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.