The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and mid-1978, when it was released. The Intel 8088, released in 1979, is a slightly modified chip with an external 8-bit data bus (allowing the use of cheaper and fewer supporting ICs), and is notable as the processor used in the original IBM PC design, including the widespread version called IBM PC XT. The 8086 gave rise to the x86 architecture, which eventually became Intel’s most successful line of processors.
All internal registers, as well as internal and external data buses, are 16 bits wide, which firmly established the “16-bit microprocessor” identity of the 8086. A 20-bit external address bus provides a 1 MB physical address space (220 = 1,048,576). This address space is addressed by means of internal memory “segmentation”. The data bus is multiplexed with the address bus in order to fit all of the control lines into a standard 40-pin dual in-line package. It provides a 16-bit I/O address bus, supporting 64 KB of separate I/O space. The maximum linear address space is limited to 64 KB, simply because internal address/index registers are only 16 bits wide. Programming over 64 KB memory boundaries involves adjusting the segment registers (see below); this difficulty existed until the 80386 architecture introduced wider (32-bit) registers (the memory management hardware in the 80286 did not help in this regard, as its registers are still only 16 bits wide).
The 8086 has eight more or less general 16-bit registers (including the stack pointer but excluding the instruction pointer, flag register and segment registers). Four of them, AX, BX, CX, DX, can also be accessed as twice as many 8-bit registers (see figure) while the other four, BP, SI, DI, SP, are 16-bit only.