Hey if you are seeing this, let me tell you, this is the best video in UA-cam if you want to understand the concept of virtual memory, and others in simple and sweet. Perfect! ❤ Edit: Coming back after long time. Writing a blog, and really needed an low level and mostly complete explanation of virtual memory. Also, following windows internals part-1 book. This video is still gold.
Fantastic video. I'm taking a OS course at university and we have a virtual memory assignment. The hand out documents left me feeling confused and a bit lost, but then this 20-minute video made all concepts clear as day. Thank you for putting effort into making these videoes, you earned a new subscriber!
Start with the problem to understand the solution !!!!! Again and again I’ve seen explanation that detail the concepts without starting with the why. If you don’t understand the problem you’ll never truly get the solution. If you can’t put your finger on the problem… that’s the reason you’re having a hard time understanding. Don’t try to skip this step. Excellent job! Amazing video.
Thanks a lot for taking the time to comment. It means a lot to hear from people that it was worth it! Exactly. I always like to start with the problem first. It makes everything much easier
I had a course back when i was in engineering school, the professor did a horrible job explaining memory and virtual memory, I ended up hating the class and i never really understood how it worked. until this day when I watched your video, you did an excellent job, I wish youtube had something like this before. Fast forward today, I work as a sysadmin/infra engineer and i really didn't have an idea how virtual memory works, again until I watched your video, thank you for the effort you put here, I am sure many will watch it and gain valuable informations.
This might be the best content I have found on UA-cam this year. You explained these concepts in ways my teachers couldn’t, and you did it for free too ! You have my respect , and subscription haha !
There's a quite good reason 4 KB is used as a standard page size. In most disk partitioning software the minimum data you can read/write to is 4KB, so it can move it in one swoop
Thanks. I didn’t know that. Do you have any material or references that provide motivation for 4KB page size? I read some articles that suggest it was chosen empirically long time ago, but I’d like to learn more about it.
This is simply untrue if you grew up during the 80’s or 90’s (or, shoot, even later). Research something called a “filesystem”. Also while you’re at it, check out an article on “Solid State Drives” or “Flash Memory” or “NVRAM” if you’re wondering why 4K is the block size. 😂😂😂
Found your channel from UA-cam recommendation. Nice work. Dude you have a huge overlap of interests as me. I was a math major and now a SW engineer for over 10 yrs, play chess, plays basketball, plays games (not sure what game you play but I mostly play an old game called StarCraft 2). I recently started to play soccer as well.
@@TechWithNikola nice! I always start the game with a cannon rush. Currently stuck in M2 for two years because of my mediocre apm. Considering how much time I spend on the game, I probably need a real StarCraft coach to break my current bottleneck. My two most watched genre of videos on UA-cam are C++ stuff and StarCraft 2 games. Somehow they just feel comforting. I also feel that StarCraft 2 is like C++ and StarCraft 1 is like C programming. In SC 1 everything is so manual.
@@ShaunYChengvery cool. I got stuck with sc2 when I had to keep up with all the build changes between seasons. I haven’t player in 5-6 years. I watch Rust videos lately, but I’ve used C++ the most throughout mu career :-)
I need this resource for my OS assignment about virtual memory this week. So glad I find this video. This helps me understand the concept of virtual memory in 20 mins more than I have ever understand from the 20 chapters we have been task to read for 4 weeks now. LOL. Thank you for this.
Thank you for this amazing video! It's truly refreshing to encounter such well-scripted and nicely illustrated content. It makes really enjoyable trying to learn such complex concepts.
Wow, I was really surprised! I was just relaxing on UA-cam to fall asleep and came across your video. I thought it might be trash, but it turned out to be very good with excellent detail and explanation. Great job! Best video on youtube so fare 🎉🎉🎉
Рік тому+8
An excellent, succint and illustrative explanation :)
Good explanation and yes 4kb is the optimal size to move , its similar like why the size of a page node of a BTree in a database is of 1024 because its optimal for I/O. But I also would like to see the explanation of exactly what does MMU do here.
Im just sitting here fascinated...and it makes sense.. knowing just the basics... nice work...a dumb thing to say but this is relaxing...straight to the point ..ask a question ...then answer it... terrific
Such a Beautiful Explanation. Thanks a lot Man. This is the Best explanation, it removed all the confusions I had for the last 6 years on this OS chapter
Great video ! I am preparing for my master degree and found this gold in UA-cam. I believe I can ace my upcoming OS class. I have liked your video and subscribed to your channel. Thanks a lot for the video. Greatly appreciate it.
