The Microcode and Hardware in the 8086 Processor That Perform String Operations

owl writes:

https://www.righto.com/2023/04/8086-microcode-string-operations.html

Intel introduced the 8086 microprocessor in 1978. This processor ended up being hugely influential, setting the path for the x86 architecture that is extensively used today. One interesting feature of the 8086 was instructions that can efficiently operate on blocks of memory up to 64K bytes long. These instructions rapidly copy, compare, or scan data and are known as "string" instructions.

In this blog post, I explain string operations in the 8086, analyze the microcode that it used, and discuss the hardware circuitry that helped it out.

[...] I'll explain the behavior of an ALU micro-operation since it is important for string operations. The Arithmetic/Logic Unit (ALU) is the heart of the processor, performing addition, subtraction, and logical operations. The ALU has three temporary input registers that are invisible to the programmer: tmpA, tmpB, and tmpC. An ALU operation takes its first argument from any temporary register, while the second argument always comes from tmpB. Performing an ALU operation requires two micro-instructions. The first micro-instruction specifies the ALU operation and source register, configuring the ALU. For instance, ADD tmpA configures the ALU to add the tmpA register to the default tmpB register. In the next micro-instruction (or a later one), the ALU result can be accessed through a special register called (SIGMA) and moved to another register.

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