User:Johnburger/Demo/Ints/Timer

The Timer interrupt handler doesn't have to do much: the Periodic Interval Timer chip doesn't need any steps to acknowledge the interrupt it generated. That means that the handler just has to:

• Save the registers it's going to use;
• Acknowledge the interrupt in the PIC;
• CALL the generic Task Switching code;
• Restore the registers;
• Return to the interrupted code.

Note that if a Task Switch occurred, the code that was resumed in the last step won't be the actual code that was interrupted: that code has been paused until the Task Scheduler gets back around to it.

Timer.inc

;
; Ints/Timer.inc
;
 
; This module implements the Timer interrupt handler.
; It simply performs some housekeeping and then calls the generic Task
; Switching code.
 
Ints.Timer:
                PUSH            EAX             ; Need this register
 
                MOV             AL,Dev.PIC.Cmd.EOI ; Acknowledge interrupt in PIC
                OUT             Dev.PIC.A.Cmd,AL   ; (Note interrupts still off)
 
                CALL            Ints.Switch     ; Switch Task
 
                POP             EAX
                IRETD                           ; And return where we left off