Thanks a lot for taking the time to comment, as well as like and sub. It means a lot to me. I’m glad that the video was helpful. Good luck with acing your classes!
At 8:12, can someone explain how does "one entry for every word" differ from "one entry for every virtual address" ? Are they the same? What does 2^32 addresses per each byte = 2^30 words mean? Edit on Sun Oct 27 01:22:37 PM +0530 2024: After some thinking and search, self understanding is, What's meant by "2^32 addresses per each byte" is each memory address holds 1 byte of actual data and, a word is 4 bytes (32 bits), So the CPU actually access 4 bytes of data at a time, therefore 4 memory locations at a time. So, 2^32 addresses (each addressing one byte / byte addressable) means 2^30 words (each addressing 4 bytes / word addressable). Therefore 2^30 entries not 2^32 entries because 1 entry per 4 addresses. ((2^32)/4)=2^30.
Thanks! You explained this better than any professor did when I was still in university. Much appreciated, will be sending this video to people who ask me about this 😅
This is a very good, concise and clear explanation, please keep the work's up, I really enjoyed it : ) PS: can you make a video about how compression algorithms work?
Glad to hear that, and you thank you for taking the time to comment. :-) Yeah, some compression algorithms are in my queue, but it will take a while before I get some free time to make them. Stay tuned!
Hey, that is a great explanation. I'd like to congratulate you on your work. Keep up the good work. Now I understand how computer memory works. I am trying to understand how the database works on saving the pages. How does the computer with 8GB ram can sort 10GB of data.
Great video. So you mention that physical address is the "memory RAM". You also mention the "page tables" are store in the "memory RAM" and how there are difference level of page tables and some can be store in disk. Is virtual memory also store in the "memory RAM"?. I know I have to rewatch this video and keep studying to fully understand and that I have gaps in my knowledge. So in a high level the OS has to do a mapping between virtual memory and page tables. And then between page tables and physical address? Thanks.
Hi, apologies for the late response. So virtual memory on its own doesn’t exist really. It’s just a number, for example from 0 to 2^32-1. As such, it is not stored anywhere. It only becomes interesting when we talk about translating that number to its physical counterpart. This translation is stored in page tables, and page tables are stored in RAM. It you rewatch the video you will see the exact details on how the translation works, but in a nutshell the first part of the address is remapped via page tables, and the last 10 bits are copied (called an offset). The mapping is actually happening in the MMU (memory management unit) which you can think of as a small chip on the CPU. You are right that OS plays an important role, and specifically, OS programs the memory management unit and specifies how to do the mapping. Does that make sense?
Thank you for providing such valuable content. The examples you shared have greatly enhanced my understanding of the working principles of virtual memory.
I don’t know to laugh, cry or to be mad. I had heard about the book What Makes it Page in this subject written by an unknown software engineer from Italy. Due to one of the cybersecurity researchers’ recommendation, I always wanted to learn this book. So, about more or less 3-4 days ago, I gave it a shot. After a couple of days into it, I can assert that the text could be written more cohesively and simply; however, the journey I started is kind of amazing. The harder it gets, the slower you get reading the text, the more enjoyable it becomes. I’m writing this not to say only that but about a dumb-looking guy who were reading with me. We spent HOURS to learn a chapter and now when I looked at him found out that he didn’t learn a piece of shirt!
Everything was nice thanks for your effort.But in multi paging,the first page table contains 1024 entries which maps to 1024 second page tables ,The address range for 1024 entries is 0x000 to 0x3FF
Great video. But I would like an explanation regarding the mapping of the physical translated hex value into binary - the hex to binary conversion doesn’t seem correct, even assuming a left or right 0 padding to fill in the extra 17th and 18th bit. Could you explain this please, thanks
Hi, there is a mistake in animation in one part. I don't know if that's what you are referring to? When the offset is kept, I accidentally used wrong values for the final result.
Correction - At 10:50, page size is 4kb(kilobytes) then last 15 bits(not 12 as mentioned in the video) of the both the Virtual address and the Physical Address should be the same. Otherwise, if you mean to be 12 then in the video you should say kilobits instead of kilobytes.
