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f8ea1e8e56
Keeping the base address of this structure in a register allows us to use offsets to load individual fields from it, without needing their individual addresses. However, the ldr instruction can only use immediate offsets relative to the low registers (r0-r7), or the stack pointer (r13). Low registers are in short supply and are needed for other instructions which can only use r0-r7, so we use the stack pointer here. It's safe to do this because we do not use the stack. There are no function calls, interrupt handlers or push/pop instructions in the M0 code. This change saves four cycles by eliminating loads of the addresses for the offset & tx registers, plus a further two by eliminating the need to stash one of these addresses in r8.