Assembly. Stop system time

I have a task for my Assembly course t开发者_Go百科o stop system time while pressing Alt-button. I do it by disabling the 8th interrupt. As I understand system time is saved in the 40:6ch cell of memory, so by reading this data we can achieve current system time, but also we can achieve current system time by using the second function of 1ah interrupt. Are they equal?

I check my program in DosBox. When i check system time in 40:6ch it doesn't change while pushing Alt button, if i check system time via 1ah interrupt - the time keeps changing though the 8th interrupt was disabled(it increments 40:6ch 18 times per second, as i understand). So what time should i really check? Or is there any other way to stop system time?

Here is the program for checking time via 40:6ch:

OutStr macro str ;макрос вывода строки  
push dx  
push ax  
mov ah,09h  
lea dx,str  
int 21h  
pop ax  
pop dx  
endm  
;----------------------------------------  
OutChar macro char ;макрос вывода символа  
push ax  
push dx  
mov ah,06h  
mov dl,char  
add dl,'0'  
int 21h  
pop dx  
pop ax  
endm  
;---------------------------------------  
;---------------------------------------  
.386  
ASSUME CS:CODE, DS:DATA  
DATA    SEGMENT USE16  
    M1 DB  13,10,':$'  
    M2 DB 13,10,'Current time',13,10,'$'  
    M3 DB  13,10,'Equal times',10,13,'$'  
    M4 DB  13,10,'Alt is not pushed',10,13,'$'  
    M5 DB  13,10,'Alt is pushed',10,13,'$'  
DATA    ENDS  

CODE    SEGMENT USE16  
begin:   
mov ax,DATA ;initialization  
mov ds,ax  

beg:  
    in al, 60h  
    cmp al,38h ; is Alt pressed?  
    jne beg ;No  
        OutStr M5 ; Alt is pressed  

cur:  
    mov ax, 40h  
    mov es, ax  
    mov ebx, dword ptr es:[6ch] ; current time  

    mov al,00000001b ;disable interrupts of the system timer  
    out 21h, al  

thisl:  
    mov ecx, dword ptr es:[6ch]  
    cmp ebx, dword ptr es:[6ch]  
    je outputTrue  
    jmp next

outputTrue:
    OutStr M3
next:   
    in  al,60h
    cmp al,38h ; is Alt pressed?
    je thisl ; Yes

tt:
    OutStr M4 ; Alt is not pressed
    mov al,0h ; enable interrupts of the system timer
    out 21h, al
    jmp beg

CODE    ENDS
end Begin

You are experiencing the difference between the RTC and the PIT.

The PIT is usually used for timing purposes in an OS, such as wait(10); for wait 10 seconds. The RTC on the otherhand is used for keeping time, such as knowing the time is 12:53pm.

The PIT's interrupt is IRQ0, or interrupt 0x08. The RTC's interrupt is IRQ8, or interrupt 0x70. For a bit more information see this OSDev wiki and wikipedia

Also, it should be noted that probably even if you disable interrupt 0x70, the interrupt 0x1A will still report the timer as being incremented. This is because it doesn't need to send interrupts to the processor to keep time. It keeps time internally, which you can read through the input and output port commands.