Symmetric Multiprocessing: Difference between revisions

Some information (which may not be present on newer machines) dedicated for multiprocessing is available. First one must find the MP Floating Pointer Structure. It is aligned on a 16 byte boundary, and contains a signature at the start "_MP_" or 0x5F504D5F. The OS must search in the EBDA, the BIOS ROM space, and last kilobyte of "base memory"; the size of base memory is specified in a 2 byte value at 0x413 in kilobytes, minus 1K.

Here is what the structure looks like:

struct mp_floating_pointer_structure {

char signature[4];

// virtual wire mode is; all other bits are reserved

}

Here is what the configuration table, pointed to by the floating pointer structure looks like:

struct mp_configuration_table {

char signature[4]; // "PCMP"

uint8_t reserved;

}

After the configuration table, there are entry_count entries describing more information about the system, then after that there is an extended table. The entries are either 20 bytes to represent a processor, or 8 bytes for something else. Here are what the processor and IO APIC entries look like.

struct entry_processor {

uint8_t type; // Always 0

uint64_t reserved;

}

Here is an IO APIC entry.

struct entry_io_apic {

uint8_t type; // Always 2

uint32_t address; // The memory mapped address of the IO APIC is memory

}

For more information, see http://www.intel.com/design/pentium/datashts/24201606.pdf, chapter 4.

 

 

=== BSP Initialization Code ===

volatile uint8_t aprunning = 0; // count how many APs have started

uint8_t bspid, bspdone = 0; // BSP id and spinlock flag

bspdone = 1;

// now you'll have the number of running APs in 'aprunning'

 

=== AP Initialization Code ===

As the application processors start up in real mode, a little Assembly is needed to enter protected mode. Modify this example to your kernel's needs.

; this code will be relocated to 0x8000, sets up environment for calling a C function

.code16

; jump into C code (should never return)

ljmp $8, $ap_startup

// this C code can be anywhere you want it, no relocation needed

void ap_startup(int apicid) {

while(1);

}

 

==See Also==