Finally ... a proper explanation! I say this because countless self-made experts get it wrong. You see, Microsoft seems to have invented the misuse of the term "virtual memory" around the time of Windows 95. They used the term VM to describe what is really demand paging. Microsoft publications at the time did NOT make this mistake. (See Jeff Richter's excellent book describing the internals of Windows NT, published at the time.) But in the settings UI ... I guess "virtual memory" sounded more appealing to Microsoft than "demand paging". So they misused the term ... and misled generations of self-made CS people who subsequently grew up unaware of the misuse. They often claim that VM size can be adjusted. (It cannot be.) They believe that the disk swap/paging space is virtual memory. (It is not.) You see, virtual memory was created as a MAPPING PROCEDURE that solved the long-standing problem of mapping a program binary into the run-time address space of a computer. It was preceded by address space paging (remember the early 80x86 paging registers?). This was present in minicomputers as far back as the late 1960s. Paged memory was, in turn, preceded by run-time relocation ... a tedious run-time editing of a binary to "patch" all of the code addresses and data addresses within a program binary before execution could begin. VM is superior to both of those methods. Meanwhile some systems implemented something called swapping ... a predecessor of demand paging. When swapping, a program's ENTIRE memory consumption is swapped to disk to make way for another program. This mechanism allows the system to get around memory size limitations ... but it performs a lot of disk swapping! Demand paging goes one better by swapping only small blocks of memory. And even better, it loads only portions of a program when needed. No need to load an entire program when most of that program code won't be executed any time soon. It's all quite efficient. Virtual memory is not demand paging, and demand paging is not virtual memory. And you don't have to use VM with demand paging. And you don't have to use demand paging with VM. But the two appeared at a similar time in computing history ... and they work hand in hand to improve performance. So you always see both of them together. This video correctly presents a unified view of demand paged/virtual memory. Nice work!
Thanks a lot for taking to time to write such a good comment. I agree, and I’m also very happy to hear that this video met your expectations. Apologies for the late reply.
Hi! at 11:37 why are the last 12 bits in the physical address not the same as the virtual address (0x678)? edit: see answer that this is bug in the animation. Thanks again for the great video!
great video. is it posible that there is a mistake in 11:39 when the animation copies the offset bits from the virtual adress to the physical adress? they arent the same first 12 bits
I don't get the Security problem, isn't one of the most important services of an Operating System is to not allow that scenario to even occur? Like Segmentation Faults(if I understand correctly...?).
I've written some 6502 assembler for the BBC Master, where the most complicated it got was having to write to a memory-mapped hardware register to switch banks of RAM so that you can access 128Kb with a 16bit address bus. It's a bit mind-boggling to think about all this indirection that goes on in modern memory systems. All this back and forth would bring 8bit systems to their knees!
Hey Nikola I really liked your video! I have a question though - how does a program know which virtual address to access? For example, how did the program know in your video to access address 0x12345678? You said a virtual page is assigned to each program, so how does it know what’s the virtual page that was assigned to it?
Hey, glad to hear that! It’s up to the program to decide which virtual address to access. For example, the program may use address 0x12345678, as shown in the video. However, when we write code, we usually don’t directly specify these addresses. Instead, we write variable names like int a and assign values to them. The compiler then generates machine code instructions that reference virtual memory addresses. These addresses are determined by the compiler and the operating system during the program’s execution. Regarding how virtual pages are determined: The virtual page number (VPN) can be computed by dividing the virtual address by the page size (or equivalently, taking the upper bits of the address after considering the page size). For example, if the virtual address is 0x12345678 and the page size is 4 KB, the VPN corresponds to the upper 20 bits of the virtual address. The operating system manages the mapping of these virtual pages to physical memory. Does that make sense?
@@TechWithNikolahmm yeah I guess it does! So if I understood correctly, when I compile my code to machine language, the operating system itself translates variables that are assigned in the program to virtual addresses? And also, is the MMU also responsible for dynamic memory allocation? Thanks a lot!
It's the compiler that assigns stack and global variables to virtual addresses. This is the stack space in a program. The OS and MMU translate virtual addresses to physical addresses. Regarding dynamic memory allocation. It depends what you mean by "responsible". The program asks OS to allocate some memory (e.g. via malloc function), and OS decides which virtual address to assign to it. The MMU itself doesn't allocate any memory, it just performs translations when those addresses are accessed.
