Notations WI and XI refer to the same register, the former as a 32-bit register and the latter as a complete, 64-bit register. Most instructions that write to W register in fact operate on the complete register, sometimes sign-extending or zero-extending their results.

Except when registers appear as arguments of instructions, herd uses the “X” notation. However, quantities are printed by herd according to their types. In the absence of explicit type and of initial value, registers initial value is zero and have the implicit type int of a signed 32bit integer.