1. A method of executing a machine instruction in a computing environment, the machine instruction for initiating a transaction, wherein stores by other processors to storage locations of the transaction cause the transaction to abort, and wherein stores by instructions in the transaction are effectively delayed until a selected transaction completes, said method comprising: obtaining, by a processor, a machine instruction for operation, 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 a control field to specify one or more controls to be used in transactional execution; and executing, by the processor, the machine instruction, the executing comprising: determining from a field of the instruction whether a transaction to be initiated by the transaction begin instruction is a constrained transaction or a nonconstrained transaction; based on the field being one value, initiating a constrained transaction, and setting a condition code to a value indicating successful execution of the machine instruction, said constrained transaction having one or more restrictions associated therewith, the one or more restrictions including at least one of a subset of general instructions is available, a limited number of instructions is to be executed, a limited number of storage-operand locations is to be accessed, or the constrained transaction is limited to a single nesting level; based on the field being another value, designating a storage location for storing diagnostic information in an aborted nonconstrained transaction, initiating the nonconstrained transaction, and setting a condition code to the value indicating successful execution of the machine instruction; and wherein the field indicates a nonconstrained transaction, and wherein the machine instruction further comprises a base field to be used in executing the transaction, and wherein based on the base field comprising one value, contents of a register specified by the base filed are combined with contents of a displacement field to create a first operand address, the first operand address designating a location of a transaction diagnostic block usable to store the diagnostic information on an abort.