Normal State; Halt And Enhanced Halt Powerdown States; Halt Powerdown State; Processor Low Power State Machine - Intel HH80552PG0962M - Pentium 4 3.4 GHz Processor Datasheet

Figure 16.

Processor Low Power State Machine

Normal State

Normal execution
STPCLK#
Asserted
Stop Grant State
BCLK running
Snoops and interrupts allowed
6.2.1 Normal State
This is the normal operating state for the processor.
6.2.2

HALT and Enhanced HALT Powerdown States

The Pentium 4 processor supports the HALT or Enhanced HALT powerdown state. The
Enhanced HALT Powerdown state is configured and enabled via the BIOS. The
Enhanced HALT state must be enabled via the BIOS for the processor to remain within
its specifications.
The Enhanced HALT state is a lower power state as compared to the Stop Grant State.
If Enhanced HALT is not enabled, the default Powerdown state entered will be HALT.
Refer to the following sections for details about the HALT and Enhanced HALT states.
6.2.2.1

HALT Powerdown State

HALT is a low power state entered when all the logical processors have executed the
HALT or MWAIT instructions. When one of the logical processors executes the HALT
instruction, that logical processor is halted; however, the other processor continues
normal operation. The processor will transition to the Normal state upon the occurrence
of SMI#, BINIT#, INIT#, or LINT[1:0] (NMI, INTR). RESET# will cause the processor to
immediately initialize itself.
86
HALT or MWAIT Instruction and
HALT Bus Cycle Generated
INIT#, BINIT#, INTR, NMI, SMI#,
RESET#, FSB interrupts
STPCLK#
De-asserted
Snoop Event Occurs
Snoop Event Serviced
Features
Enhanced HALT or HALT State
BCLK running
Snoops and interrupts allowed
Snoop Snoop
Event Event
Occurs Serviced
Enhanced HALT Snoop or HALT
Snoop State
BCLK running
Service snoops to caches
Stop Grant Snoop State
BCLK running
Service snoops to caches
Datasheet