So you have a stand-alone (you said "pre-boot") program, like a bootloader, running in real mode? And this is on a PeeCee with the usual BIOS?
In that case you have only one CPU running. In order to spin-up the other CPU units an operating system will typically execute what is called the universal startup algorithm which goes like this:
BSP sends AP an INIT IPI
BSP DELAYs (10mSec)
If (APIC_VERSION is not an 82489DX) {
BSP sends AP a STARTUP IPI
BSP DELAYs (200μSEC)
BSP sends AP a STARTUP IPI
BSP DELAYs (200μSEC)
}
BSP verifies synchronization with executing AP
The BSP is the Boot Processor. An AP is an Application Processor. An IPI is an inter-processor interrupt. In order to do an IPI, you need to enable the APIC, an interrupt controller extension to the PC architecture which is not enabled at bootup. That's why the code is worried about what kind of ICU version it is running. All of this is fairly deep kernel magic. You might try looking at Linux, NetBSD, or other *BSD source code for an example, but it won't be easy to read. If you really win, you might find a small kernel or standalone SMP test program out there somewhere.
For more information, see the Intel Multiprocessor Specification.