is sketch loaded at address 0 or at high address on UNO?

doughboy:
Thanks for the reply.

My sketch size is less than 30720 and I get the error. As far as I can tell, using trial and error, my upload fails if size greater than 28672 bytes.

Are all UNO boards, including clones, use the 512 byte optiboot bootloader?

I believe so

How about chips sold by adafruit which claims to have their own bootloader, does that take up 2k or 512 bytes?

I think the Uno's bootloader is the only 512 byte size, all other arduino type bootloaders for the 328p chip use 2K, at least that is my understanding.
Lefty