Programmable Interval Timer: Difference between revisions
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convert exotic assembler syntax to NASM |
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The PIT's generating a hardware interrupt every ''n'' milliseconds allows you to create a simple timer. Start with a global variable x that contains the delay: |
The PIT's generating a hardware interrupt every ''n'' milliseconds allows you to create a simple timer. Start with a global variable x that contains the delay: |
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<source lang="asm"> |
<source lang="asm"> |
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section .data |
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CountDown |
CountDown: dd 0 |
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</source> |
</source> |
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Next, every time the timer interrupt is called, decrement this variable until 0 is stored. |
Next, every time the timer interrupt is called, decrement this variable until 0 is stored. |
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<source lang="asm"> |
<source lang="asm"> |
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section .text |
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global TimerIRQ |
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TimerIRQ: |
TimerIRQ: |
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push eax |
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mov eax, [CountDown] |
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or eax, or eax ; quick way to compare to 0 |
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jz TimerDone |
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mov eax, [CountDown] |
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dec eax |
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mov [CountDown], eax |
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TimerDone: |
TimerDone: |
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pop eax |
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iretd |
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</source> |
</source> |
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Finally, create a function <tt> |
Finally, create a function <tt>sleep</tt> that waits the interval, in milliseconds. |
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<source lang="asm"> |
<source lang="asm"> |
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[GLOBAL |
[GLOBAL sleep] |
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sleep: |
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push ebp |
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mov ebp, esp |
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push eax |
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mov eax, [ebp + 8] ; eax has value of sole argument |
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mov [CountDown], eax |
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SleepLoop: |
SleepLoop: |
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cli ; can't be interrupted for test |
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mov eax, [CountDown] |
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or eax, eax |
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jz SleepDone |
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sti |
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nop ; nop a few times so the interrupt can get handled |
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nop |
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nop |
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nop |
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nop |
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nop |
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jmp SleepLoop |
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SleepDone: |
SleepDone: |
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sti |
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pop eax |
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pop ebp |
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ret |
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</source> |
</source> |
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