1. When a program does not fit into the main memory, the parts of the program that are currently not being executed are stored on the hard disk and are transferred to main memory as and when required. Thus simply put, Virtual memory is just the extension of the main memory in the disk where parts of very big programs that are currently being executed but cannot be stored completely in the main memory itself are stored.
2. This shows that the CPU no longer generates the physical address directly. This is translated to the physical address by the MMU which is then used to address the main memory as shown in Figure 6:

Paging:

1. Programs getting divided into fixed size partitions or pieces named pages. This led to the development of the important concept of virtual memory. With the use of paging, truly effective multiprogramming systems came into being. Virtual Memory uses demand paging, which simply means that each page of a process is brought in only when it is needed, that is, on demand. It is clear that we can make better use of memory by loading in just a few pages.

2. Then, if the program branches to an instruction on a page not in main memory, or if the program references data on a page not in memory, a page fault is triggered. This tells the OS to bring in the desired page.

3. The basic mechanism for reading a word from memory involves the translation of a virtual, or logical, address, consisting of page number and offset, into a physical address, consisting of frame number and offset, using a page table. Because the page table is of variable length, depending on the size of the process, we cannot expect to hold it in registers. Instead, it must be in main memory to be accessed.

4. When a particular process is running, a register (Page Table Register) holds the starting address of the page table for that process. The page number of a virtual address is used to index that table and look up the corresponding frame number. This is combined with the offset portion of the virtual address to produce the desired real address. Physical address generation from virtual addresses is shown in Figure 7.

Segmentation:

• Paging was invisible to the programmer and serves the purpose of providing the programmer with a larger address space, segmentation is usually visible to the programmer and is provided as a convenience for organizing programs and data and as a means for associating privilege and protection attributes with instructions and data. Segments basically contain many pages.

• Thus Segmentation allows the programmer to view memory as consisting of multiple address spaces or segments. Segments are of variable, indeed dynamic, size while pages were fixed size fragments. OS will assign programs and data to different segments. There may be a number of program segments for various types of programs as well as a number of data segments. Each segment may be assigned access and usage rights. Memory references consist of a (segment number, offset) form of address.

• The address translation mechanism when segmentation and paging is used with paging is shown in Figure 8.

• The virtual address is now composed of three parts: The higher order bits give the segment number, the bits after that give the page number and the bits after that give the offset.

• Thus an extra Segment table is used. This along with the page number is used to index the page table.

• The process after that is similar to the address translation mechanism when paging was used.

• Segmentation is optional and can be disabled.