Why My Computer Wants to Forget (How Dynamic Memory Works) - Computerphile

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Shared March 18, 2020

To save silicon, RAM is built to forget. Dr Steve Bagley explains how dynamic memory saves on space but at a cost.

Computer Memory Playlist: https://www.youtube.com/playlist?list...
How Autofocus Works: https://youtu.be/B-TOUPXytw4
Why CPUs have Caches: https://youtu.be/6JpLD3PUAZk


This video was filmed and edited by Sean Riley.

Computer Science at the University of Nottingham: https://bit.ly/nottscomputer

Computerphile is a sister project to Brady Haran's Numberphile. More at http://www.bradyharan.com


Just looked inside my computer for the first time... I cant beleive there is a finger in there pushing a button that fast!

2 weeks ago | [YT] | 160

Susan Wojcicki

That is why your computer does forget. Why your computer wants to forget can be answered by looking at your browser history.

2 weeks ago | [YT] | 152

Reckless Roges

Ben Eater has entered the conversation

2 weeks ago | [YT] | 77


11:20 yeah, that's a funny irony, the RAM based on constantly forgetting remembers for an extra bit of time when power goes out, as opposed to the RAM based on permanently remembering, which forgets immediately.

2 weeks ago | [YT] | 7

Yolo Swaggins

Wait what? That's how memory is built? It's like constantly reminding someone with dementia what they're supposed to do.

2 weeks ago | [YT] | 69

Paul Miner

A long time ago when I was a kid in the 90s, I had written a memory viewer that allowed me to scroll through memory and display it. In the famous PORTS.LST listing I found out that the memory refresh timer was accessible through the PIT. I turned it off, ran my memory viewer, and could see blocks of memory alternately decaying to 00 and FF. It was a neat visual demonstration of the DRAM refresh cycle's function. Naturally the computer would later crash when it used a corrupted area that hadn't been accessed in a while (frequently accessed areas were maintained).

2 weeks ago | [YT] | 45

Orion Aerospace

It’s so cool to see how these things work at the low level! 🤯

2 weeks ago (edited) | [YT] | 50

David Jameson

Minecraft redstone comparison:
Persistent memory: RS NOR latch
Dynamic memory: repeater

2 weeks ago | [YT] | 22

Teddye Kuma

The bit at the end about the data staying for a short time is why you could pull up the level select cheat in Sonic 2 and then quickly switch the cart to Sonic 3. The flag that controlled access to the cheat was stored at the exact same address in both, and ram stayed there for a few seconds.

2 weeks ago | [YT] | 10


Now I kind of want dr. Bagley to make a vid where he recovers data from RAM on a pc that was turned off. :/

2 weeks ago | [YT] | 4


The refresh rate of the 1970’s was 2 milliseconds. The Z80 was designed to incorporate DRAM refresh. An issue with that is the complexity required for other bus masters to continue refreshing the DRAM.

2 weeks ago | [YT] | 2

Lawrence D’Oliveiro

3:58 LEDs always need a resistor to stop them exploding, because they are prone to an effect known as “thermal runaway”. Most substances exhibit higher electrical resistance as their temperature rises. So as current flows through them and heats them up, this increases the resistance and acts as an automatic limit on how much more current can flow. LEDs exhibit the opposite behaviour: their resistance goes down as they heat up. Thus, if you put too much current through them, they heat up, lowering their resistance, and this means even more current flowing through, until they blow up, die etc.

1 week ago | [YT] | 1

Amiibo Alec

i've been waiting for a video to explain this for so long, especially how DRAM refresh works. Thank you, very interesting video.

2 weeks ago | [YT] | 1

Dann NnNn

I’m forgetting a bit

2 weeks ago | [YT] | 15

Laura Halliday

In one of my university courses we implemented a DRAM controller in an FPGA. Three main pieces: the 68k bus interface, address decoding, and the refresh logic. Great fun!

2 weeks ago | [YT] | 12


Also, "resistor. it's the component that prevents electrical circuit from exploding" =D

2 weeks ago | [YT] | 1

Tyler Brown

Love to hear from Dr. Bagley!!

2 weeks ago | [YT] | 2

Joseph Holland

Sir Clive Sinclair designed his first calculators with this concept in mind.

2 weeks ago | [YT] | 0

Jakub Beránek

What an incredible way of explaining Dynamic RAM! Bravo :-)

2 weeks ago | [YT] | 0


Following this logic isn't breathing also just constant resetting of a very short countdown.

2 weeks ago | [YT] | 1