Re: [edk2-devel] Access 64bit address space in 32bit mode

["vincent zimmer" <vincent.zimmer@gmail.com>]

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we have the challenge of 32-bit PEI needing to access 64-bit addresses to
support 64-bit DXE/UEFI OS's in the capsule use-case scenario.  This is
described in
https://raw.githubusercontent.com/tianocore-docs/Docs/master/White_Papers/A_Tour_Beyond_BIOS_Capsule_Update_and_Recovery_in_EDK_II.pdf
page 22 with code
https://github.com/tianocore/edk2/tree/master/MdeModulePkg/Universal/CapsulePei

On Wed, Nov 9, 2022 at 9:21 AM Andrew Fish via groups.io <afish=
apple.com@groups.io> wrote:

>
> On Nov 7, 2022, at 7:16 PM, Yoshinoya <yoshinoyatoko@163.com> wrote:
>
> Hello
> Is it possible to access 64bit address space in 32bit mode?
>
>
> I assume you are talking about x86?
>
>
> For example, opcode prefix 0x66/67 could let code running in 16bit mode to
> access 32bit data/address.
>
>
> This is more complex than just instruction prefix. You need the CPU to be
> setup in big real mode via the GDT, so you basically have a 32-bit
> environment setup via GDT etc. Also the prefix opcodes have different
> meaning in different modes. I don’t think there is a way to make 32-bit
> code access 64-bit data via instruction prefix even if a 64-bit GDT was
> setup with paging enabled.
>
> Or, establishing page table is a must requirement for accessing 64bit
> address space.
>
>
> For x86 you have to have 64-bit versions of the IDT, GDT, and you need to
> enable paging to enter 64-bit Long Mode.
>
> In a 32-bit x86 world you can access up to 64 GB of physical memory via
> using 32-bit page table using PAE [1]. PAE is a 32-bit virtual address
> space, but with support for a 36-bit physical address. I think in the olden
> days of 32-bit x86 EFI servers would have custom EFI code that enabled
> paging in 32-bit and carved out a chunk of the 32-bit memory space that
> could be mapped to 36-bit physical addresses. I think this was platform
> specific code and I don’t know of any open source version. The 32-bit Long
> Mode EFI does not have paging enabled, so adding PAE means enabling paging
> yourself.
>
> The edk2 has the opposite version of this code so you can call 16-bit
> really mode (Legacy BIOS) from 32-bit Protected mode, or 64-bit Long Mode.
> This is the code to Thunk for 32-bit/64-bit mode to 16-bit code [2]/
>
> [1] https://en.wikipedia.org/wiki/Physical_Address_Extensio
> <https://en.wikipedia.org/wiki/Physical_Address_Extension>n
> <https://en.wikipedia.org/wiki/Physical_Address_Extension>
> [2]
> https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/X86Thunk.c
> [image: edk2.png]
>
> edk2/Thunk16.nasm at master · tianocore/edk2
> <https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/X64/Thunk16.nasm>
> github.com
> <https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/X64/Thunk16.nasm>
>
> <https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/X64/Thunk16.nasm>
> [image: edk2.png]
>
> edk2/Thunk16.nasm at master · tianocore/edk2
> <https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/Ia32/Thunk16.nasm>
> github.com
> <https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/Ia32/Thunk16.nasm>
>
> <https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/Ia32/Thunk16.nasm>
>
> Thanks,
>
> Andrew Fish
>
>
> Thanks
>
>
>
> 
>
>

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