accession/102630350 Digital at Work], Pearson, 1992, {{ISBN|1-55558-092-0}}, pp. 4, 23.</ref> was quite possibly the first-ever 16-bit computer.  Other early ({{Circa}} 1965–70) 16-bit computers include the [[Engineering:IBM 1130|IBM 1130]],<ref>IBM Archives, [http://www-03.ibm.com/ibm/history/exhibits/1130/1130_intro.html The IBM 1130 computing system].</ref> the [[HP 2100]],<ref>Computer History Museum, [http://www.computerhistory.org/revolution/minicomputers/11/337/2385 "HP 2116"].</ref> the [[Engineering:Data General Nova|Data General Nova]],<ref>Computer History Museum, [http://www.computerhistory.org/collections/accession/102646102 "Data General Nova minicomputer"].</ref> and the [[Engineering:PDP-11|DEC PDP-11]].<ref>Digital Press, [http://www.computerhistory.org/collections/accession/102630350 Digital at Work], Pearson, 1992, {{ISBN|1-55558-092-0}}, pp. 58–61.</ref>  Early ({{Circa}} 1973–75) multi-chip 16-bit microprocessors include the [[Engineering:IMP-16|National Semiconductor IMP-16]] and the [[Engineering:MCP-1600|Western Digital MCP-1600]].  Early ({{Circa}} 1975–76) single-chip 16-bit microprocessors include the Panafacom MN1610,<ref>{{cite web|title=16-bit Microprocessors|url=http://www.cpu-museum.com/161x_e.htm|publisher=CPU Museum|accessdate=5 October 2010}}</ref><ref name="fujitsu">{{cite web|title=History|url=http://www.pfu.fujitsu.com/en/profile/history.html|publisher=PFU|accessdate=5 October 2010}}</ref> [[Engineering:National Semiconductor PACE|National Semiconductor PACE]], the [[HP 2100#Descendants and variants|HP BPC]], and the [[Engineering:Texas Instruments TMS9900|TI TMS9900]].  Other notable 16-bit processors include the [[Engineering:Intel 8086|Intel 8086]], the [[Engineering:Intel 80286|Intel 80286]], the WDC 65C816, and the Zilog Z8000. The [[Engineering:Intel 8088|Intel 8088]] was binary compatible with the Intel 8086, and was 16-bit in that its registers were 16 bits wide, and arithmetic instructions could operate on 16-bit quantities, even though its external bus was 8 bits wide.

A 16-bit integer can store 2<sup>16</sup> (or 65,536) distinct values.  In an [[Signedness|unsigned]] representation, these values are the integers between 0 and 65,535; using [[Two's complement|two's complement]], possible values range from −32,768 to 32,767. Hence, a processor with 16-bit [[Memory address|memory address]]es can directly access 64&nbsp;KB of [[Byte addressing|byte-addressable]] memory.

16-bit processors have been almost entirely supplanted in the [[Engineering:Personal computer|personal computer]] industry, and are used less than 32-bit (or 8-bit) CPUs in embedded applications<!-- Outdated info?: ARM has supplanted most all 16-bit (and in iPhone for many generations now?); for example, the 16-bit [[Engineering:XAP processor|XAP processor]] is used in many [[Engineering:Application-specific integrated circuit|ASICs]]-->.

===16/32-bit Motorola 68000 and Intel 386SX===
The [[Engineering:Motorola 68000|Motorola 68000]] is sometimes called ''16-bit'' because its internal and external data buses were 16 bits wide; however, it could be considered a [[32-bit|32-bit]] processor in that the general purpose registers were 32 bits wide and most arithmetic instructions supported 32-bit arithmetic.  The 68000 was a [[Microcode|microcode]]d processor with three internal 16-bit ALUs. Only [[24-bit|24 bits]] of the [[Program counter|program counter]] (PC) were available on original [[Engineering:Dual in-line package|DIP]] packages, with up to 16 megabytes of addressable [[Random-access memory|RAM]].  68000 software is 32-bit in nature and forward-compatible with other 32-bit processors in the same family.<ref>Motorola, Inc., [http://cache.freescale.com/files/archives/doc/ref_manual/M68000PRM.pdf Motorola M68000 Family, Programmer's Reference Manual], 1992, sec. 2.4, p. 2–21.</ref>  The 68008 was a version of the 68000 with 8-bit external data path and 1 megabyte addressing for the 48-pin DIP version and 4 megabyte for the 52-pin PLCC version. Several [[Company:Apple Inc.|Apple Inc.]] [[Engineering:Macintosh|Macintosh]] models; e.g., LC series, used 32-bit 68020 and 68030 processors on a 16-bit data bus to save cost.  

Similar analysis applies to Intel's 80286 CPU replacement called the [[Engineering:Intel 80386|386SX]] which is a 32-bit processor with 32-bit [[Arithmetic logic unit|ALU]] and internal 32-bit data paths with a 16-bit external bus and 24-bit addressing of the processor it replaced.

