Dell OptiPlex G1 - Manual (Page 18)

IRQ Assignments: Dell ™ OptiPlex ™ G1 Systems
1 IRQ3 is available provided that no other device in the computer (such as a modem) is using COM2 or COM4.
2 If an ISA expansion card is configured for IRQ11, the NIC or PCI IRQ is assigned to another available IRQ line.
IRQ Line System Resource
IRQ0 System timer
IRQ1 Keyboard port
IRQ2 Enables IRQ8 through IRQ15
IRQ3 1 Available
IRQ4 Serial port
IRQ5 Available
IRQ6 Diskette/tape drive controller
IRQ7 Parallel port
IRQ8 RTC
IRQ9 Available
IRQ10 Available
IRQ11 2 NIC/default PCI IRQ
IRQ12 Mouse port
IRQ13 Math coprocessor
IRQ14 Primary EIDE channel
IRQ15 Secondary EIDE channel