16-bit computing

In computer architecture, 16-bit integers, memory addresses, or other data units are those that are 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 microcomputers are computers in which 16-bit microprocessors were the norm.

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

16-bit architecture

The MIT Whirlwind (c. 1951)[1][2] was quite possibly the first-ever 16-bit computer. Other early 16-bit computers (c. 1965–70) include the IBM 1130,[3] the HP 2100,[4] the Data General Nova,[5] and the DEC PDP-11.[6] Early multi-chip 16-bit microprocessors (c. 1973–76) include the five-chip National Semiconductor IMP-16 (1973),[7] the two-chip NEC μCOM-16 (1974),[8][7] the three-chip Western Digital MCP-1600 (1975), and the five-chip Toshiba T-3412 (1976).[7] Early single-chip 16-bit microprocessors (c. 1975–76) include the Panafacom MN1610 (1975),[9][10][7] National Semiconductor PACE (1975), General Instrument CP1600 (1975), Texas Instruments TMS9900 (1976),[7] and the HP BPC. 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 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 KB of byte-addressable 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 microcoded processor with three internal 16-bit ALUs. Only 24 bits of the program counter (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.[11] The 68008 was a version of the 68000 with an 8-bit external data path and 1-megabyte addressing for the 48-pin DIP version, and 4 megabytes for the 52-pin PLCC version. Several Apple Macintosh models--e.g., the 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.

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