NvRAM & BIOS
What is NvRam?
NvRam: Acronym ---- Non-Volatile Random Access Memory.
What are the properties about NvRam?
NvRam Chip always retains its data, even when power to the chip is turn off.
Of course, NvRam is a type of RAM that also called read/write memory, can be used to store data that change.
NvRam in BIOS：
We can define a variable in NvRam that keep value after power loss.For example: BIOS Setup Data.
How to use the variable in NvRam?
The Run time service support some functions to Read/Write NvRam data that named Variable Services: GetVariable and SetVariable.
ps:The Run Time service defined in UEFI SPEC.
About GetVariable：
typedef
EFI_STATUS
(EFIAPI *EFI_GET_VARIABLE)(
IN CHAR16 *VariableName,
IN EFI_GUID *VendorGuid,
OUT UINT32 *Attributes, OPTIONAL
IN OUT UINTN *DataSize,
OUT VOID *Data OPTIONAL
);This function will get a set of Structural data.
About SetVariable：
typedef
EFI_STATUS
(EFIAPI *EFI_SET_VARIABLE)(
IN CHAR16 *VariableName,
IN EFI_GUID *VendorGuid,
IN UINT32 Attributes,
IN UINTN DataSize,
IN VOID *Data
);
This function will Set a set of Structural data.
Let’s examine each component of the function in more detail.
*VariableName : The structure name you want to access
*VendorGuid : The GUID for this structure
Attributes : Attributes of variable
Attributes Enumeration:
EFI_VARIABLE_NON_VOLATILE //Store in NvRam
EFI_VARIABLE_BOOTSERVICE_ACCESS //Just be valid in boot service
EFI_VARIABLE_RUNTIME_ACCESS //It’s vaild in RT Service
EFI_VARIABLE_HARDWARE_ERROR_RECORD
EFI_VARIABLE_AUTHENTICATED_WRITE_ACCESS
EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS
EFI_VARIABLE_APPEND_WRITE
DataSize : The Data Size of the structure
*Data : The pointer to the structure
How to use it?
First:
You should have a structure and GUID.
#define JUST_FOR_TEST_GUID \
{0xc1439232, 0xbfac, 0x412b, 0x97, 0x9a, 0xc, 0xbd, 0x12, 0xb6, 0x1f, 0xf7}
#pragma pack(1)
typedef struct _JUST_FOR_TEST_DATA
{
UINT8 JUSTFORDATA;
} JUST_FOR_TEST_DATA;
#pragma pack()
Second:
JUST_FOR_TEST_DATA JustForTestData;
EFI_GUID gJustForTestDataGuid = JUST_FOR_TEST_GUID;
Last:Modify variable and setvariable:
JustForTestData.JUSTFORDATA = 0;
VarSize = sizeof(JUST_FOR_TEST_DATA);
Status = gRT->SetVariable (
L" JustForTestData ",
&gAdvHidePolicyVariableGuid,
EFI_VARIABLE_NON_VOLATILE,
VarSize,
& JustForTestData);
Note: You can access Setup Data by GetVariable and SetVariable anytime.
You must first use SetVariable if the variable defined by self.