Vol. 3A 8-49

MULTIPLE-PROCESSOR MANAGEMENT

supporting Intel Hyper-Threading Technology are described in the Intel® 64 and IA-32 Architectures Optimization 
Reference Manual.

8.10.6.1   Use the PAUSE Instruction in Spin-Wait Loops

Intel recommends that a PAUSE instruction be placed in all spin-wait loops that run on Intel processors supporting 
Intel Hyper-Threading Technology and multi-core processors. 
Software routines that use spin-wait loops include multiprocessor synchronization primitives (spin-locks, sema-
phores, and mutex variables) and idle loops. Such routines keep the processor core busy executing a load-compare-
branch loop while a thread waits for a resource to become available. Including a PAUSE instruction in such a loop 
greatly improves efficiency (see Section 8.10.2, “PAUSE Instruction”). The following routine gives an example of a 
spin-wait loop that uses a PAUSE instruction:

Spin_Lock:

CMP lockvar, 0

;Check if lock is free

JE Get_Lock
PAUSE

;Short delay

JMP Spin_Lock

Get_Lock:

MOV EAX, 1
XCHG EAX, lockvar ;Try to get lock
CMP EAX, 0 

;Test if successful

JNE Spin_Lock

Critical_Section:

<critical section code>
MOV lockvar, 0
...

Continue:
The spin-wait loop above uses a “test, test-and-set” technique for determining the availability of the synchroniza-
tion variable. This technique is recommended when writing spin-wait loops.
In IA-32 processor generations earlier than the Pentium 4 processor, the PAUSE instruction is treated as a NOP 
instruction.

8.10.6.2   Potential Usage of MONITOR/MWAIT in C0 Idle Loops

An operating system may implement different handlers for different idle states. A typical OS idle loop on an ACPI-
compatible OS is shown in Example 8-24: 

Example 8-24.  A Typical OS Idle Loop

// WorkQueue is a memory location indicating there is a thread 
// ready to run.  A non-zero value for WorkQueue is assumed to
// indicate the presence of work to be scheduled on the processor.
// The idle loop is entered with interrupts disabled.

WHILE (1) {

IF (WorkQueue) THEN {

// Schedule work at WorkQueue.
} 

ELSE {

// No work to do - wait in appropriate C-state handler depending 
// on Idle time accumulated
IF (IdleTime >= IdleTimeThreshhold) THEN {

// Call appropriate C1, C2, C3 state handler, C1 handler