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.