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RealView Compilation Tools Assembler Guide
Version 4.0
Home > ARM and Thumb Instructions > Miscellaneous instructions > NOP, SEV, WFE, WFI, and YIELD
4.10.8. NOP, SEV, WFE, WFI, and YIELD
No Operation, Set Event, Wait For Event, Wait for Interrupt, and Yield.
Syntax
NOP{cond}
SEV{cond}
WFE{cond}
WFI{cond}
YIELD{cond}
where:
cond
is an optional condition code (see Conditional execution).
Usage
These are hint instructions. It is optional whether they are implemented or not. If any one of them is not implemented, it behaves as a NOP.
NOP
NOP does nothing. If NOP is not implemented as a specific instruction on your target architecture, the assembler treats it as a pseudo-instruction and generates an alternative instruction that does nothing, such as MOV r0, r0(ARM) or MOV r8, r8 (Thumb).
NOP is not necessarily a time-consuming NOP. The processor might remove it from the pipeline before it reaches the execution stage.
You can use NOP for padding, for example to place the following instruction on a 64-bit boundary.
SEV
SEV causes an event to be signaled to all cores within a multiprocessor system. If SEV is implemented, WFE must also be implemented.
WFE
If the Event Register is not set, WFE suspends execution until one of the following events occurs:
an IRQ interrupt, unless masked by the CPSR I-bit
an FIQ interrupt, unless masked by the CPSR F-bit
an Imprecise Data abort, unless masked by the CPSR A-bit
a Debug Entry request, if Debug is enabled
an Event signaled by another processor using the SEV instruction.
If the Event Register is set, WFE clears it and returns immediately.
If WFE is implemented, SEV must also be implemented.
WFI
WFI suspends execution until one of the following events occurs:
an IRQ interrupt, regardless of the CPSR I-bit
an FIQ interrupt, regardless of the CPSR F-bit
an Imprecise Data abort, unless masked by the CPSR A-bit
a Debug Entry request, regardless of whether Debug is enabled.
YIELD
YIELD indicates to the hardware that the current thread is performing a task, for example a spinlock, that can be swapped out. Hardware can use this hint to suspend and resume threads in a multithreading system.
Architectures
These ARM instructions are available in ARMv6K and above.
These 32-bit Thumb instructions are available in ARMv6T2 and above.
These 16-bit Thumb instructions are available in ARMv6T2 and above.
NOP is available on all other ARM and Thumb architectures as a pseudo-instruction.
SEV, WFE, WFI, and YIELD execute as NOP instructions in ARMv6T2.
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