The origin of the 80x86 family of processors began in 1978 with the introduction of the 8086 processor. Shortly thereafter, the 8088 processor was added to the family. Both processors have 16-bit registers and use 20 bits (little-endian) to address memory; this permits addressing of a megabyte of memory. Instructions provided for three type of operands: memory, register, and immediate. Instructions could combine these operand types in any manner, except that two memory operands could not be included in the same instruction. The important distinction between the 8086 and the 8088 is that the 8086 processor had a 16-bit external data bus and a 16-bit internal data bus whereas the 8088 processor had an 8-bit external data bus and a 16-bit internal data bus.
The 80186/80188 architectures, like the 8086 and 8088, are nearly identical. The only difference between the 80186 and 80188 is the width of their data buses. The internal registers of the 80186/80188 are identical to the 8086/8088. The only difference is that the 80186/80188 contains additional reserved interrupt vectors and some very powerful built in I/O features.
The 80286 introduced several new features as compared to the 8086. The 80286 has two different types of operating mode: real address mode and protected address mode. The real address mode was introduced in the 80286 to permit backward compatibility with the 80186 and the 8086 processors. In real mode the 80286 uses 24 bits to address up to 16 megabytes of memory. In the protected mode the 80286 uses 32 bits to address up to 1 gigabyte of memory. The advanced architectural features and full capabilities of the 80286 are realized in its native protected mode. Among these features are sophisticated mechanisms to support data protection, system integrity, task concurrency, and memory management, including virtual storage.
The 80386 added memory paging and introduced 32-bit processing. The size of data registers were increased to 32 bits.
No substantial changes to memory addressing and registers have been made in processors that followed the 80386. Rather, subsequent 80x86 processors have concentrated on fine tuning the micro-architecture of the processor to increase performance. The 80486 performs aggressive pipelining and integrated the CPU and FPU ( Floating Point Unit ) on one chip.
