timer.asm

    bits 16
    cpu 386

segment _TEXT class=CODE

    global timer_init_
timer_init_:
    push ax
    push es
    push ds

    mov ax, 3508h
    int 21h
    mov [prev_isr], bx
    mov [prev_isr + 2], es

    cli

    mov ax, 2508h
    mov dx, timer_isr ; ds is already equal to cs
    int 21h

    ; Set to 1165.215 Hz
    mov dx, 0x43
    mov al, 0x36
    out dx, al
    mov dx, 0x40
    mov al, 0x00 ; Divisor LSB
    out dx, al
    mov al, 0x04 ; MSB
    out dx, al

    sti

    pop ds
    pop es
    pop ax
    ret

    global timer_cleanup_
timer_cleanup_:
    push ax
    push dx
    push ds

    cli

    mov dx, [prev_isr]
    mov ds, [prev_isr + 2]
    mov ax, 2508h
    int 21h

    mov dx, 0x43
    mov al, 0x36
    out dx, al
    mov dx, 0x40
    mov al, 0
    out dx, al
    out dx, al

    sti

    pop ds
    pop dx
    pop ax
    ret

    extern _timer_ticks
    extern timer_update_

timer_isr:
    inc dword [cs:_timer_ticks]

    pushad
    push ds
    push es

    mov ax, cs
    mov ds, ax
    mov es, ax

    call timer_update_

    pop es
    pop ds
    popad

    dec word [cs:clock_counter]
    jz clock_update

    push ax
    mov al, 0x20
    out 0x20, al
    pop ax

    iret

clock_update:
    mov word [cs:clock_counter], 0x40
    jmp far [cs:prev_isr]

    align 4
prev_isr dd 0
clock_counter dw 0x40