Notes on the Intel 8086 processor's arithmetic-logic unit

2 months ago

The Intel 8086 processor, introduced in 1978, was a 16-bit chip that laid the foundation for modern x86 architecture.
The 8086's ALU performs 28 operations, including arithmetic (addition, subtraction) and logic operations (AND, OR, XOR).
ALU control involves microcode instructions: one to configure the ALU and another to retrieve results.
The 8086 uses a two-part microcode architecture where each micro-instruction performs two unrelated operations.
Special cases in the 8086 instruction set (e.g., compare, BCD adjustments) require additional ALU control signals.
The ALU uses lookup tables (similar to FPGAs) to generate carry and output signals, enabling reconfiguration for different operations.
Control circuitry includes flip-flops to remember ALU operations and temporary registers between micro-instructions.
The 8086's complexity contrasts with RISC designs, featuring many corner cases and specialized operations.
Bootstrap driver circuits boost control signal voltages to improve performance in the ALU.

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