r8 = last allocated environment
r9 = env memory limit (bottom)

              frame:  +0     +1     +9   +17 | +25
| @r9 ........ @r8 | -25    -24    -16    -8 | ........
|     ........     | 'c'  val.a  val.e  <up> | ........

<up> points at parent environment, or NULL for the first frame

r8 < r9  -->  allocation fail, out of space