Memory management - Part 1 : Sharing

The OS provides 4 major functions to manage the virtual memory:

• Replacement : which page should be deleted to make room for other pages
• Allocation : how many pages per process should be allocated
• Fetching : when bring a page into memory
• Placement : where in memory we should place a page

Memory management has 2 purposes :

• memory protection
• memory sharing

Some challenges need to be considered while sharing memory :

• Transparency : processes must co-exist
• Safety
• Efficiency
• Relocation : program should run in different memory locations

To co-exist, memory is divided into categories:

• program & data
• read-only & read-write
• addresses & data

To execute the program, the system follows some procedures:

• compile (translate) : creates the object code
• link : combine into executable code
• load : copy code into memory
• execute : dynamic memory allocation

The main challenges with the linker are about :

• relocation : where to put pieces
• resolving references : where to find pieces
• re-organization

Relocation, one of the issues that might be encountered in memory sharing, is also a synonym of address binding: it’s mapping instructions and data into virtual memory.

There are 2 types of address binding:

• static : locations are predefined before execution
• dynamic : locations determined during execution

When the address binding is of a static type, 2 moments are noteworthy:

• compile time : translates symbolic addresses into absolute addresses
• load time : absolute addresses are mapped to relocatable addresses.

On a dynamic type address binding, the mapping it’s done by hardware components on running time.

We can have 3 types of loading :

• absolute loading : always in a designated place
• relocatable loading : loading in different locations
• dynamic loading : loads function when called.

The memory loading can occur in 3 ways:

• Direct placement : no relocation
• Overlays : tree-like structure (root is in the memory, sub-trees loaded as needed)
• Partitioning : several programs in memory in the same time (static: memory divided in fixed number of partitions, dynamic : loaded as long as there is room).

In the share memory process, fragmentation can occur. Fragmentation is unused memory that the memory management can’t allocate. You have 2 types of fragmentation:

• internal (severe in static partitioning schemes)
• external (severe in dynamic partitioning schemes)

A swapper has the responsibility of picking processes to swap in and out and manage the swap space.