问题
If I have a Memory-mapped I/O device, and I want to write to a register for this device located at address 0x16D34, the 0x16D34 address is actually a virtual address, and the CPU will translate it to a physical address first, and then write the data to the physical address.
But what about Port-mapped I/O devices (for example: a serial port), so if I want to write to a register for a serial port located at address 0x3F8, is the 0x3F8 address a physical address or a virtual address?
Edit: I am on x86 architecture.
回答1:
Port-mapped I/O on x86/x86-64 (most other modern architectures don't even support it) happens in an entirely separate address space. This address space is not subject to memory mapping, so there are no virtual port addresses, only physical ones. Special in and out instructions must be used to perform port I/O, simple memory access (e.g. with mov) can't access this separate address space. Access protection based on privilege level is possible; most modern OSes prevent user space processes from accessing I/O ports by default.
For details, you can for example check the chapter "INPUT/OUTPUT" of Intel's "Intel® 64 and IA-32 Architectures Developer's Manual: Vol. 1" (chapter 18 as of this writing).
Note that in the early days of x86, port addresses were hardwired in each device, including ISA add-in cards. If you were lucky, the card had a set of jumpers for selecting one of a limited set of possible port ranges for the device, in order to avoid range clashes between devices. Later, Plug & Play was introduced to make the selection dynamically during system boot. PCI further refined this, so that I/O BARs can pretty much be mapped anywhere within the 0x0000-0xffff address space by the operating system and/or firmware. Port-mapped I/O is now strongly discouraged when designing new hardware due to its many inherent limitations.
回答2:
It seems your question would be the differences between memory-mapped I/O and Port-mapped IO. There normally two methods for processor to connect external devices, which are memory-mapped or port mapped I/O.
Memory-mapped I/O
Memory-mapped I/O uses the same address space to address both memory and I/O devices. So when an address is accessed by the CPU, it may refer to a portion of physical RAM, but it can also refer to memory of the I/O device (Based on Memory-mapped I/O on Wiki).
The value 0x16D34 in your first example would be virtual memory, and would be mapped to the physical memory. The I/O device would refer the same physical memory as well to allow the access from CPU.
Port mapped I/O
Port mapped I/O uses a separate, dedicated address space and is accessed via a dedicated set of microprocessor instructions. For 0x3F8 in your second example, it's the address of the own address specific to Memory and I/O devices. It's not the address shared between memory and I/O devices as we previously mentioned in Memory-mapped I/O. You might get more detailed in Memory-mapped IO vs Port-mapped IO
来源：https://stackoverflow.com/questions/44342543/is-virtual-memory-used-when-using-port-mapped-i-o