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Design of control unit
Question 1) What is horizontal micro instruction?
Answer) Each line in a micro program is called a micro instruction. It is used to control the flow of data in the processor. It also controls the instruction execution. The micro instructions are classified into two classes: horizontal and vertical.
In a horizontal micro instruction, every bit in the control word is attached to the control line. As each bit in the control word is connected to the control line, the control bits are independent of each other. Hence, in horizontal microprogramming, a wider code word is required.
The horizontal micro-instructions do not use any encoding. Rather a sequential approach is used to specify an instruction. So, the horizontal micro instructions are much faster than the vertical micro instructions.
The horizontal micro instruction can be understood clearly from the diagram given below:
The difference between horizontal and vertical micro instructions can be understood from the diagram given below:
Question 2) What are the features of control words?
Answer) The control variables are called control words. The following are the features of the control words:
The control word is a string of 1’s and 0’s
The 1’s and 0’s in the control word represent the control variables which in turn represent the microinstructions.
Each step of the instruction execution is represented by a control word.
Each bit in the control signal represents the control signals.
If a bit in the control word is set to 1, it means that the corresponding control signal is required for the current instruction.
The control word is also known as the microinstruction.
Question 3) What are the functions of control unit?
Answer) The component which converts the user input into the CPU control signals is called the control unit. The functions of the control unit are given below:
The control unit generates the control signals based on the user instructions.
It controls the data flow in the CPU
The control unit interprets the CPU instructions by fetching, decode and executing them.
The control unit just transfers the data from one device to another. It does not have memory.
It controls the components in the CPU such as the registers, ALU etc.,
Question 4) How is control logic implemented in hard wired control?
Answer) In hardwired control, the control signals are generated by the hardware circuit. The basic block diagram of a hardwired control unit is shown below:
The opcode in the instruction is given as input to the instruction decoder. The instruction decoder is a combinational circuit which converts the opcode into a micro-memory address. This address consists of the corresponding micro-code for the given opcode. The output of the decoder is given to the control signal generation matrix which generates the required control signals.
The control signals that are generated by the Generation Matrix are then given to the CPU and the Next State Generator. In the next state generator, the control signals are combined with the timing signals.
The above process is continued till the END instruction of the program is executed.
Question 5) Explain micro programming control.
Answer) Microprogramming is the technique in which the bits in the control word are set to ‘1’ in order to generate the corresponding control signals.
A micro program is a set of micro instructions. It is stored in the Control Memory (ROM). The micro programs are almost as same as the machine language programs. The micro instructions specify the micro operations to be performed for the execution of a program.
The basic block diagram given below explains the basic working of the micro-programmed control unit.
Control Memory (ROM)
The control memory (ROM) is used to specify the address of the current micro instruction.
Control Data Register
The Control Data Register contains the microinstruction at the address specified by the Control Memory.
The Control Data Register is also called ‘pipeline register’, because it enables the execution of micro instructions simultaneously.
Next Address Generator
The Next Address Generator computes the address of the next instruction to be executed. The address of the next instruction is fed to the Control Address Register.
The Next Address Generator reads the address sequence from the Control Memory.
Control Address Register
The Control Address Register reads the micro instruction to be executed next.
Question 6) What is the difference between hard wired and programming control?
The control unit uses hardware circuits to generate the control signals.
The control unit uses instructions (software) to generate the control signals.
Does not require frequent memory access
Requires frequent memory access
It is faster than programming control
It is slower than hardwired control
More expensive. Because it involves the usage of hardware components.
Modification in the circuit is tough.
Modifications can be done easily.
Does not require a large instruction set.
Requires a large instruction set.
Can handle only simple instructions.
Can handle complex instructions.