README.smp

                                      SMP
                                      ===

The bootstrap processor (BSP) brings up a single processor in the target mode
(32 or 64 bit).  Before that, during startup in real-mode, the BIOS tables from
ACPI and Multiprocessor Specification are checked and their information about
available processors and APICs is stored in the local hw_info structure. In the main
kernel, the subsystems are initialized. Among them, the module apic.c initializes
the local APIC and brings up the application processors (APs).

Via the local APIC, interprocessor interrupts (IPIs) are sent from the BSP to
the APs.  In the IPI message, a startup page is sent. This can be any 4k Page
below 1 MB (obviously minus the BIOS ROM areas, effectively below 640 kB). 

Page 0 (0x00000..0x01000) is skipped. After that, the first page tables for the
booting process are placed until the memory management is initialized.
Currently, Page 0x88 is selected as booting page (config.h).

The kernel is loaded by multiboot above 1 MB which is not accessible in 16 bit
mode, so the bootstap code must be copied to that page. In the 64 bit kernel,
the code is placed in jump64.asm, because the early 32 bit code is not
accessible in the actual kernel. Further on, the code must be position
independent code or already be relocated to the target page. This is realized
by using segments in 16 bit mode and calculating the offset from the beginning
label of the code. In 32 bit mode, the offset of the smp start page is
included. The common code for both architectures is in start_smp.inc which is
included directly in start32.asm and jump64.asm. The file ends with a label so
that the APs leave the included file in 32 bit protected mode. The 32 bit
kernel just jumps to main_smp() function. The 64 bit kernel activates long mode
and jumps also to the same function.