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Question about LPAE(Large Physical Address Extensions) for ARMv7

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I read the ARMv7 architecture reference manuals. The spec. says LPAE allows 32-bits VA to be translated into 40-bits PA. The 40-bits PA means the width of address bus is 40-bits or greater than 40 bits, is it true? So my first question is what the width of address bus width?

If the width of address bus width is 32-bits, I think the 40-bits PA is useless, because the PA whose address is greater than 4GB can't map to the 32-bits address bus.

If the width of address bus is greater than 40-bits, a new question is coming. We know that VA is 32-bits. If we turn off the MMU, the PA is equal VA. So we can't be allowed to access the PA which is greater than 4GB without MMU opening. I don't know whether my understanding is true or not. 

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  • +1
    LPAE allows the physical address to be up to 40 bits wide. If take a look at the TRMs (Technical Reference Manual) for the processors that implement LPAE you can see this. For example, the Cortex-A17:

    AWADDRM[39:0] (Address signal for Writes)
    ARADDRM[39:0] (Address signal for Reads)
    infocenter.arm.com/.../BABBIFIB.html

    On your second question, when the MMU is off you can only access the bottom 32 bits of the address space. This does mean that the chip designer needs layout the address space so the things you need before the MMU is enabled is in this range.

    However, turning the MMU on is usually very early in the boot process.
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  • +1
    LPAE allows the physical address to be up to 40 bits wide. If take a look at the TRMs (Technical Reference Manual) for the processors that implement LPAE you can see this. For example, the Cortex-A17:

    AWADDRM[39:0] (Address signal for Writes)
    ARADDRM[39:0] (Address signal for Reads)
    infocenter.arm.com/.../BABBIFIB.html

    On your second question, when the MMU is off you can only access the bottom 32 bits of the address space. This does mean that the chip designer needs layout the address space so the things you need before the MMU is enabled is in this range.

    However, turning the MMU on is usually very early in the boot process.
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