Intel 8086

Intel 8086
8086

   <processor> 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)