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I have an Android smartphone and would like to understand what is the difference between RAM, SD Card and Device Memory. When I check Memory Usage of my smartphone, I see those components:
RAM: 174 used and 52 MB free
On SD Card: 492 MB used and 3.16GB free
Device Memory: 143 MB used and 47 MB free
I understand that "SD Card" is some external memory that you put in the Smartphone. But how do I understand RAM and Device Memory?
RAM - Random Accessed Memory - When running an application on your phone, the application is loaded from your device memory into the RAM, just like on a computer, the main purpose for this is access speed is far greater in RAM. You can clear the apps that is in the RAM on your phone depending on your smartphone (Samsung for example hold home button down and click ram manager > clear RAM - this will clear what's in RAM however most background apps will automatically be loaded back into the RAM hence the free space decreasing straight away
As you mentioned SD Card is a external memory card which you can switch and swap with a variety of different sizes if needed
Device Memory is your phones built in memory, just like SD Card has memory however this one cannot be swapped or taken out of the phone. It will hold all of your phones files that make it work, just like a PC, as standard you cannot edit these files as they are read only, however through rooting your phone you may access these files and change if needed.
These files as mentioned take up space on your smartphone, so you may have a 16GB device memory or Device Storage, but you could actually only have 15GB free (for example not exact) to use as 1GB is used for the files I mentioned above
Hope this clear it up for you, and I'll happily provide links for further reading for you if needed
Addition Note:
Modern Android phones (4.0+) use an "emulated SD card" for their internal memory. Which means that your phone tricks the OS into seeing the internal memory as an SD card, which can cause some confusion as to the difference between SD memory and internal memory.
I had the same question, and I found this answer to be helpful. The short summary is that "device memory", AKA "internal storage", is non-volatile: in other words it's persistent across device reboots, unlike RAM.
Internal storage is flash memory, similar to a solid-state drive or a microSD card. One way to look at it is that device memory and a microSD card use (practically) the same technology, just that the microSD card is removable.
Flash memory is much slower than RAM, and has to be written in large chunks, unlike RAM. Its role would be more like that of a hard drive than like RAM.
