|Inside Intel Core Microarchitecture|
|Advanced Power Gating|
With advanced power gating, Core microarchitecture brought CPU power saving to a totally new level. This feature enables the CPU to shut down units that aren’t being used at the moment. This idea goes even further, as the CPU can shut down specific parts inside each CPU unit in order to save energy, to dissipate less power and to provide a greater battery life (in the case of mobile CPUs).
Another power-saving capability of Core microarchitecture is to turn on only the necessary bits in the CPU internal busses. Many of the CPU internal busses are sized for the worst-case scenario – i.e., the largest x86 instruction that exists, which is a 15-byte wide instruction (480 bits)*. So, instead turning on all the 480 data lanes of this particular bus, the CPU can turn on only 32 of its data lanes, all that is necessary for transferring a 32-bit instruction, for example.
* You can find yourself quite lost by this statement, since you were always told that Intel architecture uses 32-bits instructions, so further explanation is necessary in order to clarify this affirmation.
Inside the CPU what is considered an instruction is the instruction opcode (the machine language equivalent of the assembly language instruction) plus all its required data. This is because in order to be executed, the instruction must enter the execution engine ”completed“, i.e., together with all its required data. Also, the size of each x86 instruction opcode is variable and not fixed at 32 bits, as you may think. For example, an instruction like mov eax, (32-bit data), which stores the (32-bit data) into the CPU’s EAX register is considered internally as a 40-bit length instruction (mov eax translates into a 8-bit opcode plus the 32 bits from its data). Actually, having instruction with several different lengths is what characterizes a CISC (Complex Instruction Set Computing) instruction set.
If you want to learn more about this subject, read AMD64 Architecture Programmer’s Manual Vol. 3: General Purpose and System Instructions (even though Intel provides the same information on their Intel Architecture Software Developer’s Manual Vol. 2A, AMD explanation and diagrams are easier to understand).
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