What about the problems virtual memory creates ? 1) memory non locality (slower bandwidth and sequential reads) 2) higher power draw - used memory is scattered all over the physical space, thus chips cannot be properly turned off when not in use. 3) translation overhead - mainly when entries are not in local translation cache and have to be fetched from main memory
I assume for 64-bit memory, it doesnt store page tables for the full address space, since that would be absurdly huge. I assume it just grows with memory requested and shrinks with frees. What I don’t understand there though is how it would shrink if you didn’t free the buffer at the end of your memory.
Starting off with the “bad old days” only being 4GB definitely lets me know I’m old! Just like running out of 32bit address space “must” have been back in the 1950s or something 😂
Hey if you are seeing this, let me tell you, this is the best video in UA-cam if you want to understand the concept of virtual memory, and others in simple and sweet. Perfect! ❤
Edit: Coming back after long time. Writing a blog, and really needed an low level and mostly complete explanation of virtual memory. Also, following windows internals part-1 book. This video is still gold.
Thank you! :)
Not really. He forget that OS claims RAM too
@@siegfriedbarfuss9379 3:54
@@siegfriedbarfuss9379 Why is that relevant to this demonstration? You could just generalise an OS as another program? It's also shown at 3:58...
Fantastic video. I'm taking a OS course at university and we have a virtual memory assignment. The hand out documents left me feeling confused and a bit lost, but then this 20-minute video made all concepts clear as day. Thank you for putting effort into making these videoes, you earned a new subscriber!
I’m glad it was helpful. You’re welcome, and thank you for taking the time to comment.
Thank you for the video. The explanation was clear, structured and concise, better than most other explanations on youtube.
Thanks a lot for the kind words and for taking the time to leave the comment. I'm so happy to hear that you've liked the video!
Start with the problem to understand the solution !!!!!
Again and again I’ve seen explanation that detail the concepts without starting with the why. If you don’t understand the problem you’ll never truly get the solution. If you can’t put your finger on the problem… that’s the reason you’re having a hard time understanding. Don’t try to skip this step.
Excellent job! Amazing video.
Thanks a lot for taking the time to comment. It means a lot to hear from people that it was worth it!
Exactly. I always like to start with the problem first. It makes everything much easier
because the people who are teaching you that way themselves have only memorised the concept.
Holy crap. UA-cam algorithm finally came through. This is some amazing content.
Thank you :-)
Diet
It sure did
yooo W disco elysium pfp
This is amazing! More detail than most Operating System courses in 20 minutes. Awestruck!
Thanks. Glad you think so! I’ll try to make more videos on similar topics in the future.
I had a course back when i was in engineering school, the professor did a horrible job explaining memory and virtual memory, I ended up hating the class and i never really understood how it worked. until this day when I watched your video, you did an excellent job, I wish youtube had something like this before. Fast forward today, I work as a sysadmin/infra engineer and i really didn't have an idea how virtual memory works, again until I watched your video, thank you for the effort you put here, I am sure many will watch it and gain valuable informations.
Thank you for the comment. I’m glad you’ve found it useful!
This might be the best content I have found on UA-cam this year. You explained these concepts in ways my teachers couldn’t, and you did it for free too ! You have my respect , and subscription haha !
I’m so glad that you like my content. It means a lot to me when people leave comments and subscribe, so thank you.
Videos like these are the only reason I cannot stop using UA-cam. You are such a great explainer, dude!
Thanks a lot! It means a lot to me when I receive such comments.
Amazing explanation, always wanted to know core knowledge. Thanks, keep up with these videos!
Thank you Dusan. I'm glad you've enjoyed it :)
There's something about the way you articulate concepts and break things down that just kept me hooked throughout. You deserve way more subs man!
There's a quite good reason 4 KB is used as a standard page size. In most disk partitioning software the minimum data you can read/write to is 4KB, so it can move it in one swoop
yeah i figured this was the reason and was wondering why he didn't say so
Thanks. I didn’t know that. Do you have any material or references that provide motivation for 4KB page size? I read some articles that suggest it was chosen empirically long time ago, but I’d like to learn more about it.
This is simply untrue if you grew up during the 80’s or 90’s (or, shoot, even later). Research something called a “filesystem”. Also while you’re at it, check out an article on “Solid State Drives” or “Flash Memory” or “NVRAM” if you’re wondering why 4K is the block size. 😂😂😂
@@TechWithNikola They probably choose it because 0x1000 is pretty nice to type. ARM macs use 16kb pages I think.
It has nothing to do with disc sector/cluster size, like at all.
And yeah, 64 bit osses use 32k pages.
