Asus P4B266-M - Manual (Page 61)

ASUS P4B266-M motherboard user guide 4-23
IRQ XX Reserved [No/ICU]
These fields indicate whether or not the displayed IRQ for each field is
being used by a legacy (non-PnP) card. The setting [No/ICU] for an IRQ
field indicates that you are using the ISA Configuration Utility (ICU), and
that this particular IRQ is NOT required by a legacy card. Set the IRQ field
to [Yes] if you install a legacy card that requires a unique IRQ and you are
NOT using ICU. Configuration options: [No/ICU] [Yes]
4.4.3.2 PCI IRQ Resource Exclusion