[W/K] :: Intel 8086

  Intel 8086
       
           A sixteen bit microprocessor chip used in early
          IBM PCs.  The Intel 8088 was a version with an eight-bit
          external data bus.
       
          The Intel 8086 was based on the design of the Intel 8080 and
          Intel 8085 (it was source compatible with the 8080) with a
          similar register set, but was expanded to 16 bits.  The Bus
          Interface Unit fed the instruction stream to the Execution
          Unit through a 6 byte prefetch queue, so fetch and execution
          were concurrent - a primitive form of pipelining (8086
          instructions varied from 1 to 4 bytes).
       
          It featured four 16-bit general registers, which could also
          be accessed as eight 8-bit registers, and four 16-bit index
          registers (including the stack pointer).  The data
          registers were often used implicitly by instructions,
          complicating register allocation for temporary values.  It
          featured 64K 8-bit I/O (or 32K 16 bit) ports and fixed
          vectored interrupts.  There were also four segment
          registers that could be set from index registers.
       
          The segment registers allowed the CPU to access 1 meg of
          memory in an odd way.  Rather than just supplying missing
          bytes, as most segmented processors, the 8086 actually shifted
          the segment registers left 4 bits and added it to the address.
          As a result, segments overlapped, and it was possible to have
          two pointers with the same value point to two different memory
          locations, or two pointers with different values pointing to
          the same location.  Most people consider this a brain
          damaged design.
       
          Although this was largely acceptable for assembly language,
          where control of the segments was complete (it could even be
          useful then), in higher level languages it caused constant
          confusion (e.g. near/far pointers).  Even worse, this made
          expanding the address space to more than 1 meg difficult.  A
          later version, the Intel 80386, expanded the design to 32
          bits, and "fixed" the segmentation, but required extra modes
          (suppressing the new features) for compatibility, and retains
          the awkward architecture.  In fact, with the right assembler,
          code written for the 8008 can still be run on the most recent
          Intel 486.
       
          The Intel 80386 added new op codes in a kludgy fashion
          similar to the Zilog Z80 and Zilog Z280.  The Intel
          486 added full pipelines, and clock doubling (like the
          Zilog Z280).
       
          So why did IBM chose the 8086 series when most of the
          alternatives were so much better?  Apparently IBM's own
          engineers wanted to use the Motorola 68000, and it was used
          later in the forgotten IBM Instruments 9000 Laboratory
          Computer, but IBM already had rights to manufacture the 8086,
          in exchange for giving Intel the rights to its bubble memory
          designs.  Apparently IBM was using 8086s in the IBM
          Displaywriter word processor.
       
          Other factors were the 8-bit Intel 8088 version, which could
          use existing Intel 8085-type components, and allowed the
          computer to be based on a modified 8085 design.  68000
          components were not widely available, though it could use
          Motorola 6800 components to an extent.
       
          Intel bubble memory was on the market for a while, but
          faded away as better and cheaper memory technologies arrived.
       
          (1994-12-23)