This is the single greatest explanation of virtual memory I've ever seen, thank you so much.
Found your channel from UA-cam recommendation.
Nice work.
Dude you have a huge overlap of interests as me. I was a math major and now a SW engineer for over 10 yrs, play chess, plays basketball, plays games (not sure what game you play but I mostly play an old game called StarCraft 2).
I recently started to play soccer as well.
Hey dude, nice to hear that. I’ve played SC2 a looot as well 😀 got to diamond 1 but never broke to masters…
@@TechWithNikola nice!
I always start the game with a cannon rush. Currently stuck in M2 for two years because of my mediocre apm. Considering how much time I spend on the game, I probably need a real StarCraft coach to break my current bottleneck.
My two most watched genre of videos on UA-cam are C++ stuff and StarCraft 2 games. Somehow they just feel comforting.
I also feel that StarCraft 2 is like C++ and StarCraft 1 is like C programming. In SC 1 everything is so manual.
@@ShaunYChengvery cool. I got stuck with sc2 when I had to keep up with all the build changes between seasons. I haven’t player in 5-6 years.
I watch Rust videos lately, but I’ve used C++ the most throughout mu career :-)
That's a very good, high quality production with a top notch explanation! Keep making more.
Thanks :)
I wanna thank the youtube algorithm for recommending this video. This is absolute gem for explaining the Virtual Memory concept.
Thank you!
Svaka cast burazeru. Ovo sam na faksu radio, ali si dosta bolje objasnio.
Hvala :) Drago mi je sto cujem da je dobro objasnjeno.
I'm amazed at how someone can explain things with such clarity, just like you did.
You just earned a subscriber. Thank you so much.
Brat objasnio u 20 minuta content sa tri predavanja System Level Programiranja na faksu. Svaka cast!
I need this resource for my OS assignment about virtual memory this week. So glad I find this video. This helps me understand the concept of virtual memory in 20 mins more than I have ever understand from the 20 chapters we have been task to read for 4 weeks now. LOL. Thank you for this.
Wow! Clear, concise and very neat animations! Subscribing to a channel after a long time!
Thank you for the sub!
the graphics really helped me visualize how this works! youre a lifesaver ^^
I'm glad! Graphics are a great tool for explain IMO :-)
@@TechWithNikola It's not just your opinion. It's a FACT! Graphics for complicated ideas are a must!
And thank you, your video was amazing!
Thank you for this amazing video! It's truly refreshing to encounter such well-scripted and nicely illustrated content. It makes really enjoyable trying to learn such complex concepts.
At 10:00 the page 2 in both virtual memory and physical memory is 8kb instead of 4kb. This is the great explanation video right now!
Great video, brilliantly explaining the concept in details ! Thank you so much!
please never stop, really good video
Thank you! I'm glad you've enjoyed it. I'll keep making them for sure :)
Wow, I was really surprised! I was just relaxing on UA-cam to fall asleep and came across your video. I thought it might be trash, but it turned out to be very good with excellent detail and explanation. Great job!
Best video on youtube so fare 🎉🎉🎉
An excellent, succint and illustrative explanation :)
Thanks a lot Andrej!
Best explanation of virtual memory ever seen. Thanks for putting this together, you are awesome.
This is the BEST explanation i have had about this topic. I am a newbie to such concepts and you have explained in such an elegant way. Liked n subbed
Brilliant videos with great explanations. You’re a natural when it comes to teaching
This is amazing. I am slowly realizing I am a visual learner. It was always hard to grasp content when it just a text. Worth a Sub... :)
Thank you :)
Thank you very much! I finally understand how virtual memory works. If only university professors could be structured like this...
The variety of content provided is much appreciated.
Excellent description on how cpu/memory/os are working together! Well done!
Thanks for the content. You really have a talent, keep this going.
the second part is a bit more complex but I love the visuals + explanation nonetheless. thanks for the effort put into making this one!
the best video to understand virtual memory! thanks a lot
wow!! never felt like a computer science before. learned a lot on this clip. keep them coming
This video is really good, the way that you simplified Virtual Memory is amazing!
Glad it was helpful!
Ahh…the swap file in Linux!!! I used to allocate a portion of my hard drive for swapping!! Wow!!! Such a refresher !!
Good explanation and yes 4kb is the optimal size to move , its similar like why the size of a page node of a BTree in a database is of 1024 because its optimal for I/O.
