Specifications

4 - 8
4.3.5 Interrupt Controllers
A pair of 8259A compatible interrupt controllers, within the board's ASIC devices, provides
fifteen prioritized interrupt levels. Of these, several are normally associated with the board's
onboard device interfaces and controllers, and several are available on the AT Expansion Bus.
Table 4-3 lists the normal interrupt usage.
Table 4-3. Interrupt Channel Assignment
INTERRUPT FUNCTION
IRQ0 ROM-BIOS clock tick function, from Timer 0
IRQ1 Keyboard controller output buffer full
IRQ2 Cascade input for IRQ8-15
IRQ3 Secondary serial port (if present)
IRQ4 Primary serial port (if present)
IRQ5 Secondary parallel printer (if present)
IRQ6 Floppy controller (if present)
IRQ7 Primary parallel printer (if present)
IRQ8 Reserved for battery-backed clock alarm
IRQ9 * EGA or VGA controller (if present)
IRQ10 Available
IRQ11 Available
IRQ12 Available
IRQ13 80287 Math Coprocessor (if present)
IRQ14 ATA or bus hard disk controller (if present)
IRQ15 Reserved for onboard SCSI controller
* Corresponds to IRQ2 on a PC’s expansion bus.
4.3.6 DMA Controllers
Two 8237A compatible DMA controllers are implemented within the board's ASIC devices,
resulting in seven DMA channels. The hardware and software implementation of these
controllers, as well as the additional address generation logic included, are functionally identical to
what is found in a standard AT. The seven DMA channels are utilized on the Little Board/286 as
indicated in Table 4-4.
Table 4-4. DMA Channel Assignment
CHANNEL FUNCTION
0 Available for 8-bit transfers
1 Available for 8-bit transfers
2 Floppy controller (if present)
3 Available for 8-bit transfers
4 Cascade for channels 0-3
5 Available for 16-bit transfers
6 Available for 16-bit transfers
7 SCSI controller (if present)