IBM SAGP-845EV Manuel D’Utilisation

 42

 

 

The Watchdog Timer is provided to ensure that standalone systems can always 

recover from catastrophic conditions that cause the CPU to crash.  This condition 

may have occurred by external EMI or a software bug.  When the CPU stops 

working correctly, hardware on the board will either perform a hardware reset 

(cold boot) or a Non-Maskable Interrupt (NMI) to bring the system back to a 

known state. 

 

A BIOS function call (INT 15H) is used to control the Watchdog Timer: 

 

INT 15H: 

 

AH – 6FH 
Sub-function: 

AL  –  2 : Set the Watchdog Timer’s period 

BL         : Time-out value(Its unit--second or minute, is 

                dependent on the item “Watchdog Timer unit  

                select” in CMOS setup). 

 

The sub-function 2 needs to be called to set the time-out period of Watchdog 

Timer first.  If the time-out value is not zero, the Watchdog Timer will start to 

count down.  While the timer value reaches zero, the system will reset.  To 

ensure that this reset condition does not occur, the Watchdog Timer must be 

periodically refreshed by calling sub-function 2.  However the Watchdog timer will 

be disabled if the user set the time-out value to zero. 

 

 

Note: when exiting a program, it is necessary to disable the Watchdog Timer; 

otherwise the system will reset. 

 

 

; 

W_LOOP: 

 

        MOV     AX, 6F02H              ;setting the time-out value 

        MOV     BL, 30                     ;time-out value is 48 seconds 

        INT       15H 

 ; 

; 

        CMP      EXIT_AP, 1            ;is your application over? 

        JNE       W_LOOP               ;No, restart your application 

 

         MOV    AX, 6F02H              ;disable Watchdog Timer 

         MOV    BL, 0                        ; 

         INT      15H 

; 

;