Linux Kernel Architecture

Mauerer app04.tex V1 - 09/04/2008 6:12pm Page 1230

Appendix D: System Startup

set_trap_gate(16,&coprocessor_error);

set_trap_gate(17,&alignment_check);

set_trap_gate(19,&simd_coprocessor_error);

...

set_system_gate(SYSCALL_VECTOR,&system_call);

...

}

As the code shows, this is also where the interrupt used for system calls is defined as a system

gate (SYSCALL_VECTORis set to0x80).

The IRQ handlers are initialized similarly.

❑ sched_initinitializes the data structures of the scheduler (for the main processor in this case)

and run queues are created.

❑ pidhash_initallocates the hash tables used by the PID allocator to manage free and assigned

PIDs.

❑ softirq_initregisters the softIRQ queues for tasklets with normal and high priority

(TASKLET_SOFTIRQandHI_SOFTIRQ).

❑ time_initreads the system time from the hardware clock. This is a processor-specific function

because different architectures use different mechanisms to read the clock.

❑ init_consoleinitializes the system consoles. Theearly printkmechanism is also disabled on

systems that provide this facility to allow messages to be output to the console before it has been

fully initialized (on other systems, messages are buffered until the console is activated).

❑ page_address_initsets up the hash table that the Persistent Kernel Map (PKMap) mechanism

uses to determine the physical page address of a permanent kernel mapping by reference to a

given virtual address.

❑ mem_initdisables the bootmem allocator (and performs a number of minor architecture-specific

actions that are of no concern), andkmem_cache_initinitializes the slab allocator in a multistage

process described in detail in Chapter 3.

❑ calibrate_delaycalculates the BogoMIPS value, which specifies how many empty loops the

CPU can run through per jiffy. The kernel requires this value to estimate the time needed for

some tasks that are performed with polling or busy waiting. The following code yields a good

approximation of the number of loops per jiffy and stores the result inloops_per_jiffy^2 :

init/calibrate.c

void __init calibrate_delay(void)

{

unsigned long ticks, loopbit;

int lps_precision = LPS_PREC;

loops_per_jiffy = (1<<12);

printk("Calibrating delay loop... ");

while (loops_per_jiffy <<= 1) {

/* wait for "start of" clock tick */

(^2) It is also possible to preset the BogoMIPS value, but this prevents the kernel from doing one of the most important operations ever,
which would clearly be boring.