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11.4 DMA data transfer commands
Commands for this class are:
•
READ DMA (with and without retry)
•
WRITE DMA (with and without retry)
Data transfers using DMA commands differ in two ways from PIO transfers:
data transfers are performed using the slave-DMA channel
the drive issues only one interrupt at the completion of each command
Initiation of the DMA transfer commands is identical to the READ SECTOR(S) or WRITE SECTOR(S)
commands except that the host initializes slave-DMA channel prior to issuing the command.
The interrupt handler for DMA transfers is different in that no intermediate sector interrupts are issued on
multi-sector transfer but issued only once at the completion of each command.
DMA data transfer protocol:
a) Host initializes the slave-DMA channel.
b) Host writes any required command parameters to the Features, Sector Count, Sector Number, Cylinder High,
Cylinder Low and Device/Head registers.
c) Host writes the command code to the Command register.
d) The drive sets the BSY bit .
e) The drive sets DMARQ, when it is ready to transfer data.
f) Host transfers the data using DMA transfer mode set by the Set Features command .
g) When all of the data has been transferred, the drive issues INTRQ.
h) The host resets the slave-DMA channel.
i) After detecting INTRQ, the host reads the Status register.
j) The drive negates INTRQ in response to the Status register being read.