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8/6/2019 Computer System2
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Lecture 2
CPU BASICS
Central Processor Unit
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FigureFigure 1-1 Block Diagram of a Digital Computer
Memory
Input Device Output DeviceInterface Port
Or IOP
CPU
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A typical CPU has three major components:
yregister set
y
arithmetic logic unit (AL
U)y control unit (CU)
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A CPU contains very fast memory called registers.The register can do one of two things: it can
parallel load the inputs, that is, it can read in allfour inputs and store the four bits into the array.Each element of the array can store a single bit.
Register
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The black box shown has four inputs or (8, 16, 32 inputs).
Data Inputs
b3 b2 b1 b0
1 0 1 0
z3 z2 z1 z0
Outputs
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Register Types
The register set differs from one computerarchitecture to another. It is usually a
combination of general-purpose andspecial purpose registers.
General-purpose registers are used for
any purpose, hence the name generalpurpose. For example (A, B, C, D, R1, R2, R3)
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Special-purpose registers have specificfunctions within the CPU.
For example of special-purpose registers:
The program counter (PC) is a special-purpose register that is used to hold theaddress of the instruction to be executed
next.The instruction register (IR) which isused to hold the instruction that is currentlyexecuted.
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Other processor registers used by a program instruction.
Instruction Decoder (ID)
MAR (Memory Address Register)
MDR (Memory Data Register) .
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The ALU provides the circuitry needed toperform the arithmetic, logical and shift
operations demanded of the instructionset.
ALU
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The control unit is the entity responsiblefor fetching the instruction to be
executed from the main memory anddecoding and then executing it.
Control Unit
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A typical and simple execution cycle can be summarized as follows:
y The next instruction to be executed, whoseaddress is obtained from the PC, is fetched fromthe memory and stored in the IR.
y The instruction is decoded.y Operands are fetched from the memory and
stored in CPU registers, if needed.
y The instruction is executed.
y Results are transferred from CPU registers to thememory, if needed.
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The CPU fetches instructions frommemory, reads and writes data from and
to memory, and transfers data from andto input/output devices.
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Question
-List the main components of CPU.
-Define the following:
y Register
y ALU
y Control Unit