Linux by Neural Lantern

6/8/2025

Get a clear, fun breakdown of data sizes in x86-64 CPUs! This video covers bytes (8 bits), words (16 bits), D words (32 bits), and quad words (64 bits), plus how they fit into assembly and CPU registers. Perfect for programmers, students, or anyone curious about low-level computing. I?ll walk you through the basics with relatable examples, no jargon overload. Subscribe for more coding & tech videos, and scan the QR code to visit my site for extra tutorials! Leave a comment with your thoughts!

Introduction 00:00:00
Data Sizes Overview 00:00:01
Byte Definition 00:00:44
Bits to Bytes Conversion 00:01:27
Word Size Explanation 00:02:20
Double Word (D Word) 00:04:03
Quad Word Definition 00:04:44
CPU Registers and Memory 00:05:21
Conclusion and Outro 00:06:07
Call to Action 00:06:32

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Master Data Sizes in x86-64 CPUs: Bytes, Words, D Words, and Quad Words Explained

Neural Lantern

6/8/2025

Transcript

00:00Hello there.

00:02I'd like to talk to you about data sizes within the context of x86,

00:0764 CPUs,

00:15you know, just like 64 bit CPUs for assembly and just whatever.

00:20So what's going on inside of the machine and how do we describe the sizes of

00:24various, you know, clumps of data. So for starters, you know,

00:27like inside of your computer,

00:28there's just a bunch of ones and zeros represented in all the wires and all

00:33the circuitry. Uh, the human beings with feelings decide, Oh, you know,

00:37I'm going to take a group of this many bits, this many ones and zeros.

00:41And I'm going to say that it is something else.

00:43I'm going to call it an abstract unit.

00:45So the first thing that we do is we have something called a byte that just

00:50means eight bits. So we can say one byte equals eight bits.

00:57I don't know why I hit the shift key there.

00:59That's probably going to mess up my annotator.

01:02Nope. Okay.

01:04So what is eight bits? Just as a little reminder, you know,

01:06we'll just say one, two, three, four, five, six, seven, eight,

01:09or it could be, you know, something more random looking one, two, three,

01:12four, five, six, seven, eight, you know,

01:14so like inside of the machine we have eight ones and zeros or eight bits.

01:21And when we group them together, we'll just call that a byte. Okay.

01:25No problem. So that means, by the way, if somebody says, Oh,

01:28my internet connection is this many megabits a second.

01:31If you want to know how many bytes a second, try dividing by eight.

01:35Or if somebody says,

01:35I have this many megabytes a second of transfer speed,

01:39try a dividing that by eight, you know,

01:41so multiply or divide by eight to get the right number. Okay.

01:45So if we had two bytes together,

01:49that's just going to be 16 bits. No problem there.

01:52I'm just going to copy paste these bits and you don't have to do this.

01:55But for me personally,

01:56I like to put a little space in between byte groupings just to help my human

02:01brain kind of see, Oh,

02:02I'm looking at two bytes right here when I'm referring to different data sizes.

02:06So then eventually we kind of need an idea of,

02:10um, you know, let's, let's come up with a different word, um,

02:16that means more than one bite. And, um, uh, I guess, ironically,

02:20based on what I just said, the, the term we're going to use is word, a word, uh,

02:25the, the size of a word just kind of depends on the system that you're on,

02:28on my system, uh, the size of a word is two bytes.

02:32So I'm going to say one word equals two bytes.

02:37Uh, and you can imagine that's a 16 bits.

02:41Maybe I should start doing the double equal sign here just to be more computer

02:45like more hackery. Okay. So equals two bytes.

02:49It also equals 16 bits.

02:52And you can imagine, uh,

02:54it's basically the same thing that we just wrote up, up above.

02:57So I'm going to erase this actually 16. Oh, I put an E there. Embarrassing.

03:05Okay. So one word is two bytes. Again, the size of a word, uh,

03:08depends on your system. If you're not sure, you should probably look it up.

03:10What's the word size on my system. But again,

03:13the typical system here in 2024 on x86,

03:1664 machines running Ubuntu, um,

03:19or just like Linux is going to be like, you know, two bytes.

03:24Okay. So now, uh, we have like words that we can do.

03:27So if I had, uh, let's say I had two words, uh,

03:31two words is going to be double of what we just wrote, right?

03:33So that means we're going to have this many, whoops, I blew it.

03:37We're going to have this many bits. So two word,

03:40two words is going to be equal to, um, 32 bits.

03:43Uh, and it's also going to be equal to four bytes because you,

03:49you know, we multiplied everything by two. So this is like four bytes.

03:57So now we need another term to represent this two words,

04:01which is four bytes, which is 32 bits. We can also call that a D word.

04:06Actually, I'll say D word like that.

04:08And what does D word mean?

04:10It just means something that is twice the size of the word, uh,

04:15length on your system.

04:16So like if we decided our, our word length is a two bytes,

04:19then a D word is four bytes and that's it.

04:23Four bytes is 32 bits.

04:24You might notice this is similar to like old school CPUs that ran in 32

04:28bits and maxed out their RAM at about four gigabytes.

04:32So let's, um, let's come up with one more term here. Let's, uh,

04:38let's come up with the term quad word. Actually,

04:40we're not coming up with the term. We're just using it.

04:43What is a quad word? A quad word is, uh, two D words.

04:49So we're just kind of doubling it again. I forgot that extra equal sign there.

04:53Quad word is two D words, which means it's also, um, uh,

05:01four words.

05:03And it also means it's eight bytes.

05:06And it also means that it is 64 bits.

05:11And then I guess if I want to represent this to you,

05:14I can just kind of like copy paste that twice.

05:17And this is the size of the memory address space, uh,

05:20that we have on modern CPUs. This is also the,

05:23the size of your general purpose registers in modern 64 bit CPUs.

05:27So if somebody says, Hey, I got a quad word,

05:32I want you to store it somewhere. You can instantly think to yourself,

05:35well, obviously I could put it in memory.

05:37You could also just put it directly into a CPU register and that's it.

05:41This also means that, um,

05:43you could put two D words into one CPU register.

05:47If you were very clever, you could also do the same thing. You know,

05:51you could put like, uh, uh, four words inside of one CPU register.

05:56Cause the, you know, the CPU registers, they're all,

05:58they're always the same size, no matter how you use them.

06:01They're kind of hardwired.

06:04So, uh, I hope this has been, you know, kind of helpful.

06:07These are the basics of different data type sizes that we have in CPUs and

06:13assembly language. I'll see you in the next video.

06:19Hey everybody.

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Master Data Sizes in x86-64 CPUs: Bytes, Words, D Words, and Quad Words Explained

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