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我们使用的是intelframeworkmodule 这个包，因此bds的入口函数是在intelframeworkmodule/universal/bdsdxe/bsdentry.c其入口函数是BdsEntry

在BdsEntry->BdsBootDeviceSelect->BdsLibBootViaBootOption 这个函数中下面这段code

      Handle = BdsLibGetBootableHandle(DevicePath);

      if (Handle != NULL) {

        //

        // Load the default boot file \EFI\BOOT\boot{machinename}.EFI from removable Media

        //  machinename is ia32, ia64, x64, …

        //

        FilePath = FileDevicePath (Handle, EFI_REMOVABLE_MEDIA_FILE_NAME);

        if (FilePath != NULL) {

          REPORT_STATUS_CODE (EFI_PROGRESS_CODE, PcdGet32 (PcdProgressCodeOsLoaderLoad));

          Status = gBS->LoadImage (

                          TRUE,

                          gImageHandle,

                          FilePath,

                          NULL,

                          0,

                          &ImageHandle

                          );

        }

      }

    }

这里的filepath是通过EFI_REMOVABLE_MEDIA_FILE_NAME 这个变量来找到的

#define EFI_REMOVABLE_MEDIA_FILE_NAME_IA32    L”\\EFI\\BOOT\\BOOTIA32.EFI”

#define EFI_REMOVABLE_MEDIA_FILE_NAME_IA64    L”\\EFI\\BOOT\\BOOTIA64.EFI”

#define EFI_REMOVABLE_MEDIA_FILE_NAME_X64     L”\\EFI\\BOOT\\BOOTX64.EFI”

#define EFI_REMOVABLE_MEDIA_FILE_NAME_ARM     L”\\EFI\\BOOT\\BOOTARM.EFI”

#define EFI_REMOVABLE_MEDIA_FILE_NAME_AARCH64 L”\\EFI\\GRUB2\\grubaa64.efi”

#if   defined (MDE_CPU_IA32)

  #define EFI_REMOVABLE_MEDIA_FILE_NAME   EFI_REMOVABLE_MEDIA_FILE_NAME_IA32

#elif defined (MDE_CPU_IPF)

  #define EFI_REMOVABLE_MEDIA_FILE_NAME   EFI_REMOVABLE_MEDIA_FILE_NAME_IA64

#elif defined (MDE_CPU_X64)

  #define EFI_REMOVABLE_MEDIA_FILE_NAME   EFI_REMOVABLE_MEDIA_FILE_NAME_X64

#elif defined (MDE_CPU_EBC)

#elif defined (MDE_CPU_ARM)

  #define EFI_REMOVABLE_MEDIA_FILE_NAME   EFI_REMOVABLE_MEDIA_FILE_NAME_ARM

#elif defined (MDE_CPU_AARCH64)

  #define EFI_REMOVABLE_MEDIA_FILE_NAME   EFI_REMOVABLE_MEDIA_FILE_NAME_AARCH64

#else

  #error Unknown Processor Type

#endif

由于我们是MDE_CPU_AARCH64，因此EFI_REMOVABLE_MEDIA_FILE_NAME 就等于L”\\EFI\\GRUB2\\grubaa64.efi”

因此在BdsLibBootViaBootOption 中通过gBS->LoadImage 来找到grubaa64.efi，最后还是在BdsLibBootViaBootOption 中通过下面的code  Status = gBS->StartImage (ImageHandle, ExitDataSize, ExitData);来跳转到grub的第一行代码。