// effective address of memory is a base register value and an offset register value.
			if a.Type == obj.TYPE_MEM {
				a.Index = arm64.REG_UXTW + Rnum
			} else {
				a.Reg = arm64.REG_UXTW + Rnum
			}
		case "UXTX":
			if a.Type == obj.TYPE_MEM {
				return errors.New("invalid shift for the register offset addressing mode")
			}
			a.Reg = arm64.REG_UXTX + Rnum
		case "SXTB":
			if a.Type == obj.TYPE_MEM {

	ADDW	R2.SXTW, R10, R12               // 4cc1220b
	ADD	R19.UXTX, R14, R17              // d161338b
	ADDSW	R19.UXTW, R14, R17              // d141332b
	ADDS	R12.SXTX, R3, R1                // 61e02cab
	SUB	R19.UXTH<<4, R2, R21            // 553033cb
	SUBW	R1.UXTX<<1, R3, R2              // 6264214b
	SUBS	R3.UXTX, R8, R9                 // 096123eb
	SUBSW	R17.UXTH, R15, R21              // f521316b

	ADDSW	R7->32, R14, R13                                 // ERROR "shift amount out of range 0 to 31"
	ADD	R1.UXTB<<5, R2, R3                               // ERROR "shift amount out of range 0 to 4"
	ADDS	R1.UXTX<<7, R2, R3                               // ERROR "shift amount out of range 0 to 4"
	ADDS	R5, R6, RSP                                      // ERROR "illegal destination register"