Intel 8086
From Computer History Wiki
The i8086 was the first generation x86 cpu. The 8086 also had a cousin part, the 8088. The 8088 had 8bit external IO bus, unlike the 8086 which was a full 16 bit cpu.
The 8086 had a 20 bit address bus, allowing for 1 megabyte of accessible memory. However to maintain compatibility with the 8080 cpu that was popular in CP/M circles, the 8086 addressed memory in 64kb segments. This would be seen as a major fault in the x86 architecture.
The next cpu based on the 8086 was the i80186, although it didn't see widespread use, unlike the i80286 cpu.
