ARM Technical Support Knowledge Articles

Can I increase the number of processes allowed by the Fast Context Switch Extensions?

Applies to: ARM926EJ-S

Answer

This value cannot be altered. The CP15 c13 register uses the top 7 bits to hold the process ID, limiting the value to 128.

The introduction of the FCSE was originally to allow WinCE to run on the ARM cores. At the time, WinCE had an upper limit of 32 processes so 128 processes was  deemed to be large enough to allow for WinCE to evolve and increase the limit.

Having said that, 128 x 32MB = 4GB, i.e. the entire address space, so it would be very impractical to use anything like the full 128 process IDs in any application.

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