220.127.116.11 Register names
Local and global registers are specified as $0 to $255. The recognized special register names are rJ, rA, rB, rC, rD, rE, rF, rG, rH, rI, rK, rL, rM, rN, rO, rP, rQ, rR, rS, rT, rU, rV, rW, rX, rY, rZ, rBB, rTT, rWW, rXX, rYY and rZZ. A leading : is optional for special register names.
Local and global symbols can be equated to register names and used in place of ordinary registers.
Similarly for special registers, local and global symbols can be used.
Also, symbols equated from numbers and constant expressions are allowed in
place of a special register, except when either of the options
specified. Then only the special register names above are allowed for the
instructions having a special register operand;