9.29.2.3 Register Names

General-purpose registers are represented by predefined symbols of the form ‘ r N ’ (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.