A Guide to PC Hardware Maintenance and Repair

1
Chapter 6 Understanding CPUs
2
Objectives

• To learn the basic structure of a CPU
• Get an understanding of a CPUs capabilities
• Learn what some of the advanced features of newer
  CPUs are and what they can do for you

3
The Lowly Transistor

• Based on the semiconductor
• A semiconductor acts as a resistor to current
  until the current reaches a threshold voltage,
  then it becomes a conductor.
• Until threshold voltage is reached the transistor
  is a switch turned off.
• When threshold voltage is reached, it is turned
  on.

4
The Micro-components of the CPU

• The Control Unit The component that directs the
  activities of all other components in the CPU.
• The Prefetch The component that locates and
  retrieves data as the CPU requires it.
• External Data Bus The wires that bring data into
  the CPU from the outside world.

5
More Micro-components

• The Instruction Cache and the Data Cache Storage
  areas for instructions that have been brought in
  by the prefetch, but not yet used by the CPU.
• The Arithmetic Logic Unit Performs basic math
  functions.
• The Floating Point Unit Performs advanced math
  functions.
• Registers A storage point for data and/or
  instructions that are in current use by the CPU.

6
Overview of Microcomponents
7
The External Data Bus

• Front-side bus
• Communicates with chipset and the local bus
  components of the expansion bus.
• Determines the bus speed of memory.
• Back-side bus
• Communicates with L2 cache.
• Speed varies with chipset.

8
The System Clock

• Works with the front-side bus to control I/O
  communication between the CPU and all devices and
  from device to device.
• The CPU works on a multiple of the system clock,
  while slower devices work on a sub-multiple.

9
The CPU and Memory

• The address bus of the CPU dictates how much
  memory it can address.
• But the address range of the chipset dictates
  overall system range.
• For the CPU to access data, that data must be in
  cache or in RAM.
• Cache reads are much faster.

10
Moving Data to the CPU

• The control unit says it needs a new command or a
  piece of data.
• The prefetch asks L1 cache if it has that data.
  If so, the data is moved to the registers. If
  not, the prefetch asks L2.
• If L2 doesnt have the data, the prefetch asks
  the northbridge to search RAM.

11
Moving Data to the CPU (cont.)

• If the data isnt found in RAM, the northbridge
  searches virtual memory.
• If virtual memory doesnt have it, the
  northbridge issues a request to the southbridge
  to request a search of the hard disk (or whatever
  drive the initiating program dictates).

12
CPU Form Factor

• Slots
• Slot 1
• Slot 2
• Slot A

13
CPU Form Factor (cont.)
PGA

• Sockets
• Pin Grid Array
• 386 and 486
• Staggered Pin Grid Array
• Pentiums and up
• ZIF
• VLIF

SPGA
14
Advanced CPU Capabilities

• Protected Mode
• Floating Point Unit
• Instruction Pipelining
• Superscalar Architecture
• Branch Prediction

• Speculative Execution
• Out of Order Execution
• Hyperthreading
• Streaming SIMD

15
Protected Mode

• Allowed the CPU to run programs or access data
  from extended memory
• Multiple programs could run on the same PC at the
  same time
• HEY! It was big news then. And we could not do
  what we do today without it.

16
Floating Point Operations

• Early CPUs could only add and subtract. They
  multiplied and divided by cascading addition and
  subtraction operations.
• They could only work with full integers.
• Anything beyond these functions required a
  separate processor called the Math Co-processor.
• The Floating Point Unit was a Math-Co built into
  the CPU.

17
Instruction Pipelining

• With early CPUs, an instruction could not enter
  the processors pipeline until the instruction
  ahead of it was all the way through. It was like
  kids in line having to wait till the one ahead
  reached the end of the hallway before proceeding.
• Instruction Pipelining allows an instruction to
  enter the CPU as soon as there are registers
  available, even if another instruction is still
  being processed.

18
Superscalar Architecture

• Until the release of the Pentium, CPUs only had
  one pipeline for instructions to follow.
• Superscalar architecture allows multiple
  pipelines.

19
Branch Prediction

• When processing a set of instructions, if the CPU
  got to a point where user input dictated what
  code it needed next, early CPUs had to wait for
  that input.
• Branch Prediction allows the CPU to pre-load
  lines of code from multiple subroutines.

20
Speculative Execution

• Takes Branch Prediction to the next level.
• It chooses the subroutine it calculates is most
  likely to be run next and processes a few
  commands.
• If its wrong, it flushes the registers and
  starts again. Nothing ventured, nothing gained.

21
Out of Order Execution

• Early CPUs had to wait until all commands in a
  routine were queued and properly sorted before
  the routine could run.
• OOE allowed the CPU to process the instructions
  in the order received and sort the results as
  needed.

22
Hyperthreading

• Early CPUs multi-tasked by time sharing the CPU.
• Hyperthreading allows multiple threads of the
  same program to run in different pipelines at the
  same time.

23
Streaming SIMD

• Single Instruction, Multiple Data
• Early CPUs had to run the same instruction over
  and over again if it applied to multiple data
  sets.
• Streaming SIMD allows the instruction to be run
  once and applied to multiple data sets.

24
CPU Cooling

• Convective
• Heat passively dissipates from the CPU.
• Heat fins attached over the CPU helped the
  process.
• CPU fans work even better.

25
Heatsinks and Cooling Fans
26
Liquid Cooling

• Liquid is pumped through tubes and passed across
  the heat sinks.
• Heated liquid then gets pumped through a
  fan-cooled grid.