EFI_ACPI_5_1_IA32_ARCHITECTURE_MACHINE_CHECK_ERROR_DATA_FORMAT_INTEL64

Undocumented in source but is binding to C. You might be able to learn more by searching the web for its name.
extern (C)
enum EFI_ACPI_5_1_IA32_ARCHITECTURE_MACHINE_CHECK_ERROR_DATA_FORMAT_INTEL64 = 0x01;

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