The AVR has 32 x 8-bit general purpose working registers r0, r1, ... r31. Six of the 32 registers can be used as three 16-bit indirect address register pointers for Data Space addressing. One of the these address pointers can also be used as an address pointer for look up tables in Flash program memory. These added function registers are the 16-bit X, Y and Z - registers.
X = r26:r27 Y = r28:r29 Z = r30:r31