But I also would like to see the explanation of exactly what does MMU do here.
Im just sitting here fascinated...and it makes sense.. knowing just the basics... nice work...a dumb thing to say but this is relaxing...straight to the point ..ask a question ...then answer it... terrific
Thanks. I’m glad you like this style.
Thank you for this video, this is really helpful. Very grateful.
Such a Beautiful Explanation. Thanks a lot Man. This is the Best explanation, it removed all the confusions I had for the last 6 years on this OS chapter
Great video ! I am preparing for my master degree and found this gold in UA-cam. I believe I can ace my upcoming OS class.
I have liked your video and subscribed to your channel.
Thanks a lot for the video. Greatly appreciate it.
Thanks a lot for taking the time to comment, as well as like and sub. It means a lot to me.
I’m glad that the video was helpful. Good luck with acing your classes!
At 8:12, can someone explain how does "one entry for every word" differ from "one entry for every virtual address" ? Are they the same?
What does 2^32 addresses per each byte = 2^30 words mean?
Edit on Sun Oct 27 01:22:37 PM +0530 2024: After some thinking and search, self understanding is,
What's meant by "2^32 addresses per each byte" is each memory address holds 1 byte of actual data and, a word is 4 bytes (32 bits), So the CPU actually access 4 bytes of data at a time, therefore 4 memory locations at a time. So, 2^32 addresses (each addressing one byte / byte addressable) means 2^30 words (each addressing 4 bytes / word addressable). Therefore 2^30 entries not 2^32 entries because 1 entry per 4 addresses. ((2^32)/4)=2^30.
Thank you so much, sir. I was also struggling to wrap my head around where 2^30 came from.
This is awesome, made things so clear to me. Thank you.
You're welcome. Glad it helped!
thank you so much for this video! I'm learning about hugepages right now, and i wanted a recap on how normal pages work.
I have to words! That video is amazing! You won yourself a new subscriber, my friend! ❤
Welcome aboard! I’m glad you’ve liked it, and thank you for taking the time to comment.
@@TechWithNikola My pleasure! Have a lovely day!
Thanks! You explained this better than any professor did when I was still in university. Much appreciated, will be sending this video to people who ask me about this 😅
Glad to hear that you’ve found it useful, and thank you for sharing the video :-)
What an excellent overview. Nice job. It clarified a few things i was unclear about.
Thanks a lot. Glad it clarified things :-)
you just earned a subscriber within a few minutes of this video!!
Wonderful illustration of Virtual Memory. Good Job !
Thanks
Thank you very much for this video, it was very detailed and very well explained!
Fantastic explanation expecting more videos from you🎉
This is a very good, concise and clear explanation, please keep the work's up, I really enjoyed it : )
PS: can you make a video about how compression algorithms work?
Glad to hear that, and you thank you for taking the time to comment. :-)
Yeah, some compression algorithms are in my queue, but it will take a while before I get some free time to make them. Stay tuned!
@@TechWithNikola I'll be waiting 4 sure :3
Wooow this video is awesome! Great work on the animations and examples 🤝😎
Thank you so much! I'm glad to hear that you've liked it.
Very time I watch it, I learn something new, many thanks!
Dude, you're videos are so helpful, thanks so much!!!
Glad you like them! Thanks for taking the time to comment.
Hey, that is a great explanation. I'd like to congratulate you on your work. Keep up the good work. Now I understand how computer memory works. I am trying to understand how the database works on saving the pages. How does the computer with 8GB ram can sort 10GB of data.
Great video. So you mention that physical address is the "memory RAM". You also mention the "page tables" are store in the "memory RAM" and how there are difference level of page tables and some can be store in disk. Is virtual memory also store in the "memory RAM"?. I know I have to rewatch this video and keep studying to fully understand and that I have gaps in my knowledge. So in a high level the OS has to do a mapping between virtual memory and page tables. And then between page tables and physical address? Thanks.
Hi, apologies for the late response.
So virtual memory on its own doesn’t exist really. It’s just a number, for example from 0 to 2^32-1. As such, it is not stored anywhere.
It only becomes interesting when we talk about translating that number to its physical counterpart. This translation is stored in page tables, and page tables are stored in RAM. It you rewatch the video you will see the exact details on how the translation works, but in a nutshell the first part of the address is remapped via page tables, and the last 10 bits are copied (called an offset).
