How to Calculate Physical Address from Virtual Address
In modern computer systems, virtual memory management is a crucial component that allows the operating system to efficiently manage memory resources. One of the key operations in virtual memory management is the translation of virtual addresses to physical addresses. This translation is essential for the CPU to access the correct data in the physical memory. In this article, we will discuss how to calculate physical address from virtual address.
Understanding Virtual and Physical Addresses
Before diving into the calculation process, it is important to understand the difference between virtual and physical addresses. A virtual address is an address generated by the CPU during program execution, while a physical address refers to the actual location in the physical memory. The operating system maintains a mapping between virtual addresses and physical addresses to facilitate this translation.
The Translation Process
The translation process involves several steps, including the use of page tables and page directories. Here’s a step-by-step guide on how to calculate the physical address from a virtual address:
1. Segmentation: The virtual address is divided into two parts: the segment selector and the offset. The segment selector is used to identify the segment, while the offset represents the position within the segment.
2. Segment Table Lookup: The segment selector is used to look up the segment table. The segment table contains the base address and limit of the segment. If the offset is within the limit, the translation process continues; otherwise, a segment fault occurs.
3. Page Table Lookup: The base address obtained from the segment table is combined with the offset to form the linear address. The linear address is then used to look up the page table. The page table contains the page frame number (PFN) and the page table entry (PTE) flags.
4. Page Directory Lookup: If the page table is not present in memory, a page fault occurs. Otherwise, the PFN is used to look up the page directory. The page directory contains the base address of the page table.
5. Physical Address Calculation: Finally, the PFN obtained from the page directory is combined with the offset to calculate the physical address. This physical address represents the actual location in the physical memory where the data is stored.
Example
Let’s consider a simple example to illustrate the process. Suppose we have a virtual address of 0x1000, and the segment selector is 0x10. The segment table entry for this selector indicates a base address of 0x10000 and a limit of 0x1000. The offset is 0x1000.
1. Segment Table Lookup: The base address is 0x10000, and the offset is 0x1000. The linear address is 0x10000 + 0x1000 = 0x11000.
2. Page Table Lookup: The linear address is used to look up the page table. Let’s assume the page table entry indicates a PFN of 0x2000.
3. Page Directory Lookup: The PFN is used to look up the page directory. The page directory entry indicates a base address of 0x30000.
4. Physical Address Calculation: The PFN is 0x2000, and the offset is 0x1000. The physical address is 0x30000 + 0x2000 + 0x1000 = 0x32000.
In this example, the physical address calculated is 0x32000, which represents the actual location in the physical memory where the data is stored.
Conclusion
Calculating the physical address from a virtual address is a critical operation in virtual memory management. By understanding the translation process and the components involved, such as segment tables, page tables, and page directories, we can ensure efficient memory access and resource management in modern computer systems.
