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Re. your questions, unfortunately it's strictly impossible with AVR.
You can only use program memory with in-system amounts but it depend
on your device.
If you want to use more than 64Kbyte, there are two possible methods.
One is you may use MEGA128 which have 128Kbyte in-system memory.
Another methods is, if you possible, seperate your program into two parts.
One is main or control logic and second is data. You can save your data,
like a fonts etc. in external memory.
Because AVR and many RISC MCUs only support internal program memory only. But they support exteranl data memories.
I hope it can be a answer.
I think you can't do like a 8051 because the adress pointer of AVR can't
reach adress of beyond in-system memory.
If you flash only for the data, as I mentioned before, you can use external
data memories like 24Cxxx/93Cxxx.
There are some big difference in program memory addressing between
8051 and AVR, it maybe make you confusion??
Hope to be a useful info.