IO-APIC support on uniprocessors

configname: CONFIG_X86_UP_IOAPIC

Linux Kernel Configuration
└─>Processor type and features
└─>IO-APIC support on uniprocessors
In linux kernel since version 2.6.12  
An IO-APIC (I/O Advanced Programmable Interrupt Controller) is an
SMP-capable replacement for PC-style interrupt controllers. Most
SMP systems and many recent uniprocessor systems have one.

If you have a single-CPU system with an IO-APIC, you can say Y here
to use it. If you say Y here even though your machine doesn't have
an IO-APIC, then the kernel will still run with no slowdown at all.