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....@groups.io]
*Sent:* Wednesday, November 9, 2022 at 11:20 AM
*To:* devel@edk2.groups.io, yoshinoyat...@163.com
*Subject:* [edk2-devel] Access 64bit address space in 32bit mode
On Nov 7, 2022, at 7:16 PM, Yoshinoya <yoshinoyat...@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
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