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In a multiprogramming environment, several processes may compete for a finite number of resources. A process requests resources; if the resources are not available at that time, the process enters into wait state. Waiting processes may never again change state, because the resources they have requested are held by other waiting processes. This situation is called a deadlock. System is deadlocked if there is a set of processes such that every process in the set is waiting for another process in the set.

Example 1
System has 2 disk drives.
P1 and P2 each hold one disk drive and each needs another one.

Example 2
- Porcess A makes a request to use the scanner
- Porcess A given the scanner
- Process B request thee CD writer
- Process B given the CD writer
- Now a requrest the CD writer
- A is denied permission until B releases the CD writer
- Also now B ask for the scanner
- Result is DEADLOCK !!

Necessary conditions for deadlock/Deadlock Characterization

A deadlock can arise if the following four conditions hold simultaneously (Necessary Condition).
1. Mutual exclusion: 
Only one process at time can use the resource.
.
2.  Hold and wait: 
A process must be holding at least one resource and waiting to acquire additional resources that are currently being held by other processes.

3. No preemption: 
A resource can be released only voluntarily by the process holding it, after that process has completed its task.

4. Circular wait:
A set {P0, P1, …, Pn} of waiting processes must exist such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and Pn is waiting for a resource that is held by P0.

Deadlocks Handling

Mainly there are four methods of handling deadlock
1. Deadlock avoidance 
To avoid deadlocks the system dynamically considers every request and decides whether it is safe to grant it at this point. The system requires additional prior information regarding the overal potential use of each resource for each process. 

2. Deadlock Prevention 
It means that we design such a system where there is no chance of having a deadlock. The goal is to ensure that at least one of the Coffman's necessary conditions for deadlock can never hold. Deadlock can be prevented deadlocks by constraining how request for resources can be made in the system and how they are handled.

3. Deadlock Detection and Recovery 
We can allow the system to enter a deadlock state, detect it and recover - If a system does not employ either a deadlock prevention or a deadlock avoidance algorithm, then a deadlock situation may occur. In this environment, the system can provide an algorithm that examines the state of the system to determine whether a deadlock has occurred and an algorithm to recover from the deadlock.

4.  Ignore the Problem (Ostrich Algorithm) : 
We can ignore the deadlock problem altogether. If the deadlock occurs, the system will stop functioning and will need to be restarted manually. This approach is used in systems in which deadlocks occur infrequently and if it is not a life critical system. This method is cheaper than other methods. This solution is used by most operating systems, including UNIX.

Deadlock Prevention 

If we simulate deadlock with a table which is standing on its four legs then we can also simulate four legs with the four conditions which when occurs simultaneously, cause the deadlock.

However, if we break one of the legs of the table then the table will fall definitely. The same happens with deadlock, if we can be able to violate one of the four necessary conditions and don't let them occur together then we can prevent the deadlock.

Deadlock prevention methods are given below
1. Attack mutual Exclusion
Must hold only for non-sharable resources. Sharable resources, on the other hand, do not require mutually exclusive access, and thus cannot be involved in a deadlock.

2. Attack Hold and Wait
Ensure that whenever a process requests a resource, it does not hold any other resources. Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none. There is a drawback of low resource utilization in this method.

3. Attack No Preemption
If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are preempted (or released). If a process requests some resources and if they are not available, we check whether they are allocated to some other process that is waiting for additional resources. If so, we preempt the desired resources from the waiting process and allocate them to the requesting process.

4. Attack Circular Wait
One way to ensure that circular wait never holds is to Impose a total ordering of all resource types, and require that each process requests resources in an increasing or decreasing order of enumeration.

Deadlock Avoidance 

Traditional memory stores data at a specific address and "recalls" that data later if the address is specified. Instead of an address, associative memory can recall data if a small portion of the data itself is specified.


Associative memory is often referred to as content. Addressable memory (CAM). In associative memory, any stored items can be accessed by using the contends of item. Items are stored in an associative memory have two field format, key and data.
Associative searching is based on simultaneous matching of key to be searched with stored key associated with each line of data.



The following diagram shows the block representation of an associative memory.

The functional registers like the argument register A and key register K each have n bits, are for each bit of a word. The match register m consist of M bits, one for each memory word.

Pipeline hazards are situations that prevent the next instruction in the instruction stream from executing during its designated clock cycles. In another word, any condition that causes a stall in the pipeline operations can be called a hazard. There are mainly three types of hazards, they are:

1. Data Hazards 
2. Structural Hazards
3. Control Hazards 

Data Hazards
It arises when instructions depend on the result of previous instruction but the previous instruction is not available yet.

Structural Hazards:
They arise when there are resource conflicts that prevents hardware to execute simultaneous execution of instruction. For e.g. Lets say the hardware has a register file which has a limitation of only one read or write in a cycle. If there is an instruction that needs to read from this register file while another instruction needs to write to this register file, only one can execute because of conflict.

Control Hazards
These hazards arise as a result of any type of branch instruction. Till the branch is completely executed. The branch is completely executed, the stream of following instructions will not be known completely.

Reduced instruction set computer (RISC).
 The main characteristics of RISC pipeline is to use an efficient instructions pipeline. In case of RISC pipeline, the instruction pipeline can be implemented with only two or three segments where segments 1 fetches the instructions from the memory.

Segment 2 executes the instruction in the ALU, and segment 3 may be used to store the results of the ALU operation in a particular register.

Parallel processing systems are designed to speed up the execution of programs by dividing the program into multiple fragments and processing these fragments simultaneously such systems are known as tightly coupled systems.


Parallel computing is an evolution of serial computing where the jobs are broken into discrete parts that can be executed currently. Each part is further broken down to a series of instructions. Instructions from each part execute simultaneously on different CPU's.


Parallel systems are more difficult to program than computers with a single processor because the architecture of parallel computers varies accordingly and the processes of multiple CPU's must be co-ordinates and synchronized.

Pipeline is the process of accumulating instruction from the process through a pipeline. It allows strong and executing instructions in an orderly process. It is also known as pipeling process.

Pipeline is a technique where multiple instructions are overlapped during execution. Pipeline is divided into stages and these stages are connected with one another to form a pipe like structure. Instructions entered from one end and exit from another end.

1. Arithmetic Pipeline 
2. Instruction Pipeline 

Here, A and B are significant digits of floating point number, while a and b are exponents.


Instruction Pipeline
In this a stream of instructions can be executed by overlapping fetch, decode and execute phases of an instruction cycle. This type of technique is used to increases the throughput of the computer system.

An instruction pipeline reads instruction from the memory while previous instructions are being executed in other segments of the pipeline. Thus we can execute multiple instructions simultaneously. The pipeline will be more efficient if the instruction cycle is divided into segment of equal duration.