The DMA sends DACK - acknowledgement signal to the peripheral when _______.

Explanation:

DMA Acknowledgement Signal

Definition: The Direct Memory Access (DMA) controller is a hardware component used in computer systems to manage the flow of data directly between memory and peripherals without involving the CPU, thereby freeing the CPU to perform other tasks. When a peripheral device requests the DMA controller for data transfer, the DMA controller sends an acknowledgment signal known as DMA Acknowledge (DACK) to the peripheral device to indicate that it is ready to handle the data transfer.

Correct Option Analysis:

The correct option is:

Option 2: MPU sends the HLDA signal

When the DMA controller initiates a data transfer, it sends a request to the Microprocessor Unit (MPU) to relinquish control of the data bus. The MPU responds by sending a Hold Acknowledge (HLDA) signal to the DMA controller, indicating that it has released control of the buses. Upon receiving the HLDA signal, the DMA controller sends the DMA Acknowledge (DACK) signal to the peripheral to start the data transfer.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Higher address bus A8-A15 is available on the address bus

This option is incorrect because the availability of the higher address bus A8-A15 does not directly relate to the DMA acknowledgment signal. The higher address bus lines are used for addressing memory locations, but they do not indicate when the DMA controller is ready to handle data transfers. The DMA acknowledgment signal is specifically related to the communication between the DMA controller and the peripheral device.

Option 3: Entire address bus A0-A15 is available on the address bus

This option is also incorrect because the availability of the entire address bus A0-A15 is not the condition for the DMA acknowledgment signal. While the address bus is crucial for specifying memory locations during data transfers, the DMA acknowledgment signal involves the control and coordination between the DMA controller and the peripheral device, indicating readiness for data transfer.

Option 4: Lower address bus A0-A7 is available on the address bus

This option is incorrect for similar reasons as options 1 and 3. The availability of the lower address bus A0-A7 does not directly relate to the DMA acknowledgment signal. The lower address bus lines are part of the overall addressing scheme, but they do not play a role in signaling the readiness of the DMA controller to the peripheral device.

Conclusion:

Understanding the role of the DMA controller and the signals involved in the data transfer process is essential for correctly identifying the conditions under which the DMA acknowledgment signal (DACK) is sent to the peripheral device. The correct option highlights the importance of the MPU sending the HLDA signal to the DMA controller, which then sends the DACK signal to the peripheral, indicating readiness for data transfer. This process ensures efficient communication and coordination between the MPU, DMA controller, and peripheral devices, optimizing the overall performance of the computer system.