In direct memory access (DMA) and I/O subsystem design, what is cycle stealing and how does it affect CPU operation?

Introduction / Context:
Direct memory access allows hardware devices to transfer data directly between memory and the device without using the CPU for every word or byte. To perform these transfers, the DMA controller must use the system bus, which can temporarily conflict with the CPU need for memory access cycles. Cycle stealing is an important concept that describes how the DMA controller borrows bus cycles from the CPU in a controlled manner. Understanding this behavior helps explain subtle performance effects when large I or O operations are in progress.

Given Data / Assumptions:

• The system uses DMA to transfer data between devices and main memory.
• Both the CPU and the DMA controller need access to the memory bus to read or write data.
• The bus cannot be used simultaneously by both the CPU and DMA controller for the same cycle.
• The question asks what cycle stealing is and how it affects CPU operation.

Concept / Approach:
Cycle stealing occurs when the DMA controller temporarily takes control of the system bus for a single or small number of memory cycles to transfer data. During these cycles, the CPU is momentarily blocked from accessing memory, even if it has instructions to execute. From the CPU perspective, some cycles are lost or "stolen" by the DMA controller. This allows I or O devices to transfer data at high speed without completely halting the CPU, but it can slightly slow down CPU execution because fewer memory cycles are available to it.

Step-by-Step Solution:
Step 1: Recognize that DMA allows data transfer directly between memory and an I or O device, bypassing constant CPU intervention.Step 2: Understand that both the CPU and the DMA controller share the same memory bus and cannot access memory simultaneously in the same bus cycle.Step 3: In cycle stealing, the DMA controller requests control of the bus, uses one or more bus cycles to transfer a data word or burst, and then releases the bus back to the CPU.Step 4: During these cycles, the CPU is effectively paused when it needs memory access, so it loses some potential execution time.Step 5: Conclude that cycle stealing slightly reduces CPU throughput in exchange for efficient I or O transfers that run concurrently with CPU computation.

Verification / Alternative check:
Hardware architecture references explain that DMA controllers can operate in different modes, including block transfer and cycle stealing. In cycle stealing mode, the controller interleaves its memory accesses with those of the CPU, "stealing" cycles but not completely locking the CPU out for long periods. Performance analyses show that while the CPU experiences some slowdown, overall system throughput can improve because the CPU and I or O operate in parallel. These descriptions are consistent with the notion of cycle stealing.

Why Other Options Are Wrong:
Option B is incorrect because CPU scheduling algorithms that assign priorities are unrelated to DMA bus usage. Option C is wrong because overclocking is about increasing clock speed, not about sharing bus cycles with a DMA controller. Option D is incorrect because disk block reuse in a file system is a storage management topic and does not involve DMA bus cycle borrowing.

Common Pitfalls:
Students sometimes think that DMA always halts the CPU entirely during transfers, but in cycle stealing mode the CPU and DMA share the bus more gracefully. Another pitfall is to confuse cycle stealing with preemptive CPU scheduling, when in reality it is about memory bus arbitration. Remembering that the DMA controller borrows or steals individual cycles from the CPU helps keep the concept clear.

Final Answer:
Cycle stealing is a DMA technique in which the controller briefly takes control of the system bus and uses memory cycles that would otherwise be available to the CPU, slightly slowing CPU execution while enabling efficient data transfer.