There has been considerable recent interest in both hardware andsoftware transactional memory (TM). We present an intermediateapproach, in which hardware serves to accelerate a TM implementation controlled fundamentally by software. Specifically, we describe an alert on update mechanism (AOU) that allows a thread to receive fast, asynchronous notification when previously-identified lines are written by other threads, and a programmable data isolation mechanism (PDI) that allows a thread to hide its speculative writes from other threads, ignoring conflicts, until software decides to make them visible. These mechanisms reduce bookkeeping, validation, and copying overheads without constraining software policy on a host of design decisions. We have used AOU and PDI to implement a hardwareacceleratedsoftware transactional memory system we call RTM. We have also used AOU alone to create a simpler "RTM-Lite". Across a range of microbenchmarks, RTM outperforms RSTM, a publicly available software transactional memory system, by as much as 8.7x (geometric mean of 3.5x) in single-thread mode. At 16 threads, it outperforms RSTM by as much as 5x, with an average speedup of 2x. Performance degrades gracefully when transactions overflow hardware structures. RTM-Lite is slightly faster than RTM for transactions that modify only small objects; full RTM is significantly faster when objects are large. In a strongargument for policy flexibility, we find that the choice between eager (first-access) and lazy (commit-time) conflict detection can lead to significant performance differences in both directions, depending on application characteristics.