There is an example program \Keil\ARM\Examples\Blinky, which runs nicely with the Simulator mode of the debugger.
This uses the chip settings STM32F103RB.
But if I try to change this to STM32F407VG (or other STM32F4 chips I tried), the simulator does not run any more. It then somehow switches to a very strange single step mode - It does not come at the start of the main function (although I set the checkbox "Run to main()" in the Target page "Debug"), and if I try to run the program, it somehow runs only one step further - I cannot convince the simulator to run it correctly.