B7
B6
B5
B4
B3
B2
B1
B0
________ ________ ________ ________ ________ ________ ________ ________
M-M
Memory-to-Memory Transfers
AH
Address Hold Feature Enable
CD
Controller for DMA
CT
Compressed Timing
RP
Rotating Priority Scheme
EW
Extended Write
DREQ
DACK
bits:
B0
M-M
A one in the bit 0 position enables Channel 0 and Channel 1 to be
used for memory-to-memory transfers.
Writing a one to bit 1 enables the address hold feature in Channel 0
when performing memory-to-memory transfers.
Bit 2 is the master disable for the DMA controller. Writing a one to
this location disables the DMA subsystem (DMA1 or DMA2). This
function is normally used whenever the CPU needs to reprogram one
of the channels to prevent DMA cycles from occuring.
Compressed Timing is enabled by writing a one to bit 3 of this
register. The default 0 condition causes the DMA to operate with
normal timing.
Writing a one to bit 4 causes the 82C836 to utilize a rotating priority
scheme for honoring DMA requests. The default condition is fixed
priority.
Extended Write is enabled by writing a one to bit 5, causing the write
command to be asserted one DMA cycle earlier during a transfer.
The read and write commands both begin in state S2 when enabled.
DREQ active level is determined by bit 6. Writing a one in this bit
position causes DREQ to become active low.
DACK active level is determined by bit 7. Programming a one in
this bit position makes DACK an active high signal.
B1
AH
B2
CD
B3
CT
B4
RP
B5
EW
B6
DREQ
B7
DACK
Mode Register
Each DMA channel has a Mode Register associated with it. All four Mode registers
reside at the same I/O address. Bits 0 and 1 of the Write Mode register command
determine which channel’s Mode register is written to. The remaining six bits control the
mode of the selected channel. Each channel’s Mode register can be read by sequentially
Figure 8-2.
Command Register
I
DMA Register Descriptions
DMA Controller
8-10
Revision 3.0
P R E L I M I N A R Y
Chips and Technologies, Inc.