RTEMS CPU Architecture Supplement
The PowerPC architecture supports a simple yet effective call
and return mechanism. A subroutine is invoked
via the "branch and link" (bl) and
"brank and link absolute" (bla)
instructions. This instructions place the return address
in the Link Register (LR). The callee returns to the caller by
executing a "branch unconditional to the link register" (blr)
instruction. Thus the callee returns to the caller via a jump
to the return address which is stored in the LR.
The previous contents of the LR are not automatically saved
by either the bl or bla. It is the responsibility
of the callee to save the contents of the LR before invoking
another subroutine. If the callee invokes another subroutine,
it must restore the LR before executing the blr instruction
to return to the caller.
It is important to note that the PowerPC subroutine call and return mechanism does not automatically save and restore any registers.
The LR may be accessed as special purpose register 8 (SPR8) using the
"move from special register" (mfspr) and
"move to special register" (mtspr) instructions.
RTEMS CPU Architecture Supplement
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