In computing, a physical address (also referred to as a real address) is a memory address in the form of a binary number that identifies a specific location in the computer’s physical memory. This address is used by the address bus to access the corresponding storage cell in main memory or a memory-mapped I/O device, facilitating data retrieval or writing.
Physical vs. Virtual Addresses
In systems employing virtual memory, the memory management unit (MMU) is responsible for translating virtual addresses to physical addresses. Virtual addresses are used by programs to reference memory, but these addresses must be converted into physical addresses by the MMU before data can be fetched from or written to actual hardware memory. This distinction is critical in systems supporting memory virtualization to prevent programs from directly interacting with physical memory, offering both security and efficiency.
Unaligned Memory Access
An important aspect of physical addresses is their handling by processors, particularly in cases of unaligned memory access. Certain processor architectures, such as those in the Intel 8086 family, can experience performance degradation when data is not aligned to an appropriate boundary. For example, a processor with a 16-bit data bus works more efficiently when data starts at even-numbered addresses. When accessed at odd-numbered addresses, the processor may need to perform multiple read operations, leading to inefficiencies. In some cases, unaligned accesses can raise exceptions that require software intervention.
Addressing in Other Devices
Apart from the central processing unit (CPU), other hardware components, such as devices utilizing Direct Memory Access (DMA), also need to interact with physical memory addresses. These devices bypass the CPU to directly read from or write to main memory, necessitating an understanding of physical addressing.
Conclusion
Physical addresses are a fundamental concept in computer architecture, enabling efficient memory management and access. They work alongside virtual addresses to ensure data is routed correctly within the system, with mechanisms like MMUs and DMA ensuring optimal performance and resource management. Understanding how physical addresses operate is essential for comprehending the underlying architecture of modern computing systems.
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