Hi Roberto,

I know you are coming from Z80 world. It was one of my favorite at his age, too.
During old times could't found technical suport better than present one.
In the past you have had to search and test the program with yours own force.

I believe it's not pleasure greater than one you will find, making yourself the program.
Think about old times and will get the point.
The people inside this forum will be glad to help you improve the code.
But give them a little bit from your thoughts.
The guru will feel more comfortable to explain you. Not me, since I'm not a guru.

Meanwhile try parsing the following docs:
http://dbserv.maxim-ic.com/appnotes....number=577
I just send the code described in document by email, since is no longer available after acquisition of Dallas by Maxim.

These belongs to Dallas Semiconductor.
With a little imagination and small code adjustments you can replace the 87C520's EPROM with 80C390's internal SRAM.
How you will load the code from external NVRAM to internal 4KB SRAM, forcing the program counter to point to the code fited inside SRAM in order to modify even the external code who helped you done all the things above......well this is piece of cake

You can start asking yourself :
where are vector table mapped when you execute the program from internal 4KB SRAM ?
( with IDM0 and IDM1, both bits set, during external NVRAM programing, starting with 0x00000 location)
Obvious if you want to use serial interrupt during downloading of code from PC to external NVRAM.

No ofence. The best exercise is practice.
Since you are not in rush (it's obvious looking at your hardware), you have plenty of time to proove programing skills.

If you are in real trouble I can send you the code, but believe me I would prefer to find yourself the solution. Even small part of it.
My teacher told me the same thing.

Best regards,
Silviu