DMA

• DMA stands for Direct Memory Access
  • It is uses same Address/Data lines on ISA bus
  • It controls the ISA bus instead of the processor ("bus master")
  • Floppy Drive (DRQ2)
  • Hard Drive (varies)
  • Sound Card (varies)

• DMA vs. Programmed I/O
  • It need less processor load
  • It can continually transfer the same block (loop indefinitely)
  • The devices which are demand-based can be serviced more efficiently

   

• DMA characteristics:
  • It is 16-bit or 8-bit transfers

HOW DMA WORKS to transfer data from memory to a peripheral:

1. The peripheral Device requests for DMA service by pulling DREQ line high

2. The DMA controller requests CPU go on hold by pulling CPU's HRQ line high

3. The CPU finished current bus cycle, then acknowledges with HLDA the CPU relinquishes ALL bus control now (3-state)

4. The DMA controller activates DACK line to acknowledge the DMA to the peripheral

5. The DMA start transferring data to the peripheral