====== AVR GCC calling conventions ======
Thanks Paulo Soares <psoares@consiste.pt>

====== Register Usage ======

^ Parameters     ^ Param 1       ^ Param 2      ^ Param 3      ^ Param 4 ^
| f(p8,p8)       | R25           | R24          | ~~           | ~~      |
| f(p8,p8,p8,p8) | R25           | R24          | R23          | R22     |
| f(p16,p16)     | R25:24#       | R23:22       | ~~           | ~~      |
| f(p16,p16,p16) | R25:24        | R23:22       | R21:20       | ~~      |
| f(p32,p32)     | R25:24:23:22# | R21:20:19:18 | ~~           | ~~      |
| f(p32,p32,p32) | R25:24:23:22  | R21:20:19:18 | R17:16:15:14 | ~~      |
| f(p8,p16,p32)  | R25           | R24:23       | R22:21:20:19 | ~~      |

# MSB left, LSB right

====== Return Value ======

^ Size ^ Registers    ^
| p8   | R25:24##     |
| p16  | R25:24       |
| p32  | R25:24:23:22 |

##  R25 is cleared

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