How a 16-bit AVR Instruction is Built

In 8-bit AVR microcontrollers (like ATmega328P on Arduino Nano), instructions are stored as 16-bit words inside the Flash memory. Let's see how the LDI (Load Immediate) instruction divides these 16 bits behind the scenes.

Example with LDI R17, 20 (where Hex 20 is binary 0010 0000 and R17 becomes index 1):
The logical 16-bit word is 1110 0010 0001 0000, which translates to the hex value E210.

⚠️ The Little Endian Reality:
Because AVR architecture uses the Little Endian format, the microcontroller stores the lowest byte first in memory. Therefore, the final 2 bytes written into your .hex file will be flipped: 10 E2!

The official AVR binary template for LDI Rd, K is:
1 1 1 0 / K K K K / d d d d / K K K K

Bits 15-12	Bits 11-8	Bits 7-4	Bits 3-0
1 1 1 0	K K K K	d d d d	K K K K
Opcode (E)	Value (High)	Register Bits	Value (Low)

• Bits 15-12 (1110 = Hex 'E'): This is the fixed Opcode for LDI. Every instruction in AVR has its own unique binary prefix (Opcode) so the CPU control unit can recognize it instantly during the fetch phase:
  • 1110 (Hex E) is the fixed prefix for LDI.
  • 0110 (Hex 6) is the fixed prefix for ORI.
  • 0000 11 (6-bit prefix) is the fixed opcode for ADD.
• Bits 7-4 (dddd): This represents the Destination Register. Since LDI only works with registers R16 to R31, the hardware subtracts 16. So, R17 becomes index 1 (binary 0001).
• Bits 11-8 and 3-0 (KKKK KKKK): This is the 8-bit Operand value. The microcontroller splits it into two chunks of 4 bits to fit them around the register bits!