Intel Pentium 4 Processor on 90 nm Process

72 Datasheet
Features
6.2 Clock Control and Low Power States
The processor allows the use of AutoHALT, Stop-Grant, and Sleep states to reduce power
consumption by stopping the clock to internal sections of the processor, depending on each
particular state. See Figure 14 for a visual representation of the processor low power states.
6.2.1 Normal State—State 1
This is the normal operating state for the processor.
6.2.2 AutoHALT Powerdown State—State 2
AutoHALT is a low power state entered when the processor executes the HALT instruction. The
processor transitions to the Normal state upon the occurrence of SMI#, BINIT#, INIT#, or
LINT[1:0] (NMI, INTR). RESET# causes the processor to immediately initialize itself.
The return from a System Management Interrupt (SMI) handler can be to either Normal Mode or
the AutoHALT powerdown state. See the Intel Architecture Software Developer's Manual, Volume
III: System Programmer's Guide for more information.
The system can generate a STPCLK# while the processor is in the AutoHALT powerdown state.
When the system de-asserts the STPCLK# interrupt, the processor will return execution to the
HALT state.
While in AutoHALT powerdown state, the processor will process FSB snoops and interrupts.
Figure 14. Stop Clock State Machine
STPCLK#
De-asserted
SLP#
De-asserted
1. Normal State
Normal execution.
3. Stop Grant State
BCLK running.
Snoops and interrupts allowed.
5. Sleep State
BCLK running.
No snoops or interrupts allowed.
2. Auto HALT Power Down State
BCLK running.
Snoops and interrupts allowed.
4. HALT/Grant Snoop State
BCLK running.
Service snoops to caches.
STPCLK#
Asserted
SLP#
Asserted
Snoop
Event
Occurs
Snoop
Event
Serviced
HALT Instruction and
HALT Bus Cycle Generated
INIT#, BINIT#, INTR, NMI,
SMI#, RESET#
Snoop Event Serviced
Snoop Event Occurs
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