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Why a kilobyte is 1000 and not 1024 bytes (zeta.one)

submitted 10 months ago by [email protected] to c/[email protected]

210 comments fedilink hide all child comments

I often find myself explaining the same things in real life and online, so I recently started writing technical blog posts.

This one is about why it was a mistake to call 1024 bytes a kilobyte. It's about a 20min read so thank you very much in advance if you find the time to read it.

Feedback is very much welcome. Thank you.

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[–] [email protected] 0 points 10 months ago* (last edited 10 months ago) (2 children)

"Kilo" means 1000 under the official International System of Units.

With some computer hardware, it's more convenient to use 1024 for a kilobyte and in the early days nobody really cared that it was slightly wrong. It has to do with the way memory is physically laid out in a memory chip.

These days, people do care and the correct term for 1024 is "Kibi" (kilo-binary). For example Kibibyte. There's also Gibi, Tebi, Exbi, etc.

It's mostly CPUs that use 1024 - and also RAM because it't tightly coupled to the CPU. The internet, hard drives, etc, usually use 1000 because they don't have any reason to use a weird numbering system.

[–] [email protected] 3 points 10 months ago* (last edited 10 months ago) (1 children)

As the article mentions, windows also uses KB/MB/GB to refer to powers of 2 when calculating disk space. AFAIK Linux somes does too, although the article says otherwise. Apparently OSX uses the KB=1000 definition.

It may be outdated, but it's still incredibly common for people to use KB/MB/GB to refer to powers of 2 in computing. Best not to assume KB is always 1000.

[–] [email protected] 1 points 10 months ago* (last edited 10 months ago)

Windows and Mac both use KB = 1000. With Linux I think it depends on the distro.

You're thinking of very old versions of Windows... old versions of MacOS were also 1024.

It's honestly irrelevant anyway - if you want to actually know how much space a file is using on disk, you should look up how many pages / sectors are being used.

A page (on an SSD) or sector (on a HDD) is 32768 bits on most modern drives. They can't store a file smaller than that and all of your files take up a multiple of that. A lot of modern filesystems quietly use zip compression though. Also they have snapshots and files that exist in multiple locations other shit going on which will really mess with your actual usage.

I'm not going to run du -h / on my laptop, because it'd take forever, but I'm pretty sure it would be a number significantly larger than my actual disk. Wouldn't surprise me if it's 10x the size of my disk. Macs do some particularly interesting stuff in the filesystem layer - to the point where it's hard to even figure out how much free space you have... my Home directory has 50 GB of available space on my laptop. Open the Desktop directory (which is in the Home directory...) and the file browser shows 1.9 TB of available space.

[–] [email protected] 1 points 10 months ago

Weird numbering system? Things are still stored in blocks of 8 bits at the end, it doesn't matter.

When it gets down to what matter on hard drives, every byte still uses 8 bits, and all other numbers for people actually working with computer science that matter are multiples of 8, not 10.

And because all internal systems use base 8, base 10 is "slower" (not that it matters any longer.