Central Processing Unit (CPU) | Computer Architecture

CPU in Computer Architecture – The central processing unit (CPU) is an unit of a compute system which performs a variety of function given by the instructions. Those are incorporated in the computer. The central processing unit perform the bulk of data processing operations in the computer. It is made up of three major parts:

1. Arithmetic Logic Unit (ALU)
2. Register Array
3. Control Unit

The ALU performs the required micro operations for executing an instruction. The register array stores the immediate data used during the execution of the given instruction. The control units is responsible to supervise the timing of information transfer between registers and ALU.

Arithmetic Logic Unit (ALU)

The ALU provides arithmetic and logic operations dictated by the incorporated instruction. In addition, the CPU can also be needed by the shift operations, the shifter circuit may be placed in the input of the ALU. In some cases shift operations are included with ALU.

Now this unit is named as arithmetic logic shift unit. The arithmetic, logic and shift circuits are introduced in pervious sections. There circuits are combined into one ALU with common section variables. One stage of an arithmetic logic shift units is shown in below figure. The subscript i designates a typical stage.

The inputs A_i and B_i are connected to both the arithmetic and logic circuits. An operation is selected by the selection lines S₁ and S₀. A 4:1 multiplexer at the output chooses between an arithmetic output, a logic output, previous input A_i-1, and next input A_i+1.

Two selection lines S₂ and S₃ are used to select the particular input at output of multiplexer. The circuit of Tri-state buffer based bus architecture must be repeated n times for an n-bit ALU. The function table for arithmetic logic shift unit is given in below table.

Selection Line S3	Selection Line S2	Selection Line S1	Selection Line S0	Carry (Cin)	Register Transfer (RTL)	Micro Operation
0	0	0	0	0	F ← A	Transfer A
0	0	0	0	1	F ← A+1	Increment A
0	0	0	1	0	F ← A+B	Addition
0	0	0	1	1	F ← A+B+1	Addition with carry
0	0	1	0	0	F ← A+B	Subtraction with borrow
0	0	1	0	1	F ← A+B+1	Subtraction
0	0	1	1	0	F ← A-1	Decrement A
0	0	1	1	1	F ← A	Transfer A
0	1	0	0	x	F ← A ∧ B	AND
0	1	0	1	x	F ← A ∨ B	OR
0	1	1	0	x	F ← A ⊕ B	Ex-OR
0	1	1	1	x	F ← A	Complement A
1	0	x	x	x	F ← shr A	Shift right A
1	1	x	x	x	F ← shl A	Shift left A

Register Array

The central processing unit needs the memory locations for storing pointers, counters, returning addresses, temporary and partial results. For such applications, referring to the main memory is time consuming. It is more convenient and more efficient to store these intermediate values in processor itself.

This necessity develops the concepts of register array in CPU. A large number of registers are included in CPU. It is not efficient to connect them through a common bus. The registers communicate with each other not only for direct data transfer, but also to perform various micro operations.

Control Unit

Control unit of CPU controls the data flow through the register and ALU by selecting the various components in the system. There are two types of control units:

1. Hard wired control unit
2. Micro programmed control unit

In the hard wired control unit, the control logic is implemented with gates, flip flops, decoders, and other digital circuits. The hard wired control unit can produce a fast mode of operation. It is the major advantage of this type of control unit. The limitation of this control unit is that as the name implies, it requires changes in the wiring among the various components if the design has to be modified.

In micro programmed control unit the control information is stored in control memory. The control memory is programmed to initiate the required sequence of micro operations. In micro programmed control unit, any required changes or modification can be done by updating the micro program in control memory.

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