Direct Memory Access (DMA) allows data to be transferred directly between peripheral devices and memory without intervention from the CPU. The DMA controller initiates transfers by reading a DMA command from memory specifying the source, count, and destination. It then operates the memory bus directly to transfer data while the CPU continues other tasks. When the transfer is complete, the DMA controller interrupts the CPU. Handshaking between the DMA and device controllers coordinates the transfer of each word of data to or from memory.

1. Introduction of
Direct Memory Access (DMA)

2. Why is DMA?
• It is wasteful to feed data into a controller
register 1 bytes at a time. (PIO)
• The DMA unit is word.
• In the high loading environment, a system
with DMA has better improvement.

3. DMA transfer
5. DMA controller transfers bytes
to buffer X, increasing memory
address and decreasing C until C=0
6. when C=0, DMA interrupts CPU
to signal transfer completion
PCI bus
CPU memory bus Memory buffer
DMA/bus/
interrupt
controller
cache
CPU
1. device driver told to transfer disk
data to buffer at address X
2. device driver tells disk controller to
transfer C bytes from disk to buffer at
address X
3. disk controller initiates
DMA transfer
4. disk controller sends each
byte to DMA controller
IDE disk
controller
disk
disk
disk
disk
X

4. DMA Progress
• To initiate a DMA transfer, the host writes a
DMA command into memory:
– A pointer to the source of a transfer
– A count of the number of bytes to be transferred
– ……..
• The CPU writes the address of the DMA
command block to the DMA controller.

5. DMA Progress (cont.)
• The DMA controller proceeds to operate the
memory bus directly without CPU help.
• Handshaking exists between DMA
controller and device controller.
• When the entire transfer is finished, the
DMA controller will interrupts the CPU.

6. Handshaking
• DMA-request and DMA_acknowledge
– When a word of data is available, the device
controller places a signal on the DMA-request
wire.
– The signal causes the DMA controller to seize
the memory bus,
• To place the desired address on the memory-address
wire
• To place a signal on the DMA-acknowledge wire

7. Handshaking (cont.)
• When the device controller receives the
DMA-acknowledge signal,
– it transfer the word of data to memory
– and remove the DMA_request signal.