Registers R26...R31 have added functions besides their general-purpose usage.
These registers can form 16-bit address pointers for addressing data memory. These three address registers are called the X-register, Y-register, and Z-register. Load and store instructions can use all X-, Y- and Z-registers, while the LPM instructions can only use the Z-register. Indirect calls and jumps (ICALL and IJMP) also use the Z-register.
Refer to the instruction set or Instruction Set Summary for more information about how the X-, Y- and Z-registers are used.
The lowest register address holds the least-significant byte (LSB), and the highest register address holds the most-significant byte (MSB). In the different addressing modes, these address registers function as fixed displacement, automatic increment, and automatic decrement.