82371AB (PIIX4)
E
158
4/9/97 2:23 PM PIIX4aDS
INTEL CONFIDENTIAL
(until publication date)
PRELIMINARY
8.3.
ISA/EIO Interface
PIIX4 can incorporate a fully ISA Bus compatible master and slave interface or a subset interface called the
Extended IO (EIO) Bus. PIIX4 can directly drive the equivalent of five ISA slots without external data buffers.
The ISA or EIO signals are independent of other functions and no external transceivers are required. The ISA or
EIO interface also provides byte swap logic, I/O recovery support, wait state generation, and SYSCLK
generation.
The ISA interface supports the following types of cycles:
PCI master-initiated I/O and memory cycles to the ISA Bus.
DMA compatible cycles between main memory and ISA I/O and between ISA I/O and ISA memory.
Enhanced DMA cycles between PCI memory and ISA I/O (for motherboard devices only).
ISA refresh cycles initiated by either PIIX4 or an external ISA master.
ISA master-initiated memory cycles to PCI and ISA master-initiated I/O cycles to the internal PIIX4 registers,
as shown in ISA-Compatible Register table in the Register Mapping section.
The EIO interface differs from ISA interface in following ways:
ISA Master cycles are not supported.
Only 20 bits of addressing allowed (No LA signals).
ISA Refresh is not supported.
8.4.
DMA Controller
The DMA circuitry incorporates the functionality of two 82C37 DMA controllers with seven independently
programmable channels (Figure 3). DMA Controller 1 (DMA-1) corresponds to DMA Channels 0–3 and DMA
Controller 2 (DMA-2) corresponds to Channels 5–7. DMA Channel 4 is used to cascade the two controllers and
will default to cascade mode in the DMA Channel Mode (DCM) Register. This channel is not available for any
other purpose. In addition to accepting requests from DMA slaves, the DMA controller also responds to requests
that are initiated by software. Software may initiate a DMA service request by setting any bit in the DMA Channel
Request Register to a 1.
Channel 0
Channel 1
Channel 2
Channel 3
Channel 4
Channel 5
Channel 6
Channel 7
DMA-1
DMA-2
dma_blk
Figure 3. Internal DMA Controller