The mapping is actually happening in the MMU (memory management unit) which you can think of as a small chip on the CPU. You are right that OS plays an important role, and specifically, OS programs the memory management unit and specifies how to do the mapping.
Does that make sense?
Yes, thanks for the reply
watched the first video in this channel, subbed!
Thank you for the sub. I'm so glad to hear that you've liked it!
From Aritmetic concepts , the virtual memóry obeys bijective function (injective and surjective ) about Domain and Image .
It is unbelievable !
absolute banger of a tech channel
thanks, will watch again
-IGN
Thanks a lot about this simple explanation
11:38 I’m wondering when you copy the last 12bits to the physical memory, why the number changes from 0110 0111 1000 to 010110011110
Oh that’s a good catch! It’s a bug in the animation. Sorry about that. It should remain unchanged.
@@TechWithNikola Thank you very much! Your videos are really helpful!!!
Thank you for providing such valuable content. The examples you shared have greatly enhanced my understanding of the working principles of virtual memory.
I’m very happy to hear that. Thank you for taking the time to comment.
Thank you for your high quality work. 👏👏👏
Very clear! It helps a lot, thank you very much!😀
You’re welcome! :-)
I don’t know to laugh, cry or to be mad. I had heard about the book What Makes it Page in this subject written by an unknown software engineer from Italy. Due to one of the cybersecurity researchers’ recommendation, I always wanted to learn this book. So, about more or less 3-4 days ago, I gave it a shot. After a couple of days into it, I can assert that the text could be written more cohesively and simply; however, the journey I started is kind of amazing. The harder it gets, the slower you get reading the text, the more enjoyable it becomes. I’m writing this not to say only that but about a dumb-looking guy who were reading with me. We spent HOURS to learn a chapter and now when I looked at him found out that he didn’t learn a piece of shirt!
Your explanations are amazing
Thank you, I hope to see more
Thank you. Yeah, definitely, I’ll try to find some time to make more videos soon
Everything was nice thanks for your effort.But in multi paging,the first page table contains 1024 entries which maps to 1024 second page tables ,The address range for 1024 entries is 0x000 to 0x3FF
Thus is the best video on internet!
Thank god I found this gem before my exam!
this is gold, thank you so much
Wow, good video. Good job and thanks for sharing.
Thank you! You’re welcome.
Great video. But I would like an explanation regarding the mapping of the physical translated hex value into binary - the hex to binary conversion doesn’t seem correct, even assuming a left or right 0 padding to fill in the extra 17th and 18th bit. Could you explain this please, thanks
Hi, there is a mistake in animation in one part. I don't know if that's what you are referring to? When the offset is kept, I accidentally used wrong values for the final result.
that music player & video game example is hilarious to think about
Correction - At 10:50, page size is 4kb(kilobytes) then last 15 bits(not 12 as mentioned in the video) of the both the Virtual address and the Physical Address should be the same. Otherwise, if you mean to be 12 then in the video you should say kilobits instead of kilobytes.
Finally ... a proper explanation!
I say this because countless self-made experts get it wrong. You see, Microsoft seems to have invented the misuse of the term "virtual memory" around the time of Windows 95. They used the term VM to describe what is really demand paging. Microsoft publications at the time did NOT make this mistake. (See Jeff Richter's excellent book describing the internals of Windows NT, published at the time.)
But in the settings UI ... I guess "virtual memory" sounded more appealing to Microsoft than "demand paging". So they misused the term ... and misled generations of self-made CS people who subsequently grew up unaware of the misuse.
They often claim that VM size can be adjusted. (It cannot be.)
They believe that the disk swap/paging space is virtual memory. (It is not.)
You see, virtual memory was created as a MAPPING PROCEDURE that solved the long-standing problem of mapping a program binary into the run-time address space of a computer.
It was preceded by address space paging (remember the early 80x86 paging registers?). This was present in minicomputers as far back as the late 1960s.
Paged memory was, in turn, preceded by run-time relocation ... a tedious run-time editing of a binary to "patch" all of the code addresses and data addresses within a program binary before execution could begin.
VM is superior to both of those methods.
Meanwhile some systems implemented something called swapping ... a predecessor of demand paging. When swapping, a program's ENTIRE memory consumption is swapped to disk to make way for another program. This mechanism allows the system to get around memory size limitations ... but it performs a lot of disk swapping!
Demand paging goes one better by swapping only small blocks of memory. And even better, it loads only portions of a program when needed. No need to load an entire program when most of that program code won't be executed any time soon. It's all quite efficient.
