How to change addressing mode

To change addressing mode you must change the value of the PSW A-mode bit. The following list includes all the ways to change addressing mode.

The mode setting instructions BASSM and BSM.
Macros (ATTACH, ATTACHX, LINK, LINKX, XCTL, or XCTLX). The system makes sure that routines get control in the specified addressing mode. Users need only ensure that parameter requirements are met. MVS™ restores the invoker's mode on return from LINK or LINKX.
SVCs. The supervisor saves and restores the issuer's addressing mode and ensures that the service routine receives control in the addressing mode specified in its SVC table entry.
SYNCH or SYNCHX with the AMODE parameter to specify the addressing mode in which the invoked routine is to get control.
The CIRB macro and the stage 2 exit effector. The CIRB macro is described in z/OS MVS Programming: Authorized Assembler Services Guide and z/OS MVS Programming: Authorized Assembler Services Reference ALE-DYN.
A PC, PT, or PR instruction. These three instructions establish the specified addressing mode.
An LPSW instruction (not recommended).

The example in Figure 1 illustrates how a change in addressing mode in a 24-bit addressing mode program enables the program to retrieve data from the ACTLB control block, which might reside above 16 megabytes. The example works correctly whether or not the control block is actually above 16 megabytes. The example uses the BSM instruction to change addressing mode. In the example, the instruction L 2,4(,15) must be executed in 31-bit addressing mode. Mode setting code (BSM) before the instruction establishes 31-bit addressing mode and code following the instruction establishes 24-bit addressing mode.

Figure 1. Mode Switching to Retrieve Data from Above 16 Megabytes

USER     CSECT
USER     RMODE 24
USER     AMODE 24
         L   15,ACTLB
         L   1,LABEL1   SET HIGH-ORDER BIT OF REGISTER 1 TO 1
                        AND PUT ADDRESS INTO BITS 1-31
         BSM 0,1        SET AMODE 31 (DOES NOT PRESERVE AMODE)
LABEL1   DC  A(LABEL2 + X'80000000')
LABEL2   DS  0H
         L   2,4(,15)   OBTAIN DATA FROM ABOVE 16 MEGABYTES
         LA  1,LABEL3   SET HIGH-ORDER BIT OF REGISTER 1 TO 0
                        AND PUT ADDRESS INTO BITS 1-31
         BSM 0,1        SET AMODE 24 (DOES NOT PRESERVE AMODE)
LABEL3   DS  0H