Uninformed: Informative Information for the Uninformed

Vol 1» 2005.May


Branching to a forward address without using NULL bytes can be tricky on PowerPC systems. If you try branching forward, but less than 256 bytes, your opcode will contain a NULL. If you obtain your current address and want to branch to an offset from it, you will need to place the target address into the count register (ctr) or the link register (lr). If you decide to use the link register, you will notice that every valid form of "blr" has a NULL byte. You can avoid the NULL byte by setting the branch hint bits (19-20) to "11" (unpredictable branch, do not optimize). The resulting opcode becomes 0x4e804820 instead of 0x4e800020 for the standard "blr" instruction.

The branch prediction bit (bit 10) can also come in handy, it is useful if you need to change the second byte of the branch instruction to a different character. The prediction bit tells the processor how likely it is that the instruction will result in a branch. To specify the branch prediction bit in the assembly source, just place '-' or '+' after the branch instruction.