1. A computer program product for controlling execution of instructions within transactions of a computing environment, a transaction effectively delaying committing transactional stores to main memory until completion of a selected transaction, said computer program product comprising: a non-transitory computer readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: obtaining, by a processor, a machine instruction for execution, the machine instruction being defined for computer execution according to a computer architecture, the machine instruction comprising: an operation code to specify a transaction begin operation; and at least one control to be used in controlling execution of one or more types of instructions in transactional processing; and executing, by the processor, the machine instruction, the executing comprising: initiating a transaction; and using a first control of the at least one control to determine a first value, the first value to control execution within the transaction of an instruction of a first type.