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: