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15.3. Assembly language power instructions

ARM assembly language includes instructions that can be used to place the core in a low-power state. The architecture defines these instructions as hints, meaning that the core is not required to take any specific action when it executes them. In the Cortex-A processor family, however, these instructions are implemented in a way that shuts down the clock to almost all parts of the core. This means that the power consumption of the core is significantly reduced so that only static leakage currents are drawn, and there is no dynamic power consumption.

The WFI instruction has the effect of suspending execution until the core is woken up by one of the following conditions:

  • An IRQ interrupt, even if the PSTATE I-bit is set.

  • An FIQ interrupt, even if the PSTATE F-bit is set.

  • An asynchronous abort.

In the event of the core being woken by an interrupt when the relevant PSTATE interrupt flag is disabled, the core implements the next instruction after WFI.

The WFI instruction is widely used in systems that are battery powered. For example, mobile telephones can place the core in standby mode many times a second, while waiting for you to press a button.

WFE is similar to WFI. It suspends execution until an event occurs. This can be one the event conditions listed or an event signaled by another core in a cluster. Other cores can signal events by executing the SEV instruction. SEV signals an event to all cores. The generic timer can also be programmed to trigger periodic events that wake up a core from WFE.

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