8-bit CPU control logic: Part 1
Why Hacker News recommends it
Recommended as the key video for understanding control logic and microcode.
What it is
Ben Eater writes a program in a made-up machine language and walks through the control logic an 8-bit breadboard CPU needs to execute it. It is part of his build-an-8-bit-computer series.
Recommended by 3 HN commenters across 3 different months, 2017 to 2023.
What Hacker News said
Strong rec bogomipz · 2017-06 · on “Build an 8-Bit Computer from Scratch”
This is really amazing. In addition to the interesting project this individual has a great gift for articulating everything in a concise manner and with a very pleasant cadence. What a great teacher. Thanks for sharing. I'm looking forward to seeing the rest of this series. If you only have a limited amount of time I would highly recommend the control logic video: [this video]
Recommends dkersten · 2023-03 · on “AMD ymm registers are rolling back”
I found these Ben Eater YouTube videos very useful to understanding what microcode is and how it works: 8-bit CPU control logic [this video] 8-bit CPU reprogramming microcode [LINK] In short, the microcode instructions are a bunch of flags that enable different parts of the processor during that clock cycle (eg is data being loaded off the bus into a register? Is the adder active? Etc), so to implement an instruction that says add value from memory a to value from memory b and store in memory c, the microcode might be: copy memory a onto bus, store bus to register, copy memory b to bus, store b to register, add both register and put result online bus, store value on bus to memory c. (In a hypothe
Recommends pests · 2023-07 · on “Undocumented 8086 instructions, explained by the microcode”
Ben Eater on YouTube has recreated an 8-bit computer on breadboards. [this video] That is the start of the videos where the control logic gets microcoded. Its pretty basic but over the next few videos he comes up with about 10 different OPCODES and programs their microcode (a series of control logic activations). Its pretty amazing to see it all come together and work in the end.
Cites dkersten · 2021-06 · on “6502 “Illegal” Opcodes Demystified”
Yeah, it was my understanding that the opcodes directly encoded what we would nowadays call microcode, that is, bits which selected which CPU component acted on which data[1], so these illegal instructions are just unused bit patterns that select various operations just by having those bits set, not something that was ever intended to be actually used. So it would make sense that they're not documented, since they're entirely incidental. [1] [this video]