Residency and addressing mode of programs

The control program ensures that each load module is loaded above or below 16 megabytes virtual as appropriate and that it is invoked in the correct addressing mode (24-bit or 31-bit). The placement of the module above or below 16 megabytes depends on the residency mode (RMODE) that you define for the module. Whether a module executes in 24-bit or 31-bit addressing mode depends on the addressing mode (AMODE) that you define for the module.

When a program is executing in 24-bit addressing mode, the system treats both instruction and data addresses as 24-bit addresses. This allows programs executing in 24-bit addressing mode to address 16 megabytes (16,777,216 bytes) of storage. Similarly, when a program is executing in 31-bit addressing mode, the system treats both instruction and data addresses as 31-bit addresses. This allows a program executing in 31-bit addressing mode to address 2 gigabytes (2,147,483,648 bytes or 128 x 16 megabytes) of storage.

You can define the residency mode and the addressing mode of a program in the source code. Figure 1 shows an example of the definition of the AMODE and RMODE attributes in the source code. This example defines the addressing mode of the load module as 31-bit and the residency mode of the load module as 24-bit. Therefore, the program will receive control in 31-bit addressing mode and will reside below 16 megabytes.

Figure 1. Assembler Definition of AMODE/RMODE

SAMPLE CSECT
SAMPLE AMODE  31
SAMPLE RMODE  24

The assembler places the AMODE and RMODE in the external symbol dictionary (ESD) of the output object module for use by the linkage editor. The linkage editor passes this information on to the control program through the directory entry for the partitioned data set (PDS) that contains the load module and the composite external symbol dictionary (CESD) record in the load module. You can also specify the AMODE/RMODE attributes of a load module by using linkage editor control cards. Understanding 31-bit addressing contains additional information about residency and addressing mode; z/OS MVS Program Management: User's Guide and Reference and z/OS MVS Program Management: Advanced Facilities contain information about the linkage editor control cards.