ATA PIO Mode: Difference between revisions

There is only one wire dedicated to selecting which drive on each bus is active. It is either electrically "high" or "low", which means that there can never be more than two devices operational on any ATA bus. They are called the master and the slave devices, for no particular reason. The terms 'master' and 'slave' have largely been abandoned as they inaccurately portray the master drive as having superiority over the slave drive. However, these therms will be used in this document. Their functionality is almost completely identical. There is a special IO port bit that allows a driver to select either drive as the target drive for each command byte.

Current disk controller chips almost always support two ATA buses per chip. There is a standardized set of IO ports to control the disks on the buses. The first two buses are called the Primary and Secondary ATA bus, and are almost always controlled by IO ports 0x1F0 through 0x1F7, and 0x170 through 0x177, respectively (unless you change it). The associated Device Control Registers/Alternate Status ports are IO ports 0x3F6, and 0x376, respectively. The standard IRQ for the Primary bus is IRQ14, and IRQ15 for the Secondary bus.