To avoid ASMA309W when referencing a low storage location using either a self-defining term (number) as the address or an expression which resolves to an offset, code an explicit base register of zero. For indexed instructions, IBM macros typically also code an explicit zero index register as well just to be clear.
Examples from SYS1.MACLIB:
L 15,CVTPTR(0,0)
L 15,X'10'(0,0)
L 15,PSATOLD-PSA(0,0)
Jonathan Scott, HLASM
IBM Hursley, UK
