Tracing branch instructions

System tracing allows you the option of tracing branch instructions, such as BALR, BASR, BASSM and BAKR, along with other system events. The mode tracing option is separate from branch tracing.

Attention: Branch tracing ON can affect your system performance and use very large amounts of storage. Do not use branch tracing as the default for system tracing on your system. Only use it for short periods of time to solve a specific problem. The default system tracing does not include branch instructions.

IBM® provides two health checks, SYSTRACE_BRANCH and SYSTRACE_MODE, to help identify when branch tracing may be affecting your system performance. For example, if branch tracing has been set on for a long time, you will receive message IEAH801E. For more information, see System trace checks (IBMSYSTRACE) in IBM Health Checker for z/OS User's Guide . For information about related messages, see z/OS MVS System Messages, Vol 6 (GOS-IEA).

When you want to trace branch instructions such as BALR, BASR, BASSM and BAKR, do the following:

Restart system tracing with branch tracing using the TRACE command from a console with master authority:
```
TRACE ST,BR=ON
```
Because tracing branch instructions tends to significantly increase the number of trace entries being generated, you can increase the size of the trace tables when you turn tracing on.
- To increase the size of the trace tables for each processor from the default 1 MB to 2 MB, issue:
```
TRACE ST,2M,BR=ON
```
- To increase the size of total storage for trace buffers (that is, the sum of the storage set aside for trace table entries on all the installed processors) to 2 megabytes:
```
TRACE ST,BUFSIZ=2M,BR=ON
```

For more information, see the following documentation:

z/Architecture® Principles of Operation describes the branch instruction trace entries and the mode trace entries that MVS™ combines with them (and are generated by the hardware). MVS enables or disables the production of these unformatted entries by manipulating control register bits by the instruction. The trace table entries that are not 'branch (or mode)' entries that are generated by MVS software through the TRACE or TRACG instructions. See the Tracing topic for information.
For a description of the TTE from mapping macro IHATTE, see z/OS® MVS Data Areas in z/OS Internet library.