Contiguous Memory Allocation

What is Contiguous Memory Allocation?

Contiguous memory allocation is a memory allocation technique in operating systems where each process is allocated a single continuous block of memory. The memory assigned to a process is in the form of a contiguous block, which means that the process is stored in adjacent memory locations.

Key Characteristics

Contiguity: All memory allocated to a process must be in a single, continuous block.
External Fragmentation: Can lead to external fragmentation where free memory exists but is not contiguous.
Address Translation: Simple address translation using base and limit registers.

Allocation Strategies

There are several strategies for allocating memory blocks to processes:

First-Fit Algorithm

Allocates the first free block that is large enough to accommodate the process.

Search the memory blocks from the beginning
Allocate the first block that is large enough
If the block is larger than needed, split it

Advantage: Fast allocation time

Disadvantage: Can lead to fragmentation at the beginning of memory

Next-Fit Algorithm

A variation of First-Fit that starts searching from the location of the last allocation.

Start searching from where the previous allocation ended
Continue the search until finding a block large enough
If the end of memory is reached, wrap around to the beginning

Advantage: Distributes allocations more evenly across memory

Disadvantage: May be slower than First-Fit for some allocation patterns

Best-Fit Algorithm

Allocates the smallest free block that is large enough to accommodate the process.

Search all memory blocks
Find the smallest block that is large enough
Allocate that block

Advantage: Minimizes wasted space

Disadvantage: Slower allocation time, can create many small unusable fragments

Worst-Fit Algorithm

Allocates the largest free block available.

Search all memory blocks
Find the largest block
Allocate that block

Advantage: Leaves larger leftover fragments that may be more useful

Disadvantage: Quickly exhausts large blocks, slower allocation time

Comparison of Allocation Strategies

Strategy	Speed	Memory Utilization	Fragmentation
First-Fit	Fast	Good	All lower ends of the memory at beginning
Next-Fit	Fast	Good	More evenly distributed
Best-Fit	Slow	Better	Many small fragments
Worst-Fit	Slow	Poor (generally)	Large fragments

Challenges in Contiguous Memory Allocation

External Fragmentation: Free memory space exists but is not contiguous, making it unusable for large processes.
Compaction: The process of moving all processes to one end of memory to create a large contiguous free block.
Memory Protection: Ensuring processes can only access their allocated memory.