What is Atmel AVR?

look at atmel.com

Hi!

Nothing special. But AVR is their own 8 bit RISC architecture.Just take a look at the description of the at90S8515.

But they also sell CPU's based on the 8051 (8 bit CISC - AT89) or ARM (32bit RISC AT91). And there is a product line with 4 bit.

Others have answered the "difference" question. WRT the "special" question:

- The instruction clock is 1:1 with the oscillator. Some other micros require four oscillator cycles for each instruction clock cycle. Some even require twelve.

- To go along with that, many frequently used instructions execute in a single instruction cycle, which results in a relatively fast chip.

- The processor register bank consists of 32 8-bit registers. There is some split in functionality (e.g., only registers 16 - 31 can use "immediate" instructions, like a literal load).

- It's a Harvard architecture with linear addressing up to 64 KB for both program and data address space. No banks or paging.

- In-system programmable with inexpensive (or even build-your-own) serial programmers.