Disk
DMA
Channel
Control
Data
is
normally transferred
to
the disk by direct memory access
(DMA).
The disk
DMA is controlled by four items:
o Pointer
to
the area into which or from which the
data
is to be moved
o Length of
data
to
be moved
by
DMA
o Direction of
data
transfer (read/write)
o DMA enable
Pointer
to
Data
You specify the 19-bit-wide byte address from which or to which the
data
is
to
be
transferred. The lowest bit (bit
0)
of this address
is
treated as a
O.
(You cannot
start
data
on an odd-byte boundary.)
This address must be written into registers named DSKPTH and DSKPTL. DSKPTH
gets the high three bits of the pointer, DSKPTL gets the
low
sixteen bits of the pointer.
These registers are positioned
at
two consecutive word addresses on a long word boun-
dary within the register space. This allows you
to
initialize both registers by a single
write of a long word
to
the address of DSKPTH.
Length,
Direction,
DMA
Enable
All of the control bits relating
to
this topic are contained in a single register, called
DSKLEN.
Its
bits are shown in table 8-7.
230 Interface Hardware
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