Functional Description
5.5.1
Channel Priority
For priority resolution, the DMA consists of two logical channel groups: channels 0–3
and channels 4–7. Each group may be in either fixed or rotate mode, as determined by
the DMA Command Register.
DMA I/O slaves normally assert their DREQ line to arbitrate for DMA service. However,
a software request for DMA service can be presented through each channel's DMA
Request Register. A software request is subject to the same prioritization as any
hardware request. See the detailed register description for Request Register
programming information in Section 13.2.
5.5.1.1
Fixed Priority
The initial fixed priority structure is as follows:
High priority Low priority
0, 1, 2, 3
5, 6, 7
The fixed priority ordering is 0, 1, 2, 3, 5, 6, and 7. In this scheme, channel 0 has the
highest priority, and channel 7 has the lowest priority. Channels [3:0] of DMA-1 assume
the priority position of channel 4 in DMA-2, thus taking priority over channels 5, 6, and
7.
5.5.1.2
Rotating Priority
Rotation allows for “fairness” in priority resolution. The priority chain rotates so that the
last channel serviced is assigned the lowest priority in the channel group (0–3, 5–7).
Channels 0–3 rotate as a group of 4. They are always placed between channel 5 and
channel 7 in the priority list.
Channel 5–7 rotate as part of a group of 4. That is, channels (5–7) form the first three
positions in the rotation, while channel group (0–3) comprises the fourth position in the
arbitration.
5.5.2
Address Compatibility Mode
When the DMA is operating, the addresses do not increment or decrement through the
High and Low Page Registers. Therefore, if a 24-bit address is 01FFFFh and increments,
the next address is 010000h, not 020000h. Similarly, if a 24-bit address is 020000h
and decrements, the next address is 02FFFFh, not 01FFFFh. However, when the DMA is
operating in 16-bit mode, the addresses still do not increment or decrement through
the High and Low Page Registers but the page boundary is now 128 K. Therefore, if a
24-bit address is 01FFFEh and increments, the next address is 000000h, not
0100000h. Similarly, if a 24-bit address is 020000h and decrements, the next address
is 03FFFEh, not 02FFFEh. This is compatible with the 82C37 and Page Register
implementation used in the PC-AT. This mode is set after CPURST is valid.
Datasheet
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INTEL [ INTEL ]
