Nonconstrained transactions

A nonconstrained transaction begins with a TBEGIN instruction, ends normally with a TEND instruction, can be aborted (ended abnormally) with a TABORT instruction and may abort for many system-defined reasons.

The TBEGIN instruction may provide a "transaction diagnostics block" (TDB, mapped by macro IHATDB).

The instruction after the TBEGIN instruction usually is a conditional relative branch (BRC instruction) that can handle the condition codes with which a TBEGIN instruction may complete:

CC=0 (transaction initiation successful) should "fall through".
CC=1 (abort due to an indeterminate condition) should branch somewhere to deal with this situation.
CC=2 (abort due to a transient condition) should branch somewhere to deal with this situation (dealing with this might retry, but should eventually "time out" and go to the fall-back path). While there is no "right number" for the question "how many times should I retry", a small number, such as 5, is considered within reason in general.
CC=3 (abort due to a persistent condition) should branch somewhere to deal with this situation, eventually winding up in a "fall-back" path because for some reason, the system believes that the transaction is unlikely ever to succeed.
Note: This applies to the current circumstances. For example, a list search on a hash table might not succeed for the current hash, but for most other hashes might succeed.
This needs a fall-back path.

Upon transaction abort, control flows to the instruction after the TBEGIN instruction.