8086

The Intel 8086 is a 16-bit microprocessor designed and manufactured by Intel. Released in 1978, it was the first member of the x86 architecture, a family that would dominate personal computing for decades. The 8086 introduced a 16-bit data path, a 20-bit address bus, and a segmented memory model that allowed access to up to one megabyte of memory, a substantial expansion over earlier 8- or 12-bit CPUs.

Architecture and features: The chip employs a two-part core consisting of a bus interface unit (BIU) that fetches instructions and accesses memory, and an execution unit (EU) that decodes and executes instructions. It incorporates a six-byte instruction prefetch queue to improve throughput and a modest instruction set with 8- and 16-bit operations. It provides general-purpose registers AX, BX, CX, DX (with AH/AL, BH/BL, etc.), SP, BP, SI, DI, and segment registers CS, DS, ES, SS, a code pointer IP, and a flags register. It uses a 16-bit data bus and a 20-bit address space, enabling a segmented addressing mode for locating memory. An optional floating-point coprocessor, the 8087, could be used to accelerate numeric calculations.

Memory model and addressing: Memory addressing is segmented, with four 16-bit segment registers defining a segment base. Each segment represents up to 4 KB of address space, and the effective address is computed as a segment base (segment << 4) plus an offset, yielding up to 1 MB of addressable memory.

Variants and impact: A variant of the 8086, the 8088, offered an 8-bit data bus and was widely used in the IBM PC line. The 8086 and 8088 laid the groundwork for the x86 lineage, influencing subsequent processors such as the 80286 and beyond, and shaping software development for decades.