Hi Michael,

Thanks for the reply. I hadn't looked in detail at the different non-volatile serial memories I could use yet - a large fast SPI flash part definitely sounds like what I want.

To be honest, the main reason I was looking at using the AT89S52 as the MCU is that it is the part that my students are familiar with from the labs on the microprocessors course I teach at my university. We have several test boards in the labs that use them so I thought it would help the students to stick with what they know initially for prototyping. That unfortunately means software emulation of the SPI or I2C protocol for talking to a serial memory so the boot will be slow but if they can prove the general concept then that's a good start and an alternative part with built in SPI support can be chosen later in the project.

Some of my other students have looked at Silabs parts with built in XDATA for their projects. However, from the data sheets it looked like their internal XDATA space cannot be set to be addressed as code so I figured they would not be appropriate for this project. Are there some MCS-51 derivatives that have built in XDATA memory that can be addressed as code that you could point me to?

I take your point about keeping a common base section of code so that the handlers/drivers don't need to be duplicated in each application - the plan was to have a main OS program that would do that job, exposing an API for the other applications that are loaded into higher parts of the code space. However, my intention was to keep that OS code in the serial memory along with the applications so that the OS can be updated in the field via a MIDI port as I mentioned in my reply to Kai (http://8052.com/forum/read/174105). The boot program would contain just enough code to load the OS into XDATA then switch over to the OS or, given the right circumstances (maybe pressing a button while the device boots) would also allow an OS update via the MIDI port using a sysex message.

Best wishes

Chris