All PowerPC instructions are four bytes long. Whenever the processor calculates the destination address of a branch, the two low-order bits are ignored, so the actual two low-order bits are always 0 in the destination address (i.e., every instruction is word-aligned).
Bits 0-5 of an instruction are specified by the op code, and also indicate the form of the instruction (how to divide up the remaining bits of the instruction into fields, and the meaning of each field).
- "Assembler Language Reference for for POWER-based architectures". (includes both Power2 and PowerPC instruction sets).