In the PowerPC/EABI application binary interface,
the general shape of activation records is as follows,
from bottom (lowest offsets) to top:
8 reserved bytes. The first 4 bytes hold the back pointer to the
activation record of the caller. The next 4 bytes are reserved
for called functions to store their return addresses.
Since we would rather store our return address in our own
frame, we will not use these 4 bytes, and just reserve them.
Space for outgoing arguments to function calls.
Local stack slots.
Saved values of callee-save registers used by the function.
Space for the stack-allocated data declared in Cminor.
The frame_env compilation environment records the positions of
the boundaries between areas in the frame part.