ARM ARM1176JZF-S Technical Reference Manual page 102

2.10.9 Reserved bits
ARM DDI 0301H
ID012310
Only Secure privileged modes can write directly to the CPSR mode bits to enter Secure
Monitor mode. If the core is in Secure User mode, Non-secure User mode, or Non-secure
privileged modes it ignores changes to the CPSR to enter the Secure Monitor. The core
does not copy mode bits in the SPSR, changed in the Non-secure world, across to the
CPSR.
The remaining bits in the PSRs are unused, but are reserved. When changing a PSR flag or
control bits, make sure that these reserved bits are not altered. You must ensure that your
program does not rely on reserved bits containing specific values because future processors
might use some or all of the reserved bits.
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