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I see the word ROM and internal memory used interchangeably in the context of Android phones. The OS seems to be stored in the internal memory itself, which is also used for installing applications. Which implies that it is not "read only".

So, just to clear my confusion, am I correct in understanding that in Android phones the OS is stored in the internal memory and word "ROM" is used for "historic" reasons, since ROM is where firmwares on non-Android phone were/are otherwise commonly installed?

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It seems like you've been misinformed. ROM means Read-Only Memory and is used in Android as such.
The ROM is the part of your system that is: able to run on its own, write protected and thus not changeable.
Many/All manufacturers implement a way to bypass Linux' restrictions/file system permissions to be able to update their ROM ie. write to /system.
The normal user is not able to write to that /system partition, which is independent form the /data partition, where all your apps and data is stored (/data is not needed to run Android).
The only thing those two partitions share is (not always) the same hardware/memory chip.

The contents stored on /system contain all software needed to run Android on that specific device including: drivers, configurations, scripts, applications, frameworks etc.
They are read-only for the user and only writable after remounting as a system user or superuser/root.
Therefore ROM can be considered a valid and correct term for the Android system's /system partition/ROM partition.

  • Write restrictions on /system are not imposed by the filesystem and permissions, the partition is mounted read-only which is a kernel feature, not filesystem one. Also while updating the system partition is usually accessed from outside the operating system hence there is no "bypass" of restrictions since there are no restrictions in the first place. – mikky Feb 1 '15 at 22:50
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ROM is just part of internal ssd-like memory which is, under normal circumstances, mounted read-only. There is no physical write restriction on the memory as it used to be on the original ROM chips a few decades before.

ROM memories used to be hardwired, the data was physically "fused" onto the chip. Then later these chips became flashable.

Today both "ROM" and "flashing" terms are used technically incorrectly but they carry a principle of how are these memories treated.

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ROM means Read Only Memory. As we know in the world of technology anything is possible. So Read only memory on any device means that the user can only READ the memory, but if you are a superuser then you can do whatever you want to that readonly memory.

Long story short: ROM is where you bootloader/OS is installed, but is considered READ-ONLY because it is so hard to write over that data.

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