9.29.2.3 Register Names

General-purpose registers are represented by predefined symbols of the form rN (for global registers), where N represents a number between 0 and 15. The leading letters may be in either upper or lower case; for example, r13 and R7 are both valid register names.

Register names PC, SP and SR cannot be used as register names and will be treated as variables. Use r0, r1, and r2 instead.