What is virtual memory? – Gary explains

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[tanishbansal]

when you are taught the same in class but Gary explains better 😀

[unlokia]

Gary, you are a marvelous teacher. Clear, concise, precise, relaxed, happy... and no irritating "YouTube persona". From an engineer, let me tell you this - you *really do* sound like you know your subjects well, so thank you for being you and for articulating so effectively and clearly. You are an asset to the engineering world.

God bless you! 😀

Matthew, England.

[trueblue]

Don't know how much AA pay you, Gary, but you should get a pay rise

[aRoamingDuck]

I never would have thought I would end up going to Android Authority to help study for my OS' exam... As always Gary your videos are probably the best on AA. Keep it up!

[thomyhr]

Knew most of this from the course in Operating Systems in the University, but it is always nice to hear Gary explain stuff. And I got to repeat the things I knew too. Please never stop making these videos, Gary! :D

[RockstarBruski]

excellent tutorial. I've been working on computers since 1980 and never have heard this explained so clearly as you did in this video. love the charts and diagrams as well as those really help! and the funny kids pics made me laugh. cheers! 😎

[BraveCarnage]

Saving this to favorites. Best explanation I’ve seen on this subject. Gonna have to rewatch 5x until everything makes sense but I’m finally understanding it.

[--Paws--]

In other words the physical representation of memory and virtual memory is equivalent to how inventory is shipped to a warehouse and memory is the amount of space the warehouse can store of that shipment. Virtual memory is a shipment being processed to go either in its storage location or to be used in either to be once again shipped out or for some function in the warehouse/facility.

[sshknt]

I'm stupid but Gary makes me smart.

[AElShahawy]

Playlist of Gary explains, the best ever !

[fezspencer]

Who are the butttrolls who dislike excellent productions such as this.

Wow.

[SteveTheTechy]

Machines have been using virtual memory since the 70's. Gary in his run down of computers did not mention the IBM Mainframes where we had the MVS and VM operating systems which ran very much as Gary explained. There was the additions of the Link Pack Area (or on DOS/VS) machines the SVA (shared Virtual Area) where common routines used by multiple programs would be loaded into memory and shared between all the address spaces, saving real memory.

The small operating systems like DOS/VS though only had one address space, so multiple processes would be loaded in to memory in several contiguous regions called partitions. So partition 1 had memory say from 100K to 200K, partition 2 from 200K to 500K, partition 3 500K to600K etc. It was still virtual because address translation between a virtual address and a real address still had to happen. There was not enough memory, so memory was swapped out to disk when it was not required. Often then machines ran with 200K of memory, and ran order processing, payroles, stock processing and even some development all at the same time.

This contiguous space for multiple partitions could lead to the two problems which Gary mentioned early on. One program accessing the memory of another, and relative addressing.

Each partition was protected from access from another by a protection key. each partition having a different key. A partition when scheduled was allocated the key for its memory. Should it try and access the memory of another partition, then it would fail.

The relative addressing was addressed by the loader program .Often a branch address would be loaded form a table to access a routine. If the program was loaded in the partition starting at 200K, or it was loaded in the partition starting at 500K, then this address had to change. That was the job of the loader program, it would correct these addresses as the program was loaded into memory.

Yes address spaces as used by MVS and VM were much easier, but they also still had loader programs as you can't predict where routines are loaded into memory in the address space.

[djknight]

I'm a computer science student and Gary has helped me pass in atleast a couple of subjects.

[noobaeon]

It's always a pleasure listening to your explanations Gary. Keep up the good work :D

[jerrymachado]

Hi Gary. I was trained as an EE, but went to the dark side of sales, so I am only dimly aware of all the subjects you discuss. Thanks for the context and thanks for putting the puzzle together for me.

[bodeghost]

Greetings world! I'm very new to, Gary's channel & all I can say is, I've been asleep at the wheel for far too long. Sir, this channel is marvelous! From the quality video/audio & editing, to your ability to explain things in a manner with which most anyone can comprehend /completely understand the inner-workings of all this technology that changes every 27 seconds, if you will...😉 I love reading this communities feedback & shared knowledge! Love & Light To You All out there in YouTube Land and Beyond the world over. Many thanks, Gary for taking what has to be a great amount of time you invest so that, we the viewers, can be educated correctly, efficiently & with absolute ease! 🍀 💜 🎶 👻

[rsdwall]

Oh my god . Mr. Simps thanks a lot for explains this. When you explained this, it seems so simple. Other people make it sound so tough. I wish I had listen to your lectures earlier. So so good.
Thanks you very much from a student trying to explore world of computers.

[clairebear]

When ever Gary explains, I understand. He is the only way I am going to get through my systems architecture exam, so thanks!!!

[MrMysticphantom]

This has been the best of Gary Explains vids

Nice job sir

[fuzz]

Gary - your ability to explain the complex is unsurpassed.