Virtual memory is not demand paging, and demand paging is not virtual memory.
And you don't have to use VM with demand paging. And you don't have to use demand paging with VM.
But the two appeared at a similar time in computing history ... and they work hand in hand to improve performance.
So you always see both of them together.
This video correctly presents a unified view of demand paged/virtual memory. Nice work!
Thanks a lot for taking to time to write such a good comment. I agree, and I’m also very happy to hear that this video met your expectations.
Apologies for the late reply.
Hi! at 11:37 why are the last 12 bits in the physical address not the same as the virtual address (0x678)?
edit: see answer that this is bug in the animation. Thanks again for the great video!
great video. is it posible that there is a mistake in 11:39 when the animation copies the offset bits from the virtual adress to the physical adress? they arent the same first 12 bits
Thanks :) Yeah, there is a mistake in the animation. Sorry about that.
bro you are amazing, love your energy
Thanks. I appreciate that!
Thank you for this videos , it is reali amazing❤
Amazing video, wish I saw it while still in college 😊
I don't get the Security problem, isn't one of the most important services of an Operating System is to not allow that scenario to even occur?
Like Segmentation Faults(if I understand correctly...?).
I've written some 6502 assembler for the BBC Master, where the most complicated it got was having to write to a memory-mapped hardware register to switch banks of RAM so that you can access 128Kb with a 16bit address bus. It's a bit mind-boggling to think about all this indirection that goes on in modern memory systems. All this back and forth would bring 8bit systems to their knees!
clear as water, thanks!
Great to hear that :) Thank you!
Amazing explanation
Hey Nikola I really liked your video! I have a question though - how does a program know which virtual address to access? For example, how did the program know in your video to access address 0x12345678? You said a virtual page is assigned to each program, so how does it know what’s the virtual page that was assigned to it?
Hey, glad to hear that!
It’s up to the program to decide which virtual address to access. For example, the program may use address 0x12345678, as shown in the video. However, when we write code, we usually don’t directly specify these addresses. Instead, we write variable names like int a and assign values to them. The compiler then generates machine code instructions that reference virtual memory addresses. These addresses are determined by the compiler and the operating system during the program’s execution.
Regarding how virtual pages are determined: The virtual page number (VPN) can be computed by dividing the virtual address by the page size (or equivalently, taking the upper bits of the address after considering the page size). For example, if the virtual address is 0x12345678 and the page size is 4 KB, the VPN corresponds to the upper 20 bits of the virtual address. The operating system manages the mapping of these virtual pages to physical memory.
Does that make sense?
@@TechWithNikolahmm yeah I guess it does! So if I understood correctly, when I compile my code to machine language, the operating system itself translates variables that are assigned in the program to virtual addresses? And also, is the MMU also responsible for dynamic memory allocation? Thanks a lot!
It's the compiler that assigns stack and global variables to virtual addresses. This is the stack space in a program.
The OS and MMU translate virtual addresses to physical addresses.
Regarding dynamic memory allocation. It depends what you mean by "responsible". The program asks OS to allocate some memory (e.g. via malloc function), and OS decides which virtual address to assign to it. The MMU itself doesn't allocate any memory, it just performs translations when those addresses are accessed.
What about the problems virtual memory creates ?
1) memory non locality (slower bandwidth and sequential reads)
2) higher power draw - used memory is scattered all over the physical space, thus chips cannot be properly turned off when not in use.
3) translation overhead - mainly when entries are not in local translation cache and have to be fetched from main memory
Bro created the perfect video
hi, what does 0x0010 in "2nd level entry" column mean at 19:28? it doesn't seem to be used in the process of finding the physical address.
I assume for 64-bit memory, it doesnt store page tables for the full address space, since that would be absurdly huge. I assume it just grows with memory requested and shrinks with frees. What I don’t understand there though is how it would shrink if you didn’t free the buffer at the end of your memory.
Starting off with the “bad old days” only being 4GB definitely lets me know I’m old! Just like running out of 32bit address space “must” have been back in the 1950s or something 😂
excellent explanation!!
one doubt isn't the page table size 16MB as there 2^22 pages and we uses word size for each page.
so 4*2^22 16MB?
32MB considering map between virtual address and physical address
such a good video, you are the man !
Thank you!
thank you, this video was very clarifying.
You’re welcome. I’m glad to hear that.
Can you next teach us about deditated WAM?