The DICT Development Group

Search for:
Search type:

Database copyright information
Server information

1 definition found
 for Intel 8086
From The Free On-line Dictionary of Computing (30 December 2018) :

  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.

Contact=webmaster@dict.org Specification=RFC 2229