Intel Extensible Firmware Interface, Specification

Page: 120 / 494

Extensible Firmware Interface Specification
102 12/12/00 Version 1.02
3.8.5 GetNextHighMonotonicCount()
Summary
Returns the next high 32 bits of the platform’s monotonic counter.
Prototype
EFI_STATUS
GetNextHighMonotonicCount (
OUT UINT32 *HighCount
);
Parameters
HighCount
Pointer to returned value.
Description
The
GetNextHighMonotonicCount()
function returns the next high 32 bits of the platform’s
monotonic counter.
The platform’s monotonic counter is comprised of two 32 bit quantities: the high 32 bits and the
low 32 bits. During boot service time the low 32 bit value is volatile: it is reset to zero on every
system reset and is increased by 1 on every call to
GetNextMonotonicCount().
The high
32 bit value is non-volatile and is increased by 1 whenever the system resets or whenever the low
32 bit count [returned by
GetNextMonoticCount()
] overflows.
The
GetNextMonotonicCount()
function is only available at boot services time. If the
operating system wishes to extend the platform monotonic counter to runtime, it may do so by
utilizing
GetNextHighMonotonicCount()
. To do this, before calling
ExitBootServices()
the operating system would call
GetNextMonotonicCount()
to
obtain the current platform monotonic count. The operating system would then provide an
interface that returns the next count by:
•
Adding 1 to the last count.
•
Before the lower 32 bits of the count overflows, call
GetNextHighMonotonicCount()
.
This will increase the high 32 bits of the platform’s non-volatile portion of the monotonic count
by 1.
This function may only be called at Runtime.
Status Codes Returned
EFI_SUCCESS The next high monotonic count was returned.
EFI_DEVICE_ERROR The device is not functioning properly.
EFI_INVALID_PARAMETER One of the parameters has an invalid value.