Buses and operation(8086)

All internal registers as well as internal and external data buses were 16 bits wide, firmly establishing the "16-bit microprocessor" identity of the 8086. A 20-bit external address bus gave an 1 MB (segmented) physical address space (220 = 1,048,576). The data bus was multiplexed with the address bus in order to fit a standard 40-pin dual in-line package. 16-bit I/O addresses meant 64 KB of separate I/O space (216 = 65,536). The maximum linear address space were limited to 64 KB, simply because internal registers were only 16 bits wide. Programming over 64 KB boundaries involved adjusting segment registers (see below) and were therefore fairly awkward (and remained so until the 80386).
Some of the control pins, which carry essential signals for all external operations, had more than one function depending upon whether the device was operated in "min" or "max" mode. The former were intended for small single processor systems whilst the latter were for medium or large systems, using more than one processor.