code: function(name) {
          if (!this.isValidRegister(name)) {
            return this.unamedRegister;
          }
          name = name.toLowerCase();
          if (!this.registers[name]) {
            this.registers[name] = new Register();
          }
          return this.registers[name];
        }
docs: Gets the register named @name.  If one of @name doesn't already exist, create it.  If @name is invalid, return the unamedRegister.