The process by which the system reads data from disk to memory

At the beginning of the DMA transfer, the host writes the DA command block to the memory, writes the address of the command block to the DMA controller, and starts the I/O device. Then, the CPU continues other work, and the DMA controller continues to operate the memory bus directly, putting the address on the bus to start transmission. When the entire transfer is complete, the DMA controller interrupts the CPU. Therefore, the correct execution sequence should be
1. Initialize the DMA controller and start the disk
2. Transfer a piece of data from the disk to the memory buffer
3. The DMA controller issues an interrupt request
4. Perform the "DMA end" interrupt service program