A PowerPC instruction a day series: 1

Notice that all instructions that compute a value use the first operand as the destination register. In all of these instructions the registers are specified only by their number. For example, the instruction for loading the number 12 into register 5 is li 5, 12. We know that 5 refers to a register and 12 refers to the number 12 because of the instruction format -- there is no other indicator.
Each PowerPC instruction is 32 bits long. The first six bits determine the instruction and the remaining portions have different functions depending on the instruction. The fact that they are fixed-length allows the processor to process them more efficiently. However, the limitation to 32 bits can cause a few headaches, which we will encounter. The solutions to most of these headaches will be discussed in part 2.
Many of the instructions above make use of the PowerPC extended mnemonics. This means that they are actually specializations of a more general instruction. For example, all of the conditional branches mentioned above are actually specializations of the bc (branch conditional) instruction. The form of the bc instruction is bc MODE, CBIT, ADDRESS. The CBIT is the bit of the condition register to test. The MODE has lots of interesting uses, but for simple uses you set it to 12 if you want to branch if the condition bit is set, and 4 if you want to branch if the condition bit is not set. Some of the important condition register bits are 8 for less than, 9 for greater than, and 10 for equal. Therefore, the instruction beq ADDRESS is really bc 12, 10 ADDRESS. Likewise li is a specialized form of addi and mr is a specialized form of or. These extended mnemonics help make PowerPC assembly language programs more readable and writable for simpler programs, while not removing the power available for more advanced programs and programmers.