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11.4 DMA commands
The following are DMA commands:
•
Read DMA
•
Read DMA Ext
•
Write DMA
•
Write DMA
Data transfers using DMA commands differ in two ways from PIO transfers:
•
Data transfers are performed using the Slave DMA channel
•
No intermediate sector interrupts are issued on multisector commands.
Initiation of the DMA transfer commands is identical to the Read Sector(s) or Write Sector(s) commands with one
exception: the host initializes the Slave DMA channel prior to issuing the command.
The interrupt handler for DMA transfers differs in two ways:
•
No intermediate sector interrupts are issued on multisector commands.
•
The host resets the DMA channel prior to reading status from the device
The DMA protocol allows high performance multitasking operating systems to eliminate processor overhead asso-
ciated with PIO transfers.
1. The host initializes the Slave DMA channel.
2. The host writes any required parameters to the Features, Sector Count, Sector Number, Cylinder, and
Device/Head registers.
3. The host writes command code to the Command Register.
4. The device sets DMARQ when it is ready to transfer any part of the data.
5. The host transfers the data using the DMA transfer protocol currently in effect.
6. When all of the data has been transferred, the device generates an interrupt to the host.
7. The host resets the Slave DMA channel.
8. The host reads the Status Register and, optionally, the Error Register.
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