Intel 386 - Manual (Page 376)

12-41
DMA CONTROLLER
Figure 12-26. DMA Mode 2 Register (DMAMOD2)
DMA Mode 2
DMAMOD2
(write only) Expanded Addr:
ISA Addr:
Reset State: F01BH
—
00H
7 0
BCO RD TD RH RI TH 0 CS
Bit
Number
Bit
Mnemonic
Function
7 BCO Bus Cycle Option:
0 = Selects the fly-by data transfer bus cycle option for the channel specified
by bit 0.
1 = Selects the two-cycle data transfer bus cycle option for the channel
specified by bit 0.
6 RD Requester Device Type:
0 = Clear this bit when the requester for the channel specified by bit 0 is in
memory space.
1 = Set this bit when the requester for the channel specified by bit 0 is in I/O
space.
This bit is ignored if BCO is cleared.
5 TD Target Device Type:
0 = Clear this bit when the target for the channel specified by bit 0 is in
memory space.
1 = Set this bit when the target for the channel specified by bit 0 is in I/O
space.
4 RH Requester Address Hold:
0 = Causes the address to be modified (incremented or decremented,
depending on DMAMOD2.3).
1 = Causes the requester’s address for the channel specified by bit 0 to
remain constant during a buffer transfer.
3 RI Requester Address Increment/Decrement:
0 = Causes the requester address to be incremented after each data transfer
in a buffer transfer.
1 = Causes the requester address for the channel specified by bit 0 to be
decremented after each data transfer in a buffer transfer. Note that it
does not decrement words. When decrementing it will do two byte
transfers for a word.
Note: When the target address is programmed to remain constant
(DMAMOD2.4 = 1), this bit is a don’t care.
2 TH Target Address Hold:
0 = Causes the address to be modified (incremented or decremented,
depending on DMAMOD1.5).
1 = Causes the target’s address for the channel specified by bit 0 to remain
constant during a buffer transfer.
1 0 Must be 0 for correct operation.
0 CS Channel Select:
0 = The selections for bits 7–2 affect channel 0.
1 = The selections for bits 7–2 affect channel 1.