1. A method for use for transaction processing on a computing system for managing memory access to a shared transactional memory location for transaction data of a first thread, the method comprising the steps of: decoding marking code for a string of instructions for execution on a processor of said computing system and determining whether said marking code marks the beginning of an active atomic instruction group (AIG) of instructions associated with a transaction of a thread, (the marks are made by insertion in the code explicitly by programmers in a high level language, added by compliers or translators as implementations of high level locking functions, added by a library call, included by a special even at runtime) (The instructions or marks may be generated by firmware, hardware or a combination of both as a response to decoding a particular sequence of instructions or receiving a special command to enter a new mode of execution.) when it is determined that the marking code marks the beginning of an active instruction group (AIG) of instructions associated with a transaction of a thread, said processor enters a millicode mode and begins execution of millicode routines written for supporting transactional memory execution.