I have a basic understanding of Memory-Mapped I/O (MMIO). Below is copied from the Wikipedia page:
Memory-mapped I/O uses the same address space to address both memory and I/O devices. The memory and registers of the I/O devices are mapped to (associated with) address values. So a memory address may refer to either a portion of physical RAM, or instead to memory and registers of the I/O device. Thus, the CPU instructions used to access the memory can also be used for accessing devices. Each I/O device monitors the CPU's address bus and responds to any CPU access of an address assigned to that device, connecting the data bus to the desired device's hardware register. To accommodate the I/O devices, areas of the addresses used by the CPU must be reserved for I/O and must not be available for normal physical memory.
My question is, suppose the address of the MMIO area is addr, if we issue a write to addr, will it be written to addr in main memory as well? Or only written to the memory within the I/O device?
My thought is as follows:
Since it is stated that "a memory address may refer to either a portion of physical RAM, or instead to memory and registers of the I/O device", the data will not be written to RAM, i.e. the data never goes to RAM, instead, it would be snooped by the memory controller of the I/O device, and written to the device memory.
If we issue a read from address addr, such read instruction will be captured by memory controller of I/O device and the data will be transferred from the device memory to the destination register in CPU. If we want the data to be in memory, then we need to issue another write, to another address addr_new (no overlap with addr).
May I know if my understanding is correct?
In memory-mapped I/O, there is no address that maps to both RAM and I/O registers -- it's one or the other.
It's really about the processor instruction set.
x86 processors have special instructions for reading and writing IO registers.
Memory-mapped I/O is the alternative. You use the same instructions to use memory or I/O, and only the address you use determines which is which.
The simplest old-time implementation of memory-mapped I/O could just use one of the address lines to select either memory or I/O ports, requiring both of those to implement similar protocols. That's not really practical today, though, because RAM is now complicated.