Processors all have status flags to keep track of conditions such as a zero value, a carry, or a negative value. Whenever you write a loop or conditional, these flags ultimately are in control. But how are these flags implemented in the chip’s silicon? I’ve reverse-engineered the flag circuits in the 8085 microprocessor and explain what is really going on.
The photograph below is a highly magnified image of the 8085’s silicon, showing the relevant parts of the chip. In the upper-left, the arithmetic logic unit (ALU) performs 8-bit arithmetic operations. The status flag circuitry is below the ALU and the flags are connected to the data bus (indicated in blue). To the right of the ALU, the control PLA decodes the instructions into control lines that control the operations of the ALU and flag circuits.
As 2022 starts, let’s take some time to share our goals for CircuitPython in 2022. Just like past years (full summary 2019, 2020, and 2021), we’d like everyone in the CircuitPython community to contribute by posting their thoughts to some public place on the Internet. Here are a few ways to post: a video on YouTub, a post on the CircuitPython forum, a blog post on your site, a series of Tweets, a Gist on GitHub. We want to hear from you. When you post, please add #CircuitPython2022 and email firstname.lastname@example.org to let us know about your post so we can blog it up here.
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