Notes on the Intel 8086 processor's arithmetic-logic unit
15 days ago
- #Intel 8086
- #Microcode
- #ALU
- 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.