As an ARM authorised reseller, we tend to use the ARM Realview tools (RVDS) whenever appropriate. With ARMs recent acquisition of Keil there is now another option available, the Microcontrol Development Kit - MDK. MDK is aimed at the lower-end market - ARM7, Cortex-M3, and some ARM9. It is also targeted towards either OS-less, or RTX-based designs. RTX is ARMs simple realtime OS. It includes all of the features normally supplied with a small embedded OS: Threads, Scheduler, Mutexing, Memory Pools, Mailboxes, Delays, Events etc... When using the RTX kernel and the MDK tools together, a large number of advanced debugging options become available.  The ability to trivially graph thread stack usage and thread execution time, which can make some aspects of profiling an application trivial. It also provides a full simulation system for a large number of CPUs (see the Keil Device List for more details). These simulated CPUs generally include the full peripheral suite as well, which makes prototype development easy, even in the absence of actual hardware. MDK also includes a full IDE, build system and large library of examples, making initial project creation a breeze. RVDS is targeted at a different market. It aims to support the latest ARM CPUs, with recent additions for the Cortex-A9. Its simulation model is geared more towards ARM cores rather than specific SOC chips, and as such it is not suitable for full system simulation. It can be used very well for algorithm simulation however, and provides all the optimisation feedback necessary for this purpose. Both of these platforms share the same compiler suite, RVCT. RVCT generally provides the best code generation and optimisation features of all the ARM compilers, certainly far better than the commonly used GCC compiler. It also supports a lot of the extensions provided by other tool chains, making it relatively easy to transition from one compiler to another. Generally, the decision between the two is quite apparent - if a more traditional embedded design is being done, such as an embedded control application, then a lower CPU, such as the Cortex-M3, or ARM7 will be chosen. In this case, MDK is more appropriate. If a more advanced design is to be used, involving a complex modern operating system such as Linux or WinCE and an ARM9 or greater CPU, then RVDS will provide faster code download & flexibility.