This allowed the 8086 to implement complex instructions (such as multiplication and string copying) without making the circuitry more complex. However, later chips such as the 800, used microcode, taking advantage of increasing chip densities. Microcode was common in mainframe computers of the 1960s,īut early microprocessors such as the 6502 and Z-80 didn't use microcodeīecause early chips didn't have room to store microcode. With microcode, building the processor's control logic becomes a programming task instead of a logic design task. To execute an instruction, the computer internally executes several simpler micro-instructions, which are specified by the microcode. In 1951, Maurice Wilkes came up with the idea of microcode: instead of building the control logic from complex logic gate circuitry, the control logic could be replaced with special code called microcode. One of the hardest parts of computer design is creating the control logic that tells each part of the processor what to do to carry out each instruction. The ALU does not implement multiplication or division these operations are performed through a sequence of shifts and adds/subtracts, The 16-bit ALU performs arithmetic operations (addition and subtraction), Boolean logical operations, and shifts. The lower registers include the general-purpose registers and index registers such as the stack pointer. The lower-left corner of the chip holds the Execution Unit, which performs data operations. The upper registers also include six bytes of instruction prefetch buffer and the program counter. For each memory access, a segment register and a memory offset wereįor performance, the 8086 had a separate adder for these memory address computations, rather than using the ALU. The 64 kilobytes allowed by a 16-bit address. The upper registers include the 8086's infamous segment registers, which provided access to a larger address space than This was implemented by the Bus Interface Unit in the upper left, which accessed external memory. One feature of the 8086 was instruction prefetching, which improved performance by fetching instructions from memory Die of the 8086 microprocessor showing main functional blocks.
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