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

Summary

In 1978, Intel introduced the 8086 processor, a revolutionary chip that led to the modern x86 architecture. Unlike modern 64-bit processors, however, the 8086 is a 16-bit chip. Its arithmetic/logic unit (ALU) operates on 16-bit values, performing arithmetic operations such as addition and subtraction, as well as logic operations including bitwise AND, OR, and XOR. The 8086's ALU is a complicated part of the chip, performing 28 operations in total.1 In this post, I discuss the circuitry that controls the ALU, generating the appropriate control signals for a particular operation. The process is more complicated than you might expect. First, a machine code instruction results in the execution of multiple microcode instructions. Using the ALU is a two-step process: one microcode instruction (micro-instruction) configures the ALU for the desired operation, while a second micro-instruction gets the results from the ALU. Moreover, based on both the microcode micro-instruction and the machine code instruction, the control circuitry sends control signals to the ALU, reconfiguring it for the desired operation. Thus, this circuitry provides the "glue" between the micro-instructions and the ALU. The die photo below shows the 8086 processor under a microscope. I've labeled the key functional blocks. Architecturally, the chip is partitioned into a Bus Interface Unit (BIU) at the top and an Execution Unit (EU) below. The BIU handles bus and memory activity as well as instruction prefetching, while the Execution Unit (EU) executes the instructions. In the lower right corner, the microcode ROM holds the micro-instructions. The ALU is in the lower left corner, with bits 7-0 above and bits 15-8 below, sandwiching the status flag circuitry. The ALU control circuitry, highlighted in red at the bottom of the chip, is the focus of this article. The die of the 8086. Click this image (or any other) for a larger version. Microcode The 8086 processor implements most machine instructions in mi...