Documentation

A source register an encoding names, by the field it is read from: a general-purpose register at 64 or 32 bits (field value 31 the zero register, or the stack pointer where the encoding says so), a SIMD&FP register at a fixed width, a vector whose width is the encoding's Q bit (bit 30), or a SIMD&FP register read whole where nothing fixes its width.

An encoding the contract covers; one it does not that is control flow or system, which reads no data the contract speaks to; or one it does not, with its source registers, each by the low bit of its field.

An encoding, by its name in the specification, its fixed bits, and what the contract says of it.