On Nov 7, 2022, at 7:16 PM, Yoshinoya <yoshinoyatoko@163.com> wrote:HelloIs 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]/
Thanks,
Andrew Fish
Thanks