Slight correction:  PAE paging can access up to 52 physical address 
bits, for 4 PBytes of memory.  Section 4.4 of the Intel® 64 and IA-32 
Architectures Software Developer’s Manual Volume 3 (3A, 3B, 3C & 3D): 
System Programming Guide covers it.

Brian J. Johnson

------------------------------------------------------------------------
*From:* Andrew Fish via groups.io [mailto:afish=apple.com@groups.io]
*Sent:* Wednesday, November 9, 2022 at 11:20 AM
*To:* devel@edk2.groups.io, yoshinoyatoko@163.com
*Subject:* [edk2-devel] Access 64bit address space in 32bit mode

>
>> 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
> 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>
> 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
>
>