Virtual Memory

• Virtual memory is a technique that allows the execution of processes which are not completely available in memory.
• The main visible advantage of this scheme is that programs can be larger than physical memory.
• Virtual memory is the separation of user logical memory from physical memory.
• This separation allows an extremely large virtual memory to be provided for programmers when only a smaller physical memory is available.
• Following are the situations, when entire program is not required to be loaded fully in main memory.

1. User written error handling routines are used only when an error occurred in the data or computation.
2. Certain options and features of a program may be used rarely.
3. Many tables are assigned a fixed amount of address space even though only a small amount of the table is actually used.
4. The ability to execute a program that is only partially in memory would counter many benefits.
5. Less number of I/O would be needed to load or swap each user program into memory.
6. A program would no longer be constrained by the amount of physical memory that is available.
7. Each user program could take less physical memory, more programs could be run the same time, with a corresponding increase in CPU utilization and throughput.

• Virtual memory is commonly implemented by demand paging. It can also be implemented in a segmentation system.
• Demand segmentation can also be used to provide virtual memory.

Paging

• External fragmentation is avoided by using paging technique.
• Paging is a technique in which physical memory is broken into blocks of the same size called pages (size is power of 2, between 512 bytes and 8192 bytes).
• When a process is to be executed, its corresponding pages are loaded into any available memory frames.
• Logical address space of a process can be non-contiguous and a process is allocated physical memory whenever the free memory frame is available.
• Operating system keeps track of all free frames. Operating system needs n free frames to run a program of size n pages.

Address generated by CPU is divided into

• Page number (p) -- page number is used as an index into a page table which contains base address of each page in physical memory.
• Page offset (d) -- page offset is combined with base address to define the physical memory address.

Segmentation

• Segmentation is a technique to break memory into logical pieces where each piece represents a group of related information.
• For example, data segments or code segment for each process, data segment for operating system and so on.
• Segmentation can be implemented using or without using paging.
• Unlike paging, segment are having varying sizes and thus eliminates internal fragmentation.
• External fragmentation still exists but to lesser extent.

Address generated by CPU is divided into

• Segment number (s) -- segment number is used as an index into a segment table which contains base address of each segment in physical memory and a limit of segment.
• Segment offset (o) -- segment offset is first checked against limit and then is combined with base address to define the physical memory address.