How can ARM processors use more than 4GB of ram?

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I recently started working on my own operating system. I am following jsandler18‘s awesome tutorial and making changes as I go to allow it to run on the raspberry pi 4.

Sadly, jsandler18 stopped updating the tutorial before he had finished the page on virtual memory. I read through some other sources, and found a little problem: The ARM l1 address translation table divides the computers RAM into 1-MB blocks. The problem here is that it only allows up to 4096 entries, or 4GB of virtual ram.

Is there some way I can use the ARM MMU to translate more than 4GB of virtual memory?

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1 Answers

The tutorial being referenced appears to be executing in ARMV7, which can be thought of as 32-bit ARM. This is roughly equivalent to running in 32-bit PAE mode in X86. Thus using this example it is not possible to to use more that 4GB of virtual memory.

ARMV8 (or AARCH64) supports 64-bit virtual addresses, and would allow mapping more that 4GB of virtual memory.

Switching into ARMV8 is done by switching Exception levels, which are usually denoted as EL0, EL1, EL2 and EL3. The one challenge you could run into is that once you enter AARCH32 mode, you can not go to a lower exception level and switch to AARCH64. For example going from EL1 64-bit -> EL0 32-bit is supported, but going EL1 32-bit -> EL0 64-bit is not. This could pose a challenge if the firmware handing execution off to your OS is in AARCH32 mode.

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