The caller shouldn’t assume that R0-3 have not been touched by the called function. Before returning, it puts its return value into R0 for the calling code to read. The calling convention says that the called function can read and modify R0-3, but must preserve the other registers by pushing them onto the stack and then popping them back once it’s work is done. Functions with more than four parameters typically use one or more of those parameter registers to hold addresses pointing to data structures in memory. Registers R0-3 are also, by convention, used to hold parameter values to be passed to a subroutine or function. The R registers are a mix of general purpose (R0-12) and special purpose (R13-15) stores. The latter is short for ‘Program Status Register’ and it’s what we used to call the condition code register: the place where the CPU records the effect of the most recently executed instruction. The M0+ contains 16 32-bit registers named R0 through R15 and one called PSR. You might also want to download the Cortex M0+ Technical Reference Manual from ARM. Note You’ll want to grab the RP2040 datasheet early on - it has all the device details you’ll need. However, all but one of these, BL, are for advanced tasks you probably won’t need to worry about yet.

The M0+ is still a 32-bit processor, and has a handful of full 32-bit instructions too. The M0+ uses the ARMv6-M Thumb instruction set, which comprises memory-saving 16-bit versions of the standard ARMv6-M ISA. The RP2040 is based on the Cortex-M0+ core design - it has two of them on board - which is well documented by ARM. It’s not a tutorial, more a ‘what you need to know’ guide. I’ve tried to condense what I’ve learned into a series of posts intended to help anyone with a little C/C++ programming experience learn how to add assembly code to existing and new software projects.

Assembly language on mac