Device drivers are software modules that can be plugged into an OS to handle a particular device. Operating System takes help from device drivers to handle all I/O devices. The Device Controller works like an interface between a device and a device driver. I-O DATA is a leading manufacturer and provider of high-quality computer peripheral products including memory card, smartmedia card, multimedia card, compact flash card, mmc card, memorystick, usb adapter, sd memory card, memory card reader and writer, lcd flat panel computer monitor used in a variety of digital applications. However, the system master can enable the I/Os as either inputs or outputs by writing to the I/O configuration bits. If no signals are applied externally to the PCA9536, the voltage level is 1, or high, because of the internal pullup resistors. The data for each input or output is stored in the corresponding Input Port or Output Port register. Each USB C Series DAQ device is a combination of a USB-9162 USB carrier and a C Series measurement I/O module. The USB-9162 provides USB connectivity back to a host PC. C Series modules combine A/D converters, signal conditioning, and signal connectivity in one package for measuring or generating one or more specific types of signal.

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Memory-mapped I/O (MMIO) and port-mapped I/O (PMIO) are two complementary methods of performing input/output (I/O) between the central processing unit (CPU) and peripheral devices in a computer. An alternative approach is using dedicated I/O processors, commonly known as channels on mainframe computers, which execute their own instructions.

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 when an address is accessed by the CPU, it may refer to a portion of physical RAM, or it can instead refer to memory 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. The reservation may be permanent, or temporary (as achieved via bank switching). An example of the latter is found in the Commodore 64, which uses a form of memory mapping to cause RAM or I/O hardware to appear in the 0xD000-0xDFFF range.

Port-mapped I/O often uses a special class of CPU instructions designed specifically for performing I/O, such as the in and out instructions found on microprocessors based on the x86 and x86-64 architectures. Different forms of these two instructions can copy one, two or four bytes (outb, outw and outl, respectively) between the EAX register or one of that register's subdivisions on the CPU and a specified I/O port which is assigned to an I/O device. I/O devices have a separate address space from general memory, either accomplished by an extra 'I/O' pin on the CPU's physical interface, or an entire bus dedicated to I/O. Because the address space for I/O is isolated from that for main memory, this is sometimes referred to as isolated I/O.^[1]

Overview[edit]

Different CPU-to-device communication methods, such as memory mapping, do not affect the direct memory access (DMA) for a device, because, by definition, DMA is a memory-to-device communication method that bypasses the CPU.

Hardware interrupts are another communication method between the CPU and peripheral devices, however, for a number of reasons, interrupts are always treated separately. An interrupt is device-initiated, as opposed to the methods mentioned above, which are CPU-initiated. It is also unidirectional, as information flows only from device to CPU. Lastly, each interrupt line carries only one bit of information with a fixed meaning, namely 'an event that requires attention has occurred in a device on this interrupt line'.

I/O operations can slow memory access if the address and data buses are shared. This is because the peripheral device is usually much slower than main memory. In some architectures, port-mapped I/O operates via a dedicated I/O bus, alleviating the problem.

One merit of memory-mapped I/O is that, by discarding the extra complexity that port I/O brings, a CPU requires less internal logic and is thus cheaper, faster, easier to build, consumes less power and can be physically smaller; this follows the basic tenets of reduced instruction set computing, and is also advantageous in embedded systems. The other advantage is that, because regular memory instructions are used to address devices, all of the CPU's addressing modes are available for the I/O as well as the memory, and instructions that perform an ALU operation directly on a memory operand (loading an operand from a memory location, storing the result to a memory location, or both) can be used with I/O device registers as well. In contrast, port-mapped I/O instructions are often very limited, often providing only for simple load-and-store operations between CPU registers and I/O ports, so that, for example, to add a constant to a port-mapped device register would require three instructions: read the port to a CPU register, add the constant to the CPU register, and write the result back to the port.

As 16-bit processors have become obsolete and replaced with 32-bit and 64-bit in general use, reserving ranges of memory address space for I/O is less of a problem, as the memory address space of the processor is usually much larger than the required space for all memory and I/O devices in a system. Therefore, it has become more frequently practical to take advantage of the benefits of memory-mapped I/O. However, even with address space being no longer a major concern, neither I/O mapping method is universally superior to the other, and there will be cases where using port-mapped I/O is still preferable.

Memory-mapped I/O is preferred in x86-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.^[2]^[3] Since any general-purpose register can send or receive data to or from memory and memory-mapped I/O devices, memory-mapped I/O uses fewer instructions and can run faster than port I/O. AMD did not extend the port I/O instructions when defining the x86-64 architecture to support 64-bit ports, so 64-bit transfers cannot be performed using port I/O.^[4]

Memory barriers[edit]

Since the caches mediate accesses to memory addresses, data written to different addresses may reach the peripherals' memory or registers out of the program order, i.e. if software writes data to an address and then writes data to another address, the cache write buffer does not guarantee that the data will reach the peripherals in that order.^[5] It is the responsibility^{[clarification needed]} of the software to include memory barrier instructions after the first write, to ensure that the cache buffer is drained before the second write is executed.

Memory-mapped I/O is the cause of memory barriers in older generations of computers, which are unrelated to memory barrier instructions. The 640 KB barrier is due to the IBM PC placing the Upper Memory Area in the 640–1024 KB range within its 20-bit memory addressing. The 3 GB barrier and PCI hole are manifestations of this with 32-bit memory addressing; with 64-bit memory addressing these are usually no longer problems on newer architectures.

Overview[edit]

Memory barriers[edit]

Examples[edit]

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