Solution

The correct answer is A, D, C, B

Key Points

• ​A. I/O Module: The simplest component. It manages basic input/output operations and communication between the CPU and peripherals.
• D. DMA (Direct Memory Access): DMA allows data to be transferred directly between memory and peripherals without involving the CPU, making it more efficient than an I/O module but still simpler than an I/O channel or processor.
• C. I/O Channel: A more complex mechanism that manages communication between memory and I/O devices. It is a more advanced solution than DMA because it controls multiple I/O operations and has its own dedicated path.
• B. I/O Processor: The most complex, as it acts as a separate processor dedicated to handling all I/O tasks, independently managing I/O devices and reducing the CPU's workload.

Thus, the increasing order of complexity is: A (I/O Module) < D (DMA) < C (I/O Channel) < B (I/O Processor).

Additional Information

•  The DMA mode of information transfer reduces CPU’s overhead in handling I/O operations. It additionally permits correspondence in CPU and I/O operations. Such correspondence is critical to avoid wastage of valuable CPU time whereas handling I/O devices whose speeds area unit abundant slower as compared to CPU.
• The construct of DMA operation is extended to alleviate the CPU away from obtaining involved in the execution of I/O operations. this provides rise to the event of special-purpose processors referred to as Input-Output Processors (IOP) or IO channels.