Example: Accessing An Array

To make clear the operation of a TLB, let’s examine a simple virtual address trace and see how a TLB can improve its performance. In this example, let’s assume we have an array of 10 4-byte integers in memory, starting at virtual address 100. Assume further that we have a small 8-bit virtual address space, with 16-byte pages; thus, a virtual address breaks down into a 4-bit VPN (there are 16 virtual pages) and a 4-bit offset (there are 16 bytes on each of those pages).

The figure below shows the array laid out on the 16 16-byte pages of the system. As you can see, the array’s first entry (a[0]) begins on (VPN=06, offset=04); only three 4-byte integers fit onto that page. The array continues onto the next page (VPN=07), where the next four entries (a[3] ... a[6]) are found. Finally, the last three entries of the 10-entry array (a[7] ... a[9]) are located on the next page of the address space (VPN=08).