The software for the real-time clock contains an interrupt service procedure that is called once per millisecond and a procedure that updates the time located in five memory locations. Example 12-17 lists both procedures. Note that in this example the time is stored at absolute memory locations 00400H - 00404H.
EXAMPLE 12-17
;Interrupt service procedure for the real-time clock ; TIME EQU 400H CLOCK PROC FAR USES AX SI DS MOV AX,0 MOV DS,AX ;address time segment = 0000H MOV SI,OFFSET TIME MOV AH,0 ;modulus = 100 CALL UPDATE ;adjust 1/1000 second counter JNZ DONE ;if NO rollover to 00 MOV AH,10H ;modulus = 10 CALL UPDATE ;adjust 1/10 second counter JNZ DONE MOV AH,60H ;modulus = 60 CALL UPDATE ;adjust seconds counter JNZ DONE CALL UPDATE ;adjust minutes counter JNZ DONE MOV AH,24H ;modulus = 24 CALL UPDATE ;adjust hours counter DONE: MOV DX,0FF02H ;address EOI register MOV AX,8000H ;EOI instruction OUT DX,AX IRET CLOCK ENDP