System support of AMODE and RMODE

The following are examples of system support of AMODE and RMODE:

The system obtains storage for the module as indicated by RMODE.
ATTACH, ATTACHX, LINK, LINKX, XCTL, and XCTLX give the invoked module control in the addressing mode specified by its AMODE.
LOAD brings a module into storage based on its RMODE and sets bit 0 in register 0 to indicate its AMODE.
CALL passes control in the AMODE of the caller.
SYNCH or SYNCHX has an AMODE parameter that you can use to specify the AMODE of the invoked module.
For SVCs, the system saves and sets the addressing mode.
SRBs are dispatched in the addressing mode indicated by the SRB specified to the SCHEDULE macro.
The cross memory instructions PC and PT establish the addressing mode for the target program.
Access methods support all valid combinations of AMODE and RMODE with these exceptions:
- AMODE 64 and RMODE 64 are not supported unless otherwise documented for the interface.
- Some devices such as unit record devices and TSO terminals have documented restrictions.
Dumping is based on the AMODE specified in the error-related PSW.

Program Fetch

The system uses RMODE information from the PDS to determine whether to obtain storage above or below 16 megabytes.

ATTACH, ATTACHX, LINK, LINKX, XCTL, and XCTLX

Issuing an ATTACH or ATTACHX macro causes the control program to create a new task and indicates the entry point to be given control when the new task becomes active. If the entry point is a member name or an alias in the PDS. ATTACH or ATTACHX gives it control in the addressing mode specified in the PDS or in the mode specified by the loader. If the invoked program has the AMODE ANY attribute, it gets control in the AMODE of its caller.

The LINK, LINKX, XCTL, and XCTLX macros also give the invoked program control in the addressing mode indicated by its PDS for programs brought in by fetch or in the AMODE specified by the loader. The entry point specified must be a member name or an alias in the PDS passed by the loader, or specified in an IDENTIFY macro. If the entry point is an entry name specified in an IDENTIFY macro, IDENTIFY sets the addressing mode of the entry name equal to the addressing mode of the main entry point.

LOAD

Issuing the LOAD macro causes MVS™ to bring the load module containing the specified entry point name into virtual storage (if a usable copy is not already there). LOAD sets the high-order bit of the entry point address in register 0 to indicate the module's AMODE (0 for 24, 1 for 31), which LOAD obtains from the module's PDS entry. If the module's AMODE is ANY, LOAD sets the high-order bit in register 0 to correspond to the caller's AMODE.

LOAD places the module in virtual storage either above or below 16 megabytes as indicated by the module's RMODE, which is specified in the PDS for the module.

Specifying the ADDR parameter indicates that you want the module loaded at a particular location. If you specify an address above 16 megabytes, be sure that the module being loaded has the RMODE ANY attribute. If you do not know the AMODE and RMODE attributes of the module, specify an address below 16 megabytes or omit the ADDR parameter.

CALL

The CALL macro passes control to an entry point via BALR. Thus control is transferred in the AMODE of the caller. CALL does not change AMODE.

SYNCH or SYNCHX

Using the AMODE parameter on the SYNCH or SYNCHX macro, you can specify the addressing mode in which the invoked module is to get control. Otherwise, SYNCH or SYNCHX passes control in the caller's addressing mode.

SVC

For SVCs (supervisor calls), the system saves and restores the issuer's addressing mode and makes sure that the invoked service gets control in the specified addressing mode.

SRB

When an SRB (service request block) is dispatched, the system sets the addressing mode based on the high-order bit of the SRBEP field. This bit, set by the issuer of the SCHEDULE macro, indicates the addressing mode of the routine operating under the dispatched SRB.

PC and PT

For a program call (PC), the entry table indicates the target program's addressing mode. The address field in the entry table must be initialized by setting the high-order bit to 0 for 24-bit addressing mode or to 1 for 31-bit addressing mode.

The PC instruction sets up register 14 with the return address and AMODE for use with the PT (program transfer) instruction. If PT is not preceded by a PC instruction, the PT issuer must set the high-order bit of the second operand register to indicate the AMODE of the program being entered (0 for 24-bit addressing mode or 1 for 31-bit addressing mode).

Data Management Access Methods

User programs can be in AMODE 24 or AMODE 31 when invoking access methods except with certain devices. The non-VSAM access methods require certain parameter lists, control blocks and user exit routines to reside in virtual storage below 16 megabytes. Some interfaces support only AMODE 24 and some support AMODE 64. See z/OS DFSMS Using Data Sets and z/OS DFSMS Macro Instructions for Data Sets.

AMODE's Effect on Dumps

The only time AMODE has an effect on dumps is when data on either side of the address in each register is dumped. If the addresses in registers are treated as 24-bit addresses, the data dumped may come from a different storage location than when the addresses are treated as 31-bit addresses. If a dump occurs shortly after an addressing mode switch, some registers may contain 31-bit addresses and some may contain 24 bit addresses, but dumping services does not distinguish among them. Dumping services uses the AMODE from the error-related PSW. For example, in dumping the area related to the registers saved in the SDWA, dumping services uses the AMODE from the error PSW stored in the SDWA.