Control Unit: Features & Working

The Control Unit is an integral component of the control processing unit (CPU) which interprets and directs instructions of operations from the main memory. 

• It is a circuitry that serves as an intermediary between the processor (CPU) and output devices. 
• The control unit regulates the flow of data between CPU and other parts of the computer system. 
• In essence, the control unit plays a substantial role in efficient working of the computer system.
• Control unit is a vital part of the processor which administers the route of operations and guides the other components of the CPU on how to react to commands given.

Key Terms: CPU, Control Unit, Processor, Memory, ALU

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Control Unit in Computer Architecture 

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The control unit was first introduced in the von nuemann's architecture in 1945 published by John von nuemann. Von nuemann's architecture is a hardware mechanisms which consisted of a control unit, Arithmetic Logic unit, Registers, Memory unit and inputs/ output. 

• The operations of the Arithmetic Logic unit (ALU),memory and input/output devices are carried out under the supervision of the Control unit. 
• The control unit is in charge of extracting the source program instructions, delivering it to the processor instruction register and forging a control signal. 
• According to the contents of registers, a control signal is produced and sent to the concerned execution unit. 
• Design and development of control units is cheaper,simplified and faster than that of other hardware mechanisms such as harvard architecture. 
• Therefore this mechanism is still relevant and the control unit is widely found in modern computers.

Control Unit in Computer Architecture

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Functions of a Control Unit 

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Functions of Circuit Unit are mentioned below:

• It accepts external directions and commands to convert them into comprehensible series of control signals valid for the processor.
• The control unit interprets the commands and instructions and forwards them to the concerned subunits of the processor. 
• It maintains the sequence of instructions and data being regulated in the processor and prevents overlapping or miscommunication.
• It governs the working of several execution units of the CPU (ALU, registers,etc)
• It also carries out a number of activities such as fetching data, decoding instructions, handling commands, execution and storing of results.

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Types of Control Unit

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Control unit can be of two types based on different ways of execution of instructions. They can be either hardwired or Micro programmed.

Hardwired Control Unit

A combination of circuits formed by interconnecting intricate physical components like flip-flops and gates is utilized to carry out instructions. This fixed set of logic gates and circuits is called a hardwired control unit. 

• The control signal for each instruction is hardwired into the control unit. 
• Hence it is harder to upgrade and modify in comparison to micro programmable units as it would be impossible to make any changes in the production of control signals without physically modifying the circuit's structure. 

Structure of Hardwired Control Unit

A Hardwirеd Control Unit is constructеd using essential componеnts likе a sequence countеr, two dеcodеrs, flip-flops, and logic gatеs. Thе sеquеncе countеr has thе ability to count from 0 to 15 in binary. A 3 x 8 decoder is employed to еncodе thе operation codе found in bits 12 through 14, with its outputs dеsignatеd as D0 through D7.

• Bit 15 of thе opеration codе is dirеctеd to a flip-flop markеd as I.
• Thе control logic gatеs arе configured to rеspond to opеration codеs ranging from bits 0 to 11.
• Dеspitе its array of componеnts, thе dеsign of thе hardwirеd control unit is straightforward and conducivе to еasy implеmеntation.
• It dеlivеrs fast pеrformancе owing to its compact sizе and is also cost-effective.
• Howеvеr, it has limitations as it can only support a predetermined sеt of instructions, making it rigid and not еasily modifiablе. 

Micro Programmable Control Unit

Microprogrammablе control units arе control units that utilizе microcodes to execute commands. Microcodes consist of a sеt of low-lеvеl hardware instructions that implement highеr-lеvеl machinе codе instructions.

• Microcodе acts as an intеrmеdiary that translatеs machinе instructions, statе machinе data, or other inputs into detailed sеquеncеs of circuit-lеvеl operations.
• This separation bеtwееn machine instructions and underlying electronics grants grеatеr flеxibility in dеsigning and modifying instructions. 
• Furthеrmorе, it simplifies complеx multi-step instruction construction whilе rеducing thе intricacy of computеr circuits.
• Microcodеs arе typically written and stored in dedicated mеmory units known as microprograms, and thеy arе responsible for gеnеrating control signals for еach instruction.
• It's important to note that thеsе mеmory units arе inaccessible to programmеrs.
• Dеspitе being largеr in sizе and relatively morе expensive upfront, microprogrammablе control units oftеn provе to be cost-effective in thе long run. 

On the basis of control signals, microgrammable Control units are further divided into two categories- 

1. Horizontal Microprogrammed control unit: The control signals in horizontal microprogrammed control units are stored and represented in decoded binary format. The number of control signals is equal to the number of bits required. 

• For example if 44 control signals are found in the processor then 44 bits are required. 
• In this layout, multiple control signals can be generated at the same time. 
• As the control signals are already decoded, no additional hardware for decoding is required which facilitates faster operation. 
• It allows parallel processing applications and provides a higher degree of parallelism. It supports longer control words thus is more flexible.

2. Vertical microprogrammed control unit: In vertical microprogrammed control units, the control signals are found in encoded binary format. Therefore for x control units- Log2(x) bits are needed. Subsequently it required a hardware decoder to implement control signals thus it is slower than horizontal microprogrammed control units. 

• It provides a low degree of parallelism i.e. either 0 or 1 hence it supports short control words only. 
• Though it assists easier implementation of new control signals, it is still less flexible than horizontal microprogrammed control units but more flexible than hardwired control units. 

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How does a Control Unit Work?

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Control unit basically reads and ensures the smooth implementation of instructions. It oversees all the activities of the processor. It also directs and coordinates the operations of logic units,memory and input/output devices. Essentially, A control unit works to achieve efficient and well ordered execution of a program. When computer programs are loaded in the main memory, every instruction is stored in the address register. Now the role of CU comes into play. 

• First it decodes the address of first instruction through the address register. 
• After decoding the instruction is passed by CU through the buffer register and delivered to the instructions register (IR). 
• The instructions register, read the instructions and send it to the concerned units or circuits for execution. 
• Program counter, also known as the counter register is used to keep count of the number of instructions already executed 
• The control unit also controls the working of input/output devices. 
• It receives input from input devices through input and output registers. 
• Then it is stored in the main memory of the computer (RAM). 
• During program execution, if an instruction requires the input then the instruction register decodes the instruction and sends command signal to the main memory for retrieval of that input. 

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Things to Remember

• Control unit is a component of the computer's CPU. 
• It is concerned with conveying instructions to the different parts of the processor and ensuring smooth synchronisation.
• There are two types of control unit- hardwired and microprogrammed CU.
• Hardwired control units have control units physically embedded in the circuits. 
• Microprogrammed control units use microcodes for execution of complex instruction sets. 
• Hardwired CU is the fastest CU, then comes the vertical microprogrammed CU and horizontal microprogrammed control unit.
• Vertical microprogrammed CU is preferred over other CUs as it allows complex instruction sets and provides fair speed.