ADDRESSING SYSTEM BOARD COMPONENTS
This section provides more details of how the BIOS uses the system board components
mentioned in the I/O port list.
DMA Channel Controllers
The system controller supports seven DMA channels, each with a page register used to extend
the addressing range of the channel to 16 MB. The only types of DMA transfer allowed are:
“I/O-to-memory” and “memory-to-I/O”
“I/O-to-I/O” and “memory-to-memory” transfers are disallowed by the hardware configuration.
The following table summarizes how the DMA channels are allocated.
First DMA controller (used for 8-bit transfers)
Channel
Function
0
Available
1
Available or ECP mode for parallel port
2
Flexible disk I/O
3
Available or ECP mode for parallel port
Second DMA controller (used for 16-bit transfers)
Channel
Function
4
Cascade from first DMA controller
5-6
Available
6-7
Available
