The PowerPC architecture includes a number of special registers
which are critical to the programming model:
Machine State Register
The MSR contains the processor mode, power management mode, endian mode,
exception information, privilege level, floating point available and
floating point excepiton mode, address translation information and
the exception prefix.
Link Register
The LR contains the return address after a function call. This register
must be saved before a subsequent subroutine call can be made. The
use of this register is discussed further in the Call and Return Mechanism section below.
Count Register
The CTR contains the iteration variable for some loops. It may also be used
for indirect function calls and jumps.
RTEMS CPU Architecture Supplement