... there is a core-type monitor (a simplistic one, providing data for a fancy user interface on PC) - which is exactly what you are looking for - as a part of the Keil suite, called FlashMon; but it's only part of the "professional" version (read, the $$$$ version).

On the other hand, a quite similar part from NXP (exPhilips), P89V51RD2, has this monitor "burned" into its bootFlash from factory (and they call it SoftICE); so you can use it with the "eval" version of Keil (if you can live with the limitations of the "eval" version).

JW

PS. Can you please fill in your location - your name is a very familiar one for me, I'd be very surprised if you don't come from near the very same place I was born (Lucenec)...