9.5.2.2 Register Names ¶

The AVR has 32 x 8-bit general purpose working registers ‘r0’, ‘r1’, ... ‘r31’ (‘r16’, ... ‘r31’ on the Reduced Core). Six of these registers can be used as three 16-bit address registers for data addressing. One of the these address registers can also be used as an address pointer for lookup tables in Flash memory. These added function registers are the 16-bit ‘X’, ‘Y’ and ‘Z’ registers.

X = r26:r27
Y = r28:r29
Z = r30:r31

• The low part of the X register, r26, can be written as ‘XL’. The high part can be written as ‘XH’. Similar notations exist for the parts of the Y and Z registers.
• Registers can be accessed by their register number and without the ‘r’ register prefix. For example, ‘20+1’ denotes r21.
• Both lowercase and uppercase register names are recognized.