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The 16-Bit Decision That Became a 64-Bit Problem

The Part That Changed the World

5,691 views

How did a tiny decision made in 1978 become a problem that still exists inside modern 64-bit computers?

This video tells the surprising history of the A20 line — from Intel’s 8086 and IBM’s original PC to the 80286, DOS, HIMEM.SYS, and modern x86-64 processors.

A simple compatibility problem led IBM engineers to create a temporary hardware fix using a spare pin on the keyboard controller. Decades later, that legacy is still part of the way x86 computers boot.

This is the story of how a small engineering shortcut became one of computing’s longest-lasting pieces of legacy technology.

#ComputerHistory #Intel #IBM #64Bit #x86 #TechnologyHistory

11 Comments

@mandolinic 13 days ago

What we need now is an AI20 gate that stops AI slop.

@bbbl67 13 days ago

44:15 Intel quietly let go of the X86S mode proposal two years later, as it would break too much compatibility which would need to be built back up through software emulation. Better to just keep the existing behaviour continue.

@robbietorkelsonn8509 4 days ago

all software in the MSDOS era was close to the metal

@bradmartisius2625 13 days ago

As AI slop goes, the info is really good. The voice of the narration is insufferable.

@bbbl67 13 days ago

I remember the "Big Real mode" or "UnReal mode" feature being mentioned in passing before. I remember thinking, that it could be used as a faster way of addressing large amounts of memory than the Protected Mode, because it wouldn't have to do the Protected Mode memory protection checks. But nothing came of it in the end.

@semicuriosity257 13 days ago

Intel no longer supports the A20 gate, starting with Haswell. Page 271 of the Intel System Programmers Manual Vol. 3A from June 2013 states: "The functionality of A20M# is used primarily by older operating systems and not used by modern operating systems. On newer Intel 64 processors, A20M# may be absent."

@profvonshredder2563 13 days ago

this was IBM’s “you’re holding it wrong” moment…nothing wrong with the AT pc

@nIghtorius 13 days ago

ffs, ppl seems to have discovered the a20 gate on the keyboard controller in order to make a wraparound on address above 1megabyte. Now we need to have like 5000 generic ai blabbering videos about it. And blowing it in a way worse situation that this actually is. Good for you you discovered this. Modern systems running uefi are booting already in a 64-bit state with the a20 gate disabled. If you published this video around the first x86-64. then this MIGHT be something.