ARM ARM926EJ-S Technical Reference Manual page 64

Programmer's Model
2.3.15 Test and Debug Register c15
2-36
You can use register c15 to provide device-specific test and debug operations in
ARM926EJ-S processors. Appendix B CP15 Test and Debug Registers describes the
registers and functions available using CP15 c15.This register is defined to be reserved
for implementation-defined purposes in the ARM Architecture Reference Manual. If
you write software that uses the device-specific facilities provided by c15, then this
software is unlikely to be either backwards or forwards compatible.
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