==Intel 16-bit memory models==
{{Main|Intel Memory Model}}
Just as there are multiple data models for 64-bit architectures, the 16-bit Intel architecture allows for different memory models—ways to access a particular memory location. The reason for using a specific memory model is the size of the assembler instructions or required storage for pointers. Compilers of the 16-bit era generally had the following type-width characteristic:

{|class="wikitable"
|+ 16-bit data model
|-
! Data model
! <code>short</code>
! <code>int</code>
! <code>long</code>
! Pointers
|-
! IP16L32 (near)
| 16
| 16
| 32
| 16
|-
! I16LP32 (far)
| 16
| 16
| 32
| 32
|}

;Tiny: Code and data will be in the same segment (especially, the registers CS,&nbsp;DS,&nbsp;ES,&nbsp;SS will point to the same segment); ''near'' (16-bit) pointers are always used. Code, [[Heap (data structure)|data]] and stack together cannot exceed 64&nbsp;KB.
;Small: Code and data will be in different segments, and near pointers are always used. There will be 64&nbsp;KB of space for code and 64&nbsp;KB for data/stack.
;Medium: Code pointers will use ''far'' pointers (16:16 bit), enabling access to 1&nbsp;MB. Data pointers remain to be of the near type.
;Compact: Data pointers will use far and code will use near pointers.
;Large/huge: Code and data pointers will be far.<ref>Borland Turbo C++ 1.01 in-program manual</ref>

==16-bit application==
In the context of IBM PC compatible and [[Wintel]] platforms, a 16-bit application is any software written for [[Software:MS-DOS|MS-DOS]], OS/2 1.x or early versions of [[Software:Microsoft Windows|Microsoft Windows]] which originally ran on the 16-bit [[Engineering:Intel 8088|Intel 8088]] and [[Engineering:Intel 80286|Intel 80286]] [[Engineering:Microprocessor|microprocessor]]s. Such applications used a 20-[[Bit|bit]] or 24-bit [[X86 memory segmentation|segment or selector-offset]] address representation to extend the range of addressable memory locations beyond what was possible using only 16-bit addresses. Programs containing more than 2<sup>16</sup> [[Byte|byte]]s (65,536 [[Byte|byte]]s) of instructions and data therefore required special instructions to switch between their 64-kilobyte segments, increasing the complexity of programming 16-bit applications.

==List of 16-bit CPUs==
{{div col|colwidth=20em}}
* Angstrem
** [[Engineering:1801 series CPU|1801 series CPU]]
* [[Company:Data General|Data General]]
** [[Engineering:Data General Nova|Nova]]
** [[Engineering:Data General Eclipse|Eclipse]]
* [[Company:Digital Equipment Corporation|Digital Equipment Corporation]]
** [[Engineering:PDP-11|PDP-11]] (for LSI-11, see Western Digital, below)
*** [[Engineering:DEC J-11|DEC J-11]]
*** [[Engineering:DEC T-11|DEC T-11]]
* [[Company:EnSilica|EnSilica]]
** eSi-1600
* [[Company:Ferranti|Ferranti]]
** Ferranti F100-L
** Ferranti F200-L
* Freescale
** [[Engineering:Freescale 68HC12|Freescale 68HC12]]
** [[Engineering:Freescale 68HC16|Freescale 68HC16]]
* [[Company:General Instrument|General Instrument]]
** [[Engineering:General Instrument CP1600|CP1600]]
* [[Company:Hewlett-Packard|Hewlett-Packard]]
** [[HP 2100|HP 21xx/2000/1000/98xx/BPC]]
** [[HP 3000]]
* Honeywell
** [[Engineering:Honeywell Level 6|Honeywell Level 6]]/DPS 6
* [[Company:IBM|IBM]]
** [[Engineering:IBM 1130|1130]]/1800
** System/7
** Series/1
** System/36
* Infineon
** [[Engineering:XE166 family|XE166 family]]
** [[Engineering:C166 family|C166 family]]
** C167 family
** [[Engineering:XC2000|XC2000]]
* [[Company:Intel|Intel]]
** [[Engineering:Intel 8086|Intel 8086]]/[[Engineering:Intel 8088|Intel 8088]]
** [[Engineering:Intel 80186|Intel 80186]]/[[Engineering:Intel 80188|Intel 80188]]
** [[Engineering:Intel 80286|Intel 80286]]
** [[Engineering:Intel MCS-96|Intel MCS-96]]
* [[Company:Lockheed Corporation|Lockheed]]
** [[Engineering:Lockheed MAC-16|MAC-16]]
*[[Company:Motorola|Motorola]]
**[[Engineering:Motorola 68000|Motorola 68000]] (32-bit registers, 16-bit bus)
**[[Engineering:Motorola 68010|Motorola 68010]] (32-bit registers, 16-bit bus)
* [[Company:National Semiconductor|National Semiconductor]]
** [[Engineering:IMP-16|IMP-16]]
** [[Engineering:National Semiconductor PACE|PACE/INS8900]]
* [[Company:NEC|NEC]]
** V20/V30
*Renesas
**Renesas M16C (16-bit registers, 24-bit address space)
*[[Company:Ricoh|Ricoh]]
**Ricoh 5A22 (WDC 65816 clone used in [[Engineering:Super Nintendo Entertainment System|SNES]])
* [[Company:Texas Instruments|Texas Instruments]]
** [[Engineering:Texas Instruments TMS9900|Texas Instruments TMS9900]]
** [[Engineering:TI MSP430|TI MSP430]]
* Western Design Center
** WDC 65816/65802
* [[Company:Western Digital|Western Digital]]
** [[Engineering:MCP-1600|MCP-1600]] (used in the [[Engineering:PDP-11#LSI-11|DEC LSI-11]])
* [[Company:Xerox|Xerox]]
** [[Engineering:Xerox Alto|Alto]]
* Zilog
** Zilog Z8000
{{div col end}}

==See also==
* [[Engineering:Microprocessor#16-bit designs|Microprocessor: 16-bit designs]]
* Influence of the IBM PC on the personal computer market: Before the IBM PC's introduction
* [[Engineering:74181#Computers|74181]] (key component of some early 16-bit and other CPUs)
* [[Audio bit depth]] – as 16-bit is the most common bit depth used, e.g. on CD audio.

==References==
{{Reflist|30em}}

{{CPU technologies}}

[[Category:Data unit|16-bit]]

{{Sourceattribution|16-bit|1}}