Introduction of Control Unit and its Design
Last Updated :
28 Dec, 2024
A Central Processing Unit is the most important component of a computer system. A control unit is a part of the CPU. A control unit controls the operations of all parts of the computer but it does not carry out any data processing operations.
What is a Control Unit?
The Control Unit is the part of the computer's central processing unit (CPU), which directs the operation of the processor. It was included as part of the Von Neumann Architecture by John von Neumann. It is the responsibility of the control unit to tell the computer's memory, arithmetic/logic unit, and input and output devices how to respond to the instructions that have been sent to the processor. It fetches internal instructions of the programs from the main memory to the processor instruction register, and based on this register contents, the control unit generates a control signal that supervises the execution of these instructions. A control unit works by receiving input information which it converts into control signals, which are then sent to the central processor. The computer's processor then tells the attached hardware what operations to perform. The functions that a control unit performs are dependent on the type of CPU because the architecture of the CPU varies from manufacturer to manufacturer.
Examples of devices that require a CU are:
- Control Processing Units(CPUs)
- Graphics Processing Units(GPUs)
Functions of the Control Unit
- It coordinates the sequence of data movements into, out of, and between a processor's many sub-units.
- It interprets instructions.
- It controls data flow inside the processor.
- It receives external instructions or commands to which it converts to sequence of control signals.
- It controls many execution units(i.e.ALU , data buffers and registers ) contained within a CPU.
- It also handles multiple tasks, such as fetching, decoding, execution handling and storing results.
Types of Control Unit
There are two types of control units:
- Hardwired
- Micro programmable control unit.
Hardwired Control Unit
In the Hardwired control unit, the control signals that are important for instruction execution control are generated by specially designed hardware logical circuits, in which we can not modify the signal generation method without physical change of the circuit structure. The operation code of an instruction contains the basic data for control signal generation. In the instruction decoder, the operation code is decoded. The instruction decoder constitutes a set of many decoders that decode different fields of the instruction opcode.
As a result, few output lines going out from the instruction decoder obtains active signal values. These output lines are connected to the inputs of the matrix that generates control signals for execution units of the computer. This matrix implements logical combinations of the decoded signals from the instruction opcode with the outputs from the matrix that generates signals representing consecutive control unit states and with signals coming from the outside of the processor, e.g. interrupt signals. The matrices are built in a similar way as a programmable logic arrays.
Control signals for an instruction execution have to be generated not in a single time point but during the entire time interval that corresponds to the instruction execution cycle. Following the structure of this cycle, the suitable sequence of internal states is organized in the control unit. A number of signals generated by the control signal generator matrix are sent back to inputs of the next control state generator matrix.
This matrix combines these signals with the timing signals, which are generated by the timing unit based on the rectangular patterns usually supplied by the quartz generator. When a new instruction arrives at the control unit, the control units is in the initial state of new instruction fetching. Instruction decoding allows the control unit enters the first state relating execution of the new instruction, which lasts as long as the timing signals and other input signals as flags and state information of the computer remain unaltered.
A change of any of the earlier mentioned signals stimulates the change of the control unit state. This causes that a new respective input is generated for the control signal generator matrix. When an external signal appears, (e.g. an interrupt) the control unit takes entry into a next control state that is the state concerned with the reaction to this external signal (e.g. interrupt processing).
The values of flags and state variables of the computer are used to select suitable states for the instruction execution cycle. The last states in the cycle are control states that commence fetching the next instruction of the program: sending the program counter content to the main memory address buffer register and next, reading the instruction word to the instruction register of computer. When the ongoing instruction is the stop instruction that ends program execution, the control unit enters an operating system state, in which it waits for a next user directive.
Micro Programmable control unit
The fundamental difference between these unit structures and the structure of the hardwired control unit is the existence of the control store that is used for storing words containing encoded control signals mandatory for instruction execution. In microprogrammed control units, subsequent instruction words are fetched into the instruction register in a normal way. However, the operation code of each instruction is not directly decoded to enable immediate control signal generation but it comprises the initial address of a microprogram contained in the control store.
- With a single-level control store: In this, the instruction opcode from the instruction register is sent to the control store address register. Based on this address, the first microinstruction of a microprogram that interprets execution of this instruction is read to the microinstruction register . This microinstruction contains in its operation part encoded control signals, normally as few bit fields. In a set microinstruction field decoders, the fields are decoded. The microinstruction also contains the address of the next microinstruction of the given instruction microprogram and a control field used to control activities of the microinstruction address generator.
The last mentioned field decides the addressing mode (addressing operation) to be applied to the address embedded in the ongoing microinstruction. In microinstructions along with conditional addressing mode, this address is refined by using the processor condition flags that represent the status of computations in the current program. The last microinstruction in the instruction of the given microprogram is the microinstruction that fetches the next instruction from the main memory to the instruction register.
- With a two-level control store: In this, in a control unit with a two-level control store, besides the control memory for microinstructions, a nano-instruction memory is included. In such a control unit, microinstructions do not contain encoded control signals. The operation part of microinstructions contains the address of the word in the nano-instruction memory, which contains encoded control signals. The nano-instruction memory contains all combinations of control signals that appear in microprograms that interpret the complete instruction set of a given computer, written once in the form of nano-instructions.
In this way, unnecessary storing of the same operation parts of microinstructions is avoided. In this case, microinstruction word can be much shorter than with the single level control store. It gives a much smaller size in bits of the microinstruction memory and, as a result, a much smaller size of the entire control memory. The microinstruction memory contains the control for selection of consecutive microinstructions, while those control signals are generated at the basis of nano-instructions. In nano-instructions, control signals are frequently encoded using 1 bit/ 1 signal method that eliminates decoding.
Advantages of a Well-Designed Control Unit
- Efficient instruction execution: A well-designed control unit can execute instructions more efficiently by optimizing the instruction pipeline and minimizing the number of clock cycles required for each instruction.
- Improved performance: A well-designed control unit can improve the performance of the CPU by increasing the clock speed, reducing the latency, and improving the throughput.
- Support for complex instructions: A well-designed control unit can support complex instructions that require multiple operations, reducing the number of instructions required to execute a program.
- Improved reliability: A well-designed control unit can improve the reliability of the CPU by detecting and correcting errors, such as memory errors and pipeline stalls.
- Lower power consumption: A well-designed control unit can reduce power consumption by optimizing the use of resources, such as registers and memory , and reducing the number of clock cycles required for each instruction.
- Better branch prediction: A well-designed control unit can improve branch prediction accuracy, reducing the number of branch mispredictions and improving performance.
- Improved scalability: A well-designed control unit can improve the scalability of the CPU, allowing it to handle larger and more complex workloads.
- Better support for parallelism: A well-designed control unit can better support parallelism, allowing the CPU to execute multiple instructions simultaneously and improve overall performance.
- Improved security: A well-designed control unit can improve the security of the CPU by implementing security features such as address space layout randomization and data execution prevention.
- Lower cost: A well-designed control unit can reduce the cost of the CPU by minimizing the number of components required and improving manufacturing efficiency.
Disadvantages of a Poorly-Designed Control Unit
- Reduced performance: A poorly-designed control unit can reduce the performance of the CPU by introducing pipeline stalls, increasing the latency, and reducing the throughput.
- Increased complexity: A poorly-designed control unit can increase the complexity of the CPU, making it harder to design, test, and maintain.
- Higher power consumption: A poorly-designed control unit can increase power consumption by inefficiently using resources, such as registers and memory, and requiring more clock cycles for each instruction.
- Reduced reliability: A poorly-designed control unit can reduce the reliability of the CPU by introducing errors, such as memory errors and pipeline stalls.
- Limitations on instruction set: A poorly-designed control unit may limit the instruction set of the CPU, making it harder to execute complex instructions and limiting the functionality of the CPU.
- Inefficient use of resources: A poorly-designed control unit may inefficiently use resources such as registers and memory, leading to wasted resources and reduced performance.
- Limited scalability: A poorly-designed control unit may limit the scalability of the CPU, making it harder to handle larger and more complex workloads.
- Poor support for parallelism: A poorly-designed control unit may limit the ability of the CPU to support parallelism, reducing the overall performance of the system.
- Security vulnerabilities: A poorly-designed control unit may introduce security vulnerabilities, such as buffer overflows or code injection attacks.
- Higher cost: A poorly-designed control unit may increase the cost of the CPU by requiring additional components or increasing the manufacturing complexity.
Similar Reads
Machine instructions and addressing modes
Computer Organization is like understanding the "blueprint" of how a computer works internally. One of the most important models in this field is the Von Neumann architecture, which is the foundation of most modern computers. Named after John von Neumann, this architecture introduced the concept of
6 min read
Computer organization refers to the way in which the components of a computer system are organized and interconnected to perform specific tasks. One of the most fundamental aspects of computer organization is the set of basic computer instructions that the system can execute.Basic Computer Instructi
6 min read
Instruction formats refer to the way instructions are encoded and represented in machine language. There are several types of instruction formats, including zero, one, two, and three-address instructions. Each type of instruction format has its own advantages and disadvantages in terms of code size,
11 min read
Based on the number of address fields, CPU organization is of three types: Single Accumulator organization, register based organization and stack based CPU organization.Stack-Based CPU OrganizationThe computers which use Stack-based CPU Organization are based on a data structure called a stack. The
4 min read
When we are using multiple general-purpose registers, instead of a single accumulator register, in the CPU Organization then this type of organization is known as General register-based CPU Organization. In this type of organization, the computer uses two or three address fields in their instruction
3 min read
The computers, present in the early days of computer history, had accumulator-based CPUs. In this type of CPU organization, the accumulator register is used implicitly for processing all instructions of a program and storing the results into the accumulator. The instruction format that is used by th
2 min read
Prerequisite - Basic Computer Instructions, Instruction Formats Problem statement: Consider a computer architecture where instructions are 16 bits long. The first 6 bits of the instruction are reserved for the opcode, and the remaining 10 bits are used for the operands. There are three addressing mo
7 min read
Addressing modes are the techniques used by the CPU to identify where the data needed for an operation is stored. They provide rules for interpreting or modifying the address field in an instruction before accessing the operand.Addressing modes for 8086 instructions are divided into two categories:
7 min read
Machine Instructions are commands or programs written in the machine code of a machine (computer) that it can recognize and execute. A machine instruction consists of several bytes in memory that tell the processor to perform one machine operation. The processor looks at machine instructions in main
5 min read
In assembly language as well as in low level programming CALL and JUMP are the two major control transfer instructions. Both instructions enable a program to go to different other parts of the code but both are different. CALL is mostly used to direct calls to subroutine or a function and regresses
5 min read
Simplified Instructional Computer (SIC) is a hypothetical computer that has hardware features that are often found in real machines. There are two versions of this machine: SIC standard ModelSIC/XE(extra equipment or expensive)Object programs for SIC can be properly executed on SIC/XE which is known
4 min read
Let's discuss about parallel computing and hardware architecture of parallel computing in this post. Note that there are two types of computing but we only learn parallel computing here. As we are going to learn parallel computing for that we should know following terms. Era of computing - The two f
3 min read
Parallel computing is a computing where the jobs are broken into discrete parts that can be executed concurrently. Each part is further broken down to a series of instructions. Instructions from each part execute simultaneously on different CPUs. Parallel systems deal with the simultaneous use of mu
4 min read
The generation of computers refers to the progression of computer technology over time, marked by key advancements in hardware and software. These advancements are divided into five generations, each defined by improvements in processing power, size, efficiency, and overall capabilities. Starting wi
6 min read
It is named after computer scientist Gene Amdahl( a computer architect from IBM and Amdahl corporation) and was presented at the AFIPS Spring Joint Computer Conference in 1967. It is also known as Amdahl's argument. It is a formula that gives the theoretical speedup in latency of the execution of a
6 min read
ALU, dataâ€Âpath and control unit
Instruction pipelining
Pipelining is a technique used in modern processors to improve performance by executing multiple instructions simultaneously. It breaks down the execution of instructions into several stages, where each stage completes a part of the instruction. These stages can overlap, allowing the processor to wo
9 min read
Please see Set 1 for Execution, Stages and Performance (Throughput) and Set 3 for Types of Pipeline and Stalling. Dependencies in a pipelined processor There are mainly three types of dependencies possible in a pipelined processor. These are : 1) Structural Dependency 2) Control Dependency 3) Data D
6 min read
Please see Set 1 for Execution, Stages and Performance (Throughput) and Set 2 for Dependencies and Data Hazard. Types of pipeline Uniform delay pipeline In this type of pipeline, all the stages will take same time to complete an operation. In uniform delay pipeline, Cycle Time (Tp) = Stage Delay If
3 min read
Introduction : Prerequisite - Execution, Stages and Throughput Registers Involved In Each Instruction Cycle: Memory address registers(MAR) : It is connected to the address lines of the system bus. It specifies the address in memory for a read or write operation.Memory Buffer Register(MBR) : It is co
11 min read
In computer organization, performance refers to the speed and efficiency at which a computer system can execute tasks and process data. A high-performing computer system is one that can perform tasks quickly and efficiently while minimizing the amount of time and resources required to complete these
5 min read
In computer organization, a micro-operation refers to the smallest tasks performed by the CPU's control unit. These micro-operations helps to execute complex instructions. They involve simple tasks like moving data between registers, performing arithmetic calculations, or executing logic operations.
3 min read
RISC is the way to make hardware simpler whereas CISC is the single instruction that handles multiple work. In this article, we are going to discuss RISC and CISC in detail as well as the Difference between RISC and CISC, Let's proceed with RISC first. Reduced Instruction Set Architecture (RISC) The
5 min read
Cache Memory
In the Computer System Design, Memory Hierarchy is an enhancement to organize the memory such that it can minimize the access time. The Memory Hierarchy was developed based on a program behavior known as locality of references (same data or nearby data is likely to be accessed again and again). The
6 min read
Cache memory is a small, high-speed storage area in a computer. The cache is a smaller and faster memory that stores copies of the data from frequently used main memory locations. There are various independent caches in a CPU, which store instructions and data. The most important use of cache memory
11 min read
Cache is close to CPU and faster than main memory. But at the same time is smaller than main memory. The cache organization is about mapping data in memory to a location in cache. A Simple Solution: One way to go about this mapping is to consider last few bits of long memory address to find small ca
3 min read
Caches are the faster memories that are built to deal with the Processor-Memory gap in data read operation, i.e. the time difference in a data read operation in a CPU register and that in the main memory. Data read operation in registers is generally 100 times faster than in the main memory and it k
7 min read
The CPU Cache and Translation Lookaside Buffer (TLB) are two important microprocessor hardware components that improve system performance, although they have distinct functions. Even though some people may refer to TLB as a kind of cache, it's important to recognize the different functions they serv
4 min read
A memory unit stores binary information in groups of bits called words. Data input lines provide the information to be stored into the memory, Data output lines carry the information out from the memory. The control lines Read and write specifies the direction of transfer of data. Basically, in the
3 min read
Prerequisite - Virtual Memory Abstraction is one of the most important aspects of computing. It is a widely implemented Practice in the Computational field. Memory Interleaving is less or More an Abstraction technique. Though it's a bit different from Abstraction. It is a Technique that divides memo
3 min read
Memory is required to save data and instructions. Memory is divided into cells, and they are stored in the storage space present in the computer. Every cell has its unique location/address. Memory is very essential for a computer as this is the way it becomes somewhat more similar to a human brain.
11 min read
Memory is a fundamental component of computing systems, essential for performing various tasks efficiently. It plays a crucial role in how computers operate, influencing speed, performance, and data management. In the realm of computer memory, two primary types stand out: Random Access Memory (RAM)
8 min read
In the computer world, memory plays an important component in determining the performance and efficiency of a system. In between various types of memory, Random Access Memory (RAM) stands out as a necessary component that enables computers to process and store data temporarily. In this article, we w
8 min read
Memory is an important part of the Computer which is responsible for storing data and information on a temporary or permanent basis. Memory can be classified into two broad categories: Primary Memory Secondary Memory What is Primary Memory? Primary Memory is a type of Computer Memory that the Prepro
7 min read
I/O interface (Interrupt and DMA mode)
The method that is used to transfer information between internal storage and external I/O devices is known as I/O interface. The CPU is interfaced using special communication links by the peripherals connected to any computer system. These communication links are used to resolve the differences betw
6 min read
The DMA mode of data transfer reduces the CPU's overhead when handling I/O operations. It also allows parallel processing between CPU and I/O operations. This parallelism is necessary to avoid the wastage of valuable CPU time when handling I/O devices whose speeds are much slower as compared to CPU.
5 min read
The kernel provides many services related to I/O. Several services such as scheduling, caching, spooling, device reservation, and error handling - are provided by the kernel's I/O subsystem built on the hardware and device-driver infrastructure. The I/O subsystem is also responsible for protecting i
7 min read
CPU needs to communicate with the various memory and input-output devices (I/O). Data between the processor and these devices flow with the help of the system bus. There are three ways in which system bus can be allotted to them:Separate set of address, control and data bus to I/O and memory.Have co
5 min read
Introduction : In a computer system, multiple devices, such as the CPU, memory, and I/O controllers, are connected to a common communication pathway, known as a bus. In order to transfer data between these devices, they need to have access to the bus. Bus arbitration is the process of resolving conf
7 min read
In I/O Interface (Interrupt and DMA Mode), we have discussed the concept behind the Interrupt-initiated I/O. To summarize, when I/O devices are ready for I/O transfer, they generate an interrupt request signal to the computer. The CPU receives this signal, suspends the current instructions it is exe
5 min read
Introduction : Asynchronous input/output (I/O) synchronization is a technique used in computer organization to manage the transfer of data between the central processing unit (CPU) and external devices. In asynchronous I/O synchronization, data transfer occurs at an unpredictable rate, with no fixed
7 min read
A port is basically a physical docking point which is basically used to connect the external devices to the computer, or we can say that A port act as an interface between the computer and the external devices, e.g., we can connect hard drives, printers to the computer with the help of ports. Featur
3 min read
A cluster is a set of loosely or tightly connected computers working together as a unified computing resource that can create the illusion of being one machine. Computer clusters have each node set to perform the same task, controlled and produced by the software. Clustered Operating Systems work si
7 min read
Introduction - The advent of a technological marvel called the “computer†has revolutionized life in the twenty-first century. From IoT to self-driving cars to smart cities, computers have percolated through the fabric of society. Unsurprisingly the methods with which we interact with computers have
4 min read