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Then the device controller reads the command register, sees the write bit set, reads the byte of data from the data-out register, and outputs the byte of data.When the device controller sees the command-ready bit set, it first sets the busy bit.The host sets the command ready bit in the command register to notify the device of the pending command.The host writes a byte of data into the data-out register, and sets the write bit in the command register ( in either order. ).The host repeatedly checks the busy bit on the device until it becomes clear.One simple means of device handshaking involves polling:.( Note: Memory-mapped I/O is not the same thing as direct memory access, DMA.A potential problem exists with memory-mapped I/O, if a process is allowed to write directly to the address space used by a memory-mapped I/O device.For example, graphics cards still use registers for control information such as setting the video mode. Memory-mapped I/O can be used either instead of or more often in combination with traditional registers.Memory-mapped I/O is suitable for devices which must move large quantities of data quickly, such as graphics cards.In this case a certain portion of the processor's address space is mapped to the device, and communications occur by reading and writing directly to/from those memory areas.Another technique for communicating with devices is memory-mapped I/O.Figure 13.2 shows some of the most common I/O port address ranges.įigure 13.2 - Device I/O port locations on PCs ( partial ).
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The control register has bits written by the host to issue commands or to change settings of the device such as parity checking, word length, or full- versus half-duplex operation.The status register has bits read by the host to ascertain the status of the device, such as idle, ready for input, busy, error, transaction complete, etc.The data-out register is written by the host to send output.The data-in register is read by the host to get input from the device.Registers may be one to four bytes in size, and may typically include ( a subset of ) the following four: One way of communicating with devices is through registers associated with each port.
#Universal bus running status Pc
A daisy-chain bus, ( not shown) is when a string of devices is connected to each other like beads on a chain, and only one of the devices is directly connected to the host.įigure 13.1 - A typical PC bus structure.The SCSI bus connects a number of SCSI devices to a common SCSI controller.The expansion bus connects slower low-bandwidth devices, which typically deliver data one character at a time ( with buffering. ).The PCI bus connects high-speed high-bandwidth devices to the memory subsystem ( and the CPU. ).Figure 13.1 below illustrates three of the four bus types commonly found in a modern PC:.Buses include rigid protocols for the types of messages that can be sent across the bus and the procedures for resolving contention issues.A common set of wires connecting multiple devices is termed a bus.Devices connect with the computer via ports, e.g.Devices communicate with the computer via signals sent over wires or through the air.I/O devices can be roughly categorized as storage, communications, user-interface, and other.
#Universal bus running status drivers