external data bus. The Intel 8086 was based on the design of the Intel 8080 and 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 registers were often used implicitly by instructions, featured 64K 8-bit I/O (or 32K 16 bit) ports and fixed 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. 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 So why did IBM chose the 8086 series when most of the alternatives were so much better? Apparently IBM's own Computer, but IBM already had rights to manufacture the 8086, 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 faded away as better and cheaper memory technologies arrived. (1994-12-23)
|
