FOR FREE CONTENT
- Introduction: OS
- Booting Process
- Command Interpreter
- Dual mode of operation
- System Calls
- Difference between system call and library call
- Interrupt and Trap
- Introduction
- Scheduling Criteria, Terms and Concepts
- Different Type Scheduling Algorithms
- Longest Job First
- Longest Remaining Time First
- Highest Response Ratio Next (HRRN)
- Priority Scheduling
- Least Completed Next (LCN) Scheduling
- Multi-Level Queue Scheduling
- Multilevel Feedback Queue Scheduling
- Guaranteed Scheduling
- Fair Share Scheduling
- Lottery Scheduling
- Multi-Processor Scheduling
- Deadline Scheduling
- Rate-Monotonic Scheduling
- POSIX Real-Time Scheduling
- Priority Inversion
- Proportional Share Scheduling
- Comparison table of Scheduling Algorithm
- Algorithm Evaluation for Scheduling
- Important concept of MM
- Memory Representation as Address
- Memory layout for a Process
- Logical and Physical Address Space
- Necessity of Memory Management
- Introduction
- Concept of Non-contiguous memory allocation
- Basic concept of Paging
- Address Translation
- Process of Paging Method
- Calculation of Offset for paging
- Calculation of Logical Address Bit and number of Pages
- Calculation of Physical Address Bit and number of Frames
- Calculation of Page Table Size
- Organization of MMU
- Why Page size power of 2
- Paging Address Calculation: example1
- Paging Numeric Problem: Example2
- Paging Address Calculation: Example3
- Paging Address Calculation: Question1
- Some Important Question 2
- Paging Address Translation: Question3
- UGC NET Paging Question
- GATE Paging Question
- Number of information at Page Entry
- Exams Question on Page Entry Information
- Exam Question: Page Entry Information
- Calculates Page Table Entry Size
- Calculates Page Size
- Calculation of Optimal Page Size
- Question on Optimal Page Size
- Hashed Page Table with Example
- Inverted Page Table with Gate Question
- Question on Inverted Page Table
- Multi-Level Paging
- Multi-level paging: Example1
- Multi-level paging: Example2
- Allocation of Frames
- Subject Mock
Compare and Swap Instruction
In comparison to Test and Set Instruction, the compare and swap instruction operates on three operands.
Here the operand value is assigned to a new value only if the expression (*value == expected) is true. Regardless, compare_and_swap() always returns the original value of the variable value. Like the test and set instruction, compare_and_swap() is integrated instruction that runs automatically.
.png)
# Let's take an example to make it more clear.
Example: Check for Mutual Exclusion
Initially Lock = 0
.png)
Assume two processes are running P0, P1 is as below:
P0: 1, 2 (CS) | P1: 1
# P0 executes instruction 1, initially lock = 0 so, it is matched with the second parameter of the function expected value of 0 and then assign new value 1 (as the third parameter) to lock, now lock = 1.
As we know the whole function compare_and_swap returns the main initial value of the lock i.e. lock = 0. So, it satisfied and executes instruction 2 and enters into its critical section. After that P0 preempted.
# P1 gets the CPU and executes instruction 1, right now lock = 1. It is not matched with the second parameter of the function expected value of 0. So, it does not assign a new value 1 (as the third parameter) to lock.
As we know whole function compare_and_swap returns the main initial value of the lock i.e. lock=1. So, it not satisfied and stuck in instructions 1 (while loop) i.e. busy waiting until lock=0.
Compare and Swap instruction satisfy mutual exclusion.
Test and Set Instructions
1. Mutual Exclusion: Yes
2. Progress: Yes
3. Bounded waiting: No
4. Portability: No (because compare_and_swap is OS instruction)
