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Memory-mapped I/O is preferred in IA-32 and x86-64 based architectures because the instructions that perform port-based I/O are limited to one register: EAX, AX, and AL are the only registers that data can be moved into or out of, and either a byte-sized immediate value in the instruction or a value in register DX determines which port is the source or destination port of the transfer.
One of the major improvements the PCI Local Bus had over other I/O architectures was its configuration mechanism. In addition to the normal memory-mapped and I/O port spaces, each device function on the bus has a configuration space, which is 256 bytes long, addressable by knowing the eight-bit PCI bus, five-bit device, and three-bit function numbers for the device (commonly referred to as the ...
Memory-mapped I/O, an alternative to port I/O; a communication between CPU and peripheral device using the same instructions, and same bus, as between CPU and memory; Virtual memory, technique which gives an application program the impression that it has contiguous working memory, while in fact it is physically fragmented and may even overflow ...
The disadvantages of having an IOMMU, compared to direct physical addressing of the memory, include: [4] Some degradation of performance from translation and management overhead (e.g., page table walks). Consumption of physical memory for the added I/O page (translation) tables. This can be mitigated if the tables can be shared with the processor.
In contrast, in direct memory access (DMA) operations, the CPU is uninvolved in the data transfer. The term can refer to either memory-mapped I/O (MMIO) or port-mapped I/O (PMIO). PMIO refers to transfers using a special address space outside of normal memory, usually accessed with dedicated instructions, such as IN and OUT in x86 architectures.
[1] Reading a hardware register in "peripheral units" — computer hardware outside the CPU — involves accessing its memory-mapped I/O address or port-mapped I/O address with a "load" or "store" instruction, issued by the processor.
An iconic example of virtual-to-physical address translation is virtual memory, where different pages of virtual address space map either to page file or to main memory physical address space. It is possible that several numerically different virtual addresses all refer to one physical address and hence to the same physical byte of RAM.
Each GPIO port on a tiny or mega AVR drives up to eight pins and is controlled by three 8-bit registers: DDRx, PORTx and PINx, where x is the port identifier. DDRx: Data Direction Register, configures the pins as either inputs or outputs. PORTx: Output port register. Sets the output value on pins configured as outputs. Enables or disables the ...