Internal fragmentation: Internal fragmentation refers to the wasted space in a page. For s segments in memory with a page size of p bytes, sp/2 bytes will be wasted on internal fragmentation. For average process size a bytes and if each page entry requires b bytes, then,
Approximate number of pages needed per process = a/p
Page table space occupied = ab/p
Wasted memory in last page of the process due to internal fragmentation = p/2
Total overhead = ab/p + p/2
p = sqrt(2ab)
In some 16 bit computers there are separate address spaces for instructions and data.
I space: It is a separate address space for program text and instructions.
D Space: It is a separate address space for data.
Mapped file: A process can map a file in its virtual address space for I/O. Two or more processes can map the same file. When they have mapped a file processes can communicate over shared memory.