A fixed size partition in an Operating System (OS) refers to a memory management technique where the main memory is divided into several sections of fixed size. As described in the reference Fixed (or static) Partitioning in Operating System, this is the oldest and simplest technique that allows more than one processes to be loaded into main memory.
In this method, the main memory (RAM) is pre-divided into a set number of non-overlapping partitions. Each of these partitions has a size that is determined before the system starts running processes. While the number of partitions is fixed and their individual sizes are fixed, they may or may not be same size.
How Fixed Partitioning Works
This static partitioning approach is straightforward:
- The OS divides the total RAM into a fixed number of partitions.
- Each partition has a specific size assigned to it (e.g., Partition 1 is 1MB, Partition 2 is 2MB, Partition 3 is 1MB).
- A process waiting to be executed is loaded into the smallest available partition that is large enough to hold it.
- Only one process can reside in a partition at any given time.
This technique is simple because the memory division is static and doesn't change during system operation.
Key Characteristics
Based on the definition and common OS principles, fixed partitioning has several key characteristics:
- Static: Partitions are created when the system starts and their sizes do not change.
- Fixed Number: The number of partitions is set beforehand.
- Non-overlapping: Partitions occupy distinct memory address ranges.
- Variable Sizes Allowed: While each partition's size is fixed, different partitions can have different sizes (e.g., 1MB, 2MB, 4MB).
- Simplest Method: It is considered the most basic memory management approach for multi-programming.
Practical Insights and Limitations
While simple, fixed partitioning has significant drawbacks in modern computing environments:
- Internal Fragmentation: If a process is smaller than the partition it is loaded into, the remaining space within that partition goes unused. This wasted space inside the partition is called internal fragmentation. For example, a 1.5MB process loaded into a 2MB partition wastes 0.5MB.
- External Fragmentation: Although the partitions themselves are contiguous, there might be small unused spaces between allocated partitions. This isn't typical external fragmentation in the strict sense for fixed partitioning itself (as partitions are contiguous blocks), but it relates to inefficient use of the total memory capacity if large processes can't fit even if enough total free space exists across different partitions. However, the primary issue is internal fragmentation.
- Limited Multiprogramming: The number of processes that can reside in memory simultaneously is limited by the fixed number of partitions.
- Process Size Restriction: A process larger than the largest partition cannot be loaded and executed using this method alone.
Despite its limitations, understanding fixed partitioning is fundamental to appreciating the evolution of more dynamic memory management techniques like variable partitioning and paging.
