Intel 386 - Manual (Page 372)

12-37
DMA CONTROLLER
12.3.7 Command 2 Register (DMACMD2)
Use DMACMD2 to select the DREQn and EOP# sampling: asynchronous or synchronous. Bus
timing diagrams that show the differences between asynchronous and synchronous sampling are
shown in Figure 12-5 on page 12-10 and Figure 12-13 on page 12-21. Also, use DMACMD2 to
assign a particular bus request to the lowest priority level for fixed priority mode.
Figure 12-24. DMA Command 2 Register (DMACMD2)
DMA Command 2
DMACMD2
(write only) Expanded Addr:
ISA Addr:
Reset State: F01AH
—
08H
7 0
— — — — PL1 PL0 ES DS
Bit
Number
Bit
Mnemonic
Function
7–4 — Reserved; for compatibility with future devices, write zeros to these bits.
3–2 PL1:0 Low Priority Level Set:
Use these bits to assign a particular bus request to the lowest priority
level in fixed priority mode.
00 = Assigns channel 0’s request (DREQ0) to the lowest priority level
01 = Assigns channel 1’s request (DREQ1) to the lowest priority level
10 = Assigns the external bus master request (HOLD) to the lowest
priority level
11 = Reserved
1 ES EOP# Sampling:
0 = Causes the DMA to sample the EOP# input asynchronously.
1 = Causes the DMA to sample the end-of-process (EOP#) input
synchronously.
0 DS DREQ n Sampling:
0 = Causes the DMA to sample the DREQ n inputs asynchronously.
1 = Causes the DMA to sample the channel request (DREQ n) inputs
synchronously.