Setting and checking the addressing mode

z/Architecture® provides three new Set Addressing Mode instructions that allow you to change addressing mode. The instructions are SAM24, which changes the current AMODE to 24, SAM31, which changes the current AMODE to 31, and SAM64, which changes the current AMODE to 64.

Starting with z/OS® V1R3, there are other ways for a program to be in AMODE 64:

If your program uses the assembler AMODE 64 statement, and is bound that way, then the load module is AMODE 64 and the system will give it control in AMODE 64.
You could use the binder AMODE(64) statement to define that your load module is AMODE 64 and the system will give it control in AMODE 64.
You could be a target PC routine and have set up the entry table entry to indicate that your routine is to be given control in AMODE 64.
Your interface could be via BASSM and you could have set up an 8-byte target with the last bit on; callers can then load and then issue BASSM. Your routine would then be entered in AMODE 64.

The AMODE bits in the PSW tell the processor what AMODE is currently in effect. You can obtain the current addressing mode of a program by using the Test Addressing Mode (TAM) instruction. In response, TAM sets a condition code based on the setting in the PSW; 0 indicates AMODE 24, 1 indicates AMODE 31, and 3 indicates AMODE 64.