Lecture-11.0: Direct Memory Access in Microprocessor (MPU)

 Direct Memory Access

DMA stands for Direct Memory Access. It is designed by Intel to transfer data at the fastest rate. It allows the device to transfer the data directly to/from memory without any interference of the CPU.

What is a DMA Controller?

The term DMA stands for direct memory access. The hardware device used for direct memory access is called the DMA controller. DMA controller is a control unit, part of I/O device’s interface circuit, which can transfer blocks of data between I/O devices and main memory with minimal intervention from the processor.

 

DMA Controller Diagram in Computer Architecture

DMA controller provides an interface between the bus and the input-output devices. Although it transfers data without intervention of processor, it is controlled by the processor. The processor initiates the DMA controller by sending the starting address, Number of words in the data block and direction of transfer of data .i.e. from I/O devices to the memory or from main memory to I/O devices. More than one external device can be connected to the DMA controller.

DMA controller contains an address unit, for generating addresses and selecting I/O device for transfer. It also contains the control unit and data count for keeping counts of the number of blocks transferred and indicating the direction of transfer of data. When the transfer is completed, DMA informs the processor by raising an interrupt. The typical block diagram of the DMA controller is shown in the figure below.

Working of DMA Controller

DMA controller has to share the bus with the processor to make the data transfer. The device that holds the bus at a given time is called bus master. When a transfer from I/O device to the memory or vice versa has to be made, the processor stops the execution of the current program, increments the program counter, moves data over stack then sends a DMA select signal to DMA controller over the address bus.

If the DMA controller is free, it requests the control of bus from the processor by raising the bus request signal. Processor grants the bus to the controller by raising the bus grant signal, now DMA controller is the bus master. The processor initiates the DMA controller by sending the memory addresses, number of blocks of data to be transferred and direction of data transfer. After assigning the data transfer task to the DMA controller, instead of waiting ideally till completion of data transfer, the processor resumes the execution of the program after retrieving instructions from the stack.

DMA controller now has the full control of buses and can interact directly with memory and I/O devices independent of CPU. It makes the data transfer according to the control instructions received by the processor. After completion of data transfer, it disables the bus request signal and CPU disables the bus grant signal thereby moving control of buses to the CPU.

When an I/O device wants to initiate the transfer then it sends a DMA request signal to the DMA controller, for which the controller acknowledges if it is free. Then the controller requests the processor for the bus, raising the bus request signal. After receiving the bus grant signal it transfers the data from the device. For n channel DMA controller n number of external devices can be connected.

How DMA Operations are Performed?

Following is the sequence of operations performed by a DMA −

·      Initially, when any device has to send data between the device and the memory, the device has to send DMA request (DRQ) to DMA controller.

·      The DMA controller sends Hold request (HRQ) to the CPU and waits for the CPU to assert the HLDA.

·      Then the microprocessor tri-states all the data bus, address bus, and control bus. The CPU leaves the control over bus and acknowledges the HOLD request through HLDA signal.

·      Now the CPU is in HOLD state and the DMA controller has to manage the operations over buses between the CPU, memory, and I/O devices.

                         

i. DREQ (DMA Request): Peripheral sends a request to the DMA controller for data transfer.

ii. HOLD (Hold Signal): DMA controller requests control of the system bus from the CPU.

iii. HLDA (Hold Acknowledge): CPU acknowledges the HOLD signal, releasing control of the system bus.

iv. DACK (DMA Acknowledge): DMA controller signals the peripheral that it is ready for data transfer.

v. DATA Transfer/Final Execution: DMA controller transfers data directly between memory and the peripheral.

The DMA transfers the data in three modes which include the following.

a) Burst Mode: In this mode DMA handover the buses to CPU only after completion of whole data transfer. Meanwhile, if the CPU requires the bus it has to stay ideal and wait for data transfer.

b) Cycle Stealing Mode: In this mode, DMA gives control of buses to CPU after transfer of every byte. It continuously issues a request for bus control, makes the transfer of one byte and returns the bus. By this CPU doesn’t have to wait for a long time if it needs a bus for higher priority task.

 

c) Transparent Mode: Here, DMA transfers data only when CPU is executing the instruction which does not require the use of buses.

 

Advantages and Disadvantages of DMA Controller

The advantages and disadvantages of DMA controller include the following.

Advantages

·        DMA speedups the memory operations by bypassing the involvement of the CPU.

·        The work overload on the CPU decreases.

·        For each transfer, only a few numbers of clock cycles are required

Disadvantages

·        Cache coherence problem can be seen when DMA is used for data transfer.

·        Increases the price of the system.

 

Direct Memory Access (DMA) is a technique that allows peripherals or devices to transfer data directly to or from a CPU's memory without involving the CPU in every data transfer, freeing it for other tasks.

 

Here’s a concise overview of DMA applications in CPUs:

1.    High-Speed Data Transfer: DMA enables fast data movement between memory and peripherals (e.g., storage devices, network cards, or GPUs), reducing CPU overhead for tasks like copying large files or streaming data.

2.    I/O Operations: DMA is used in input/output operations, such as reading from or writing to hard drives, SSDs, or network interfaces, allowing the CPU to focus on processing while data is transferred in the background.

3.    Multimedia Processing: In audio/video applications, DMA handles continuous data streams (e.g., for sound cards or video capture devices), ensuring smooth playback or recording without CPU intervention.

4.    Network Communication: DMA accelerates packet processing in network interface cards (NICs), enabling efficient data transfer for high-bandwidth tasks like streaming or server communications.

5.    Graphics Processing: In systems with GPUs, DMA facilitates rapid data exchange between system memory and GPU memory, critical for rendering graphics or running compute-intensive tasks like AI model training.

6.    Embedded Systems: In microcontrollers or IoT devices, DMA offloads repetitive data tasks (e.g., sensor data collection), allowing the CPU to handle control logic or enter low-power modes.

 

Benefits:

• Reduces CPU workload, improving system efficiency.
• Speeds up data transfers for large or continuous datasets.
• Enhances multitasking in real-time systems.

Example: In a computer, when transferring a large file from an SSD to RAM, the DMA controller handles the transfer, allowing the CPU to execute other processes like running applications or handling user inputs.