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PC memory addressing is the way a computer identifies locations for instructions and data. On modern PCs, programs usually use virtual addresses; the processor’s memory-management unit (MMU) translates them into physical addresses that identify locations in RAM, using mappings managed by the operating system.
What is a memory address?
A memory address is an identifier for a location where data or an instruction can be found. An address space is the range of addresses available to a program or system. It is distinct from the computer’s physical memory: a program’s address space describes the locations it can refer to, while physical memory consists of actual RAM locations.
On a modern general-purpose PC, application code typically works with virtual addresses rather than directly naming physical RAM locations. That layer lets the operating system manage memory and keep processes separated.
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| Address type | What it identifies | Typical role |
|---|---|---|
| Virtual address | A location in a process’s virtual address space | Used by application code and processor operations before translation |
| Physical address | A location in physical memory | Used after translation to access RAM |
A virtual address is not a globally unique name for a RAM location. Different processes can use the same virtual address value, with each process’s mappings directing it to different physical memory. This separation helps prevent ordinary user-mode code from freely accessing another process’s memory or protected system space. Microsoft explains that processors use virtual addresses for memory reads and writes, then translate them into physical addresses in its Virtual Address Spaces documentation.
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How does a PC translate an address?
- A program refers to data or an instruction. The reference is expressed in the process’s virtual address space.
- The MMU translates the virtual address. The processor’s memory-management unit uses the applicable address mapping to determine the corresponding physical address.
- The system accesses the physical location. The physical address identifies the RAM location used for the memory access.
The operating system manages the mappings that support this translation. It organizes memory in pages, so a program can see a continuous virtual range even if the physical pages backing it are not contiguous. Microsoft’s Virtual Address Spaces describes this operating-system context and the distinction between virtual and physical addresses.
How virtual memory relates to RAM and paging
Virtual address space is not the same as installed RAM. The operating system can keep some pages in physical memory and move others between RAM and a paging file on disk. Applications continue to use virtual addresses while the operating system manages where the contents are backed.
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A page that is on disk is not equivalent to one resident in RAM: disk storage has different performance characteristics. Page size also depends on the system. For example, Microsoft documents 4-kilobyte pages as an x86 example, not as a universal page size. Its Virtual Address Space and Physical Storage explains pages, working sets, and paging.
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Windows address-space examples are not universal PC limits
Address-space size depends on processor architecture, operating system, and configuration. Microsoft’s Windows documentation, last updated June 28, 2024, gives these examples:
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| Windows process context | Documented virtual address range | Qualification |
|---|---|---|
| 32-bit process | Typically 2 gigabytes, from 0x00000000 through 0x7FFFFFFF | A typical range in the documented Windows context; actual usable range depends on platform and system. |
| 64-bit process on 64-bit Windows | 128 terabytes | A documented Windows process range, not a universal limit for every 64-bit system. |
These figures describe virtual address ranges, not installed RAM. A 64-bit processor does not imply that every possible 64-bit address is usable; implementations and operating systems define bounded ranges. See Microsoft’s Virtual Address Spaces for the Windows-specific details.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why memory addressing matters
- Isolation: Separate virtual address spaces help keep processes’ memory distinct.
- Flexible placement: A program can use a virtual range without requiring its backing physical pages to sit together in RAM.
- Operating-system control: The OS manages mappings and paging rather than letting applications freely choose physical memory locations.
These points describe CPU-managed virtual memory for programs. Other platform address spaces, such as device DMA addressing, are separate topics and should not be confused with the program-facing virtual-to-physical translation described here.
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