ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

Fast Context Switch Extension (FCSE)
E.1 About the FCSE
The Fast Context Switch Extension (FCSE) modifies the behavior of an ARM memory system. This
modification permits multiple programs running on the ARM processor to use identical address ranges,
while ensuring that the addresses they present to the rest of the memory system differ.
Normally, a swap between two software processes whose address ranges overlap requires changes to be
made to the virtual-to-physical address mapping defined by the MMU translation tables, see Translation
tables on page B3-7. It also typically causes cache and TLB contents to become invalid (because they relate
to the old virtual-to-physical address mapping), and so requires caches and TLBs to be flushed. As a result,
each process swap has a considerable overhead, both directly because of the cost of changing the translation
tables and indirectly because of the cost of subsequently reloading caches and TLBs.
By presenting different addresses to the rest of the memory system for different software processes even
when they are using identical addresses, the FCSE avoids this overhead. It also permits software processes
to use identical address ranges even when the rest of the memory system does not support virtual-to-physical
address mapping.
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