The transfer of data to and from peripheral devices in Windows PCs. The first PCs used port-mapped I/O (PMIO) addressing, which uses an address space separate from main memory to read and write peripherals. In addition, IN and OUT machine instructions were used. See
port-mapped I/O.
From PMIO to MMIO
As 32-bit and 64-bit Windows computers became popular, peripheral addressing began to change to memory-mapped I/O (MMIO), which dedicates regular memory to each device. The same LOAD and STORE instructions that read and write main memory are used to read and write peripheral data. Today, MMIO is the predominant form of I/O addressing, but Mac computers always used MMIO.
A Form of MMIO in Early PCs
Long before PCs had gigabytes of memory, pre-defined areas of regular memory were used, which were a form of MMIO. The upper memory area (UMA) region in a PC (above 640KB) was used to send data to the screen using CGA, EGA and VGA graphics. See
UMA.