VTAM® supports an application program in either 24- or 31-bit addressing mode, depending upon the addressing mode of the application program at the time the ACB is opened. The OPEN macroinstruction is used to identify the application program to VTAM.

Control block fields referenced by the OPEN and CLOSE macroinstruction can reside in either 24-bit or 31-bit storage. Use of 31-bit storage should be consistent with the addressing mode of the application program. MODE=31 must be coded on the OPEN and CLOSE macroinstructions if the ACB or other control blocks reside in 31-bit storage.

For an application program defined with an RMODE attribute of 24, the OPEN macroinstruction creates a parameter list in 24-bit storage; for an application program defined with an RMODE attribute of ANY or 31, the application program must build a parameter list in 24-bit storage and then issue the execute form of the OPEN macroinstruction. See OPEN—Open one or more ACBs, for a detailed description of the OPEN macroinstruction.