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Each byte in main memory is specified by an integer physical address. The set of valid physical addresses is called the physical address space. It usually runs from 0 to \( N - 1 \), where \( N \) is the size of main memory. On a system with 1 GiB of physical memory, the highest valid address is \( 2^{30} - 1 \), which is 1,073,741,823 in decimal, or 0x3fff ffff in hexadecimal (the prefix 0x indicates a hexadecimal number).
However, most operating systems provide virtual memory, which means that programs never deal with physical addresses, and don’t have to know how much physical memory is available.
Instead, programs work with virtual addresses, which are numbered from 0 to \( M - 1 \), where \( M \) is the number of valid virtual addresses. The size of the virtual address space is determined by the operating system and the hardware it runs on.
You have probably heard people talk about 32-bit and 64-bit systems. These terms indicate the size of the registers, which is usually also the size of a virtual address. On a 32-bit system, virtual addresses are 32 bits, which means that the virtual address space runs from 0 to 0xffff ffff. The size of this address space is \( 2^{32} \) bytes, or 4 GiB.
On a 64-bit system, the size of the virtual address space is \( 2^{64} \) bytes, or \( 2^{4} \cdot 1024^{6} \) bytes. That’s 16 exbibytes, which is about a billion times bigger than current physical memories. It might seem strange that a virtual address space can be so much bigger than physical memory, but we will see soon how that works.
When a program reads and writes values in memory, it generates virtual addresses. The hardware, with help from the operating system, translates to physical addresses before accessing main memory. This translation is done on a per-process basis, so even if two processes generate the same virtual address, they would map to different locations in physical memory.
Thus, virtual memory is one important way the operating system isolates processes from each other. In general, a process cannot access data belonging to another process, because there is no virtual address it can generate that maps to physical memory allocated to another process.
