Vincent, Thanks! I’d forgotten about that path. The other answer is defer the work to a DXE driver that runs 64-bit x86. Thanks, Andrew Fish > On Nov 9, 2022, at 10:58 AM, vincent zimmer wrote: > > 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 > wrote: >> >>> On Nov 7, 2022, at 7:16 PM, Yoshinoya > 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 n >> [2] https://github.com/tianocore/edk2/blob/master/MdePkg/Library/BaseLib/X86Thunk.c >> >> edk2/Thunk16.nasm at master · tianocore/edk2 >> github.com >> edk2/Thunk16.nasm at master · tianocore/edk2 >> github.com >> >> edk2/Thunk16.nasm at master · tianocore/edk2 >> github.com >> edk2/Thunk16.nasm at master · tianocore/edk2 >> github.com >> >> Thanks, >> >> Andrew Fish >> >> >>> Thanks >>> >> >> >> > > >