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In computer architecture, 16-bit integers, memory addresses, or other data units are those that are at most 16 bits (2 octets) wide. Also, 16-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size. 16-bit is also a term given to a generation of microcomputers in which 16-bit microprocessors were the norm.
The MIT Whirlwind (c. 1951) was quite possibly the first-ever 16-bit computer. Other early (c. 1965-1970) 16-bit computers include the IBM 1130, the HP 2100, the Data General Nova, and the DEC PDP-11. Early (c. 1973-1975) multi-chip 16-bit microprocessors include the National Semiconductor IMP-16 and the Western Digital MCP-1600. Early (c. 1975-1976) single-chip 16-bit microprocessors include the 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 program-compatible with the Intel 8086, and was 16-bit in that its registers were 16 bits long and arithmetic instructions, even though its external bus was 8 bits wide.
A 16-bit integer can store 216 (or 65,536) distinct values. In an unsigned 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 memory addresses can directly access 64 KiB of byte-addressable memory.
16-bit processors have been almost entirely supplanted in the personal computer industry, but remain in use in a wide variety of embedded applications; for example, the 16-bit XAP processor is used in many ASICs.
The 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 MC68000 was a microcoded processor with three internal 16-bit ALU units. Only 24-bits of the Program Counter were available on original DIP packages, with up to 16 megabytes of addressable RAM. MC68000 software is 32-bit in nature and forward-compatible with other 32-bit processors in the same family. The MC68008 was a version of the 68000 with 8-bit external data path and 1 megabyte addressing. Several Apple Inc. Macintosh models; e.g., LC series, used 32-bit MC68020 and MC68030 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.
The 68000 processor of the Sega Mega Drive was a highly advertised feature of the video game system. Due to the saturation of this advertising, the 1988-1995 era (fourth generation) of video game consoles is often called the 16-bit era.
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:
- 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 64K.
- Code and data will be in different segments, and near pointers are always used. There will be 64K of space for code and 64K for data/stack.
- Code pointers will use far pointers (16:16 bit), enabling access to 1 MB. Data pointers remain to be of the near type.
- Data pointers will use far and code will use near pointers.
- Code and data pointers will be far.
In the context of IBM PC compatible and Wintel platforms, a 16-bit application is any software written for MS-DOS, OS/2 1.x or early versions of Microsoft Windows which originally ran on the 16-bit Intel 8088 and Intel 80286 microprocessors. Such applications used a 20-bit 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 bytes (64 kilobytes) 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
- Data General
- Digital Equipment Corporation
- PDP-11 (for LSI-11, see Western Digital, below)
- General Instrument
- Honeywell Level 6/DPS 6
- National Semiconductor
- Texas Instruments
- Western Design Center
- Western Digital
- Microprocessor: 16-bit designs
- Influence of the IBM PC on the personal computer market: Before the IBM PC's introduction
- 74181 (key component of some early 16-bit and other CPUs)
- Computer History Museum, Year 1951 (see also Year 1943).
- Digital Press, Digital at Work, Pearson, 1992, ISBN 1-55558-092-0, pp. 4, 23.
- IBM Archives, The IBM 1130 computing system.
- Computer History Museum, "HP 2116".
- Computer History Museum, "Data General Nova minicomputer".
- Digital Press, Digital at Work, Pearson, 1992, ISBN 1-55558-092-0, pp. 58-61.
- Motorola, Inc., Motorola M68000 Family, Programmer's Reference Manual, 1992, sec. 2.4, p. 2-21.
- Borland Turbo C++ 1.01 in-program manual