When writing AVR assembly code, there are protocols that must be followed when using and abusing the various registers available in the ATmega328P device, which is what I’m using, built in to many Arduino boards.

The assembly language written can be compiled in the Arduino IDE using the GCC/G++ compiler controller and this, in turn, will call the avr-as assembler to do the actual work. All you need to do is add a new file to your Arduino sketch, and name it *.S where the name can be anything, but the extension must be an upper case ‘S’.

Special Registers

R0: Temporary Register

R0 is the temporary register used by the C++ compiler generated code. To this end, it must be preserved by the assembly code if it is used there. Obviously, it should also be restored before calling or returning to C++.

R1: Zero Register

R1 is the zero register. The code generated by the C++ compiler expects the register toalways hold the value zero. If your assembly code uses this register for any reason, it must be cleared back to zero before calling or returning to C++.

Call Saved Registers

R2-R17 plus R28 and R29 are the Call Saved Registers. What this means is that these must be saved if they are used in your assembly code, and restored before calling or returning to C++.

Call Clobbered Registers

R18-R27 plus R30 and R31 are the Call Used/Clobbered Registers. What this means is that these can be used freely in your assembly code, however, they are clobbered by the C++ compiler’s generated code, so if you call a C++ function, you must preserve them if you have values in them which you will use on return from the C++ call.

Summary