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   » » Wiki: 16-bit
Tag Wiki '16-bit'.

16-bit [[microcomputer]]s are computers in which 16-bit [[microprocessor]]s were the norm.

A 16-bit register can store 216 different values. The range of values that can be stored in 16 bits is −32,768 (−1 × 215) through 32,767 (215 − 1); the range is 0 through 65,535 (216 − 1). Since 216 is 65,536, a processor with 16-bit can directly access 64 (65,536 bytes) of memory. If a system uses segmentation with 16-bit segment offsets, more can be accessed.

16-bit architecture
The ( 1951)Computer History Museum, Year 1951 (see also Year 1943).Digital Press, Digital at Work, Pearson, 1992, , pp. 4, 23. was quite possibly the first-ever 16-bit computer. Other early ( 1965–70) 16-bit computers include the IBM 1130,IBM Archives, The IBM 1130 computing system. the HP 2100,Computer History Museum, "HP 2116". the Data General Nova,Computer History Museum, "Data General Nova minicomputer". and the DEC PDP-11.Digital Press, Digital at Work, Pearson, 1992, , pp. 58–61. Early ( 1973–75) multi-chip 16-bit microprocessors include the National Semiconductor IMP-16 and the Western Digital MCP-1600. Early ( 1975–76) single-chip 16-bit microprocessors include the MN1610, National Semiconductor PACE, the HP BPC, and the TI TMS9900. Other notable 16-bit processors include the Intel 8086, the Intel 80286, the WDC 65C816, and the Zilog Z8000. The 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 216 (or 65,536) distinct values. In an representation, these values are the integers between 0 and 65,535; using two's complement, possible values range from −32,768 to 32,767. Hence, a processor with 16-bit can directly access 64 KB of memory.

16-bit processors have been almost entirely supplanted in the personal computer industry, and are used less than 32-bit (or 8-bit) CPUs in embedded applications.

16/32-bit Motorola 68000 and Intel 386SX
The 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 processor in that the general purpose registers were 32 bits wide and most arithmetic instructions supported 32-bit arithmetic. The 68000 was a processor with three internal 16-bit ALUs. Only 24 bits of the (PC) were available on original DIP packages, with up to 16 megabytes of addressable RAM. 68000 software is 32-bit in nature and forward-compatible with other 32-bit processors in the same family.Motorola, Inc., Motorola M68000 Family, Programmer's Reference Manual, 1992, sec. 2.4, p. 2–21. 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 Apple Inc. 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 386SX which is a 32-bit processor with 32-bit 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
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:

+ 16-bit data model

Code and data will be in the same segment (especially, the registers CS, DS, ES, SS will point to the same segment); near (16-bit) pointers are always used. Code, data and stack together cannot exceed 64 KB.
Code and data will be in different segments, and near pointers are always used. There will be 64 KB of space for code and 64 KB for data/stack.
Code pointers will use far pointers (16
Data pointers will use far and code will use near pointers.
Code and data pointers will be far.Borland Turbo C++ 1.01 in-program manual

16-bit application
In the context of IBM PC compatible and platforms, a 16-bit application is any software written for , OS/2 1.x or early versions of Microsoft Windows which originally ran on the 16-bit Intel 8088 and Intel 80286 . Such applications used a 20- or 24-bit 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 216 (65,536 ) of instructions and data therefore required special instructions to switch between their 64-kilobyte , increasing the complexity of programming 16-bit applications.

List of 16-bit CPUs
  • Angstrem
    • 1801 series CPU
    • Nova
    • Eclipse
  • Digital Equipment Corporation
    • PDP-11 (for LSI-11, see Western Digital, below)
      • DEC J-11
      • DEC T-11
    • Ferranti F100-L
    • Ferranti F200-L
    • Freescale 68HC12
    • Freescale 68HC16
  • General Instrument
    • CP1600
    • HP 21xx/2000/1000/98xx/BPC
    • HP 3000
  • Honeywell
    • Honeywell Level 6/DPS 6
    • 1130/1800
    • System/7
    • Series/1
    • System/36
    • XE166 family
    • C166 family
    • C167 family
    • XC2000
    • Intel 8086/Intel 8088
    • Intel 80186/Intel 80188
    • Intel 80286
    • Intel MCS-96
  • Lockheed
    • MAC-16
    • Motorola 68000 (32-bit registers, 16-bit bus)
    • Motorola 68010 (32-bit registers, 16-bit bus)
  • National Semiconductor
    • IMP-16
    • PACE/INS8900
    • V20/V30
    • Renesas M16C (16-bit registers, 24-bit address space)
    • Ricoh 5A22 (WDC 65816 clone used in SNES)
  • Texas Instruments
    • Texas Instruments TMS9900
    • TI MSP430
  • Western Design Center
    • WDC 65816/65802
    • MCP-1600 (used in the DEC LSI-11)
    • Zilog Z8000

See also
  • 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 .

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