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 computers in which 16-bit processors were the norm.
The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor.
Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816. 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. Other notable 16-bit processors include the Texas Instruments TMS9900 and the Zilog Z8000.
A 16-bit integer can store 216 (or 65536) unique values. In an unsigned representation, these values are the integers between 0 and 65535; using two's complement, possible values range from −32768 to 32767. 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, 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 forwards-compatible with other 32-bit processors. 
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