Direct Memory Access (DMA) allows data to be transferred directly between peripheral devices and memory without intervention from the central processing unit (CPU). This improves efficiency by avoiding the need to transfer data into CPU registers one byte at a time. To initiate a DMA transfer, the CPU writes a DMA command to memory specifying the source, number of bytes, and address. The DMA controller then operates the memory bus directly to transfer data while handshaking with the device controller. When complete, the DMA controller interrupts the CPU.

1. Computer organization PRA
Direct Memory Access
21P61A1233
D.SAI TEJA IT-A

2. Introduction of
Direct Memory Access (DMA)

3. 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.

4. 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

6. 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.

7. 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.

9. 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

10. 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.

11. THANK YOU