I am playing with some assembly language in the Arduino environment and are looking to do the same in the Arduboy environment.
I am making progress with the actual code itself on a ‘bare-bones’ Arduino but struggling to understand how I move this to the Arduboy environment. Obviously I can use the asm volatile command and code away but how do I know what registers are being used at the point in time so I do not overwrite values that may still be needed?
I can see in the Arduboy2 docs that R28 and R29 are being used in the draw…() functions. How were these determined to be free? I assume that I can use them too but what if I need more?
Nice … this explains it well. Its interesting that in the drawBitmap ‘Sprite_Plus_Mask’ uses R28 and R29 which must be restored to their original values when they could have used R30 and R31. I wonder why? A little more digging and I might understand it.
This looks really useful as I am looking at how to inline some functions.
Actually, all 3 pointer registers, X (r27:r26), Y (r29:r28) and Z (r31:r30) are being used. X and Z are declared as output operands.
I didn’t write the code (I only cleaned it up a bit) but I assume trying to also declare the usage of Y caused problems with running out of registers, so instead it is preserved by pushing and popping. I can’t say for certain but it may be that Y could have been declared and either X or Z saved instead.
Just a suggestion:
It’s helpful that whenever you mix in assembly, that you also include the functionally equivalent code for the high level language (C++) you are using, as commented out code. This makes it easier to quickly port to a different processor instruction set. The person porting can then use the HLL source to get things going, and then re-code in assembly later if necessary.
The HLL source also sometimes helps document what the function does in a more readable way.
I am going to spend a little time pulling this code apart - not because I think I can do better but more that it is complex enough example that I think I will learn from it. I need to read up more on the use of registers overall.
This will probably be my approach anyway … write something in C++ before trying to optimise it with some assembly. I haven’t really got any specific uses in mind for the assembly. I am just interested as I haven’t done any since my 6502 days …