APIC Timer: Difference between revisions

=== Prerequisites ===

Before we start, let's define some constant and functions.

apic = the linear address where you have mapped the APIC registers

 

writegate: ...

ret

I will also assume that you have a working [[IDT]], and you have a function to write a gate for a specific interrupt: writegate(intnumber,israddress).

Furthermore, to make things simple, I'll assume that you did not change the default interrupt mapping found in almost every tutorial:

=== Example code in ASM ===

Here's a possible way to initialize APIC timer in fasm syntax assembly:

;you should read MSR, get APIC base and map to "apic"

;you should have used lidt properly

;although I have found buggy hardware that required it

mov dword [apic+APIC_TMRDIV], 03h

 

=== Example code in C ===

This code is an example of how to initialize the APIC timer so that it ticks every 10 milliseconds. This is done by letting the APIC timer run, waiting for 10ms using the PIT and then getting the number of ticks that were done from the APIC timer. It assumes that you have functions to "write"/"read" the APIC's registers and "pit_prepare_sleep"/"pit_perform_sleep" to perform an as accurate as possible measurement of the timer's frequency.

void apic_start_timer() {

// Tell APIC timer to use divider 16

write(APIC_REGISTER_TIMER_INITCNT, ticksIn10ms);

}

 

== See also ==

 

[[Category:Interrupts]]