AVRs are 8-bit microprocessors. The operands for most instructions are single bytes.

For example, 2 8 bit values can be added with a single instruction. Summing 2 16-bit integers requires multiple instructions.

AVRs have 32 general purpose registers all 8 bits (r0, r1, r2 ... r30, r31)

AVRs have a number of special purpose registers including

• A 16-bit program counter (PC) - address in program memory of next instruction to be executed
• A 16-bit stack pointer (SP) - address in data space of area to save function call information
• A 8-bit status register (SREG) - indicates when conditions such as arithmetic overflow occur

Some instructions treat certain pair of registers as 16 bit quantities (e.g. to form an address).

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