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Computing (FOLDOC) dictionary
Intel 8086
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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 indexregisters (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 segmentregisters 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 braindamaged 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
The Intel 80386 added new op codes in a kludgy fashion
similar to the Zilog Z80 and Zilog Z280. The Intel486 added full pipelines, and clock doubling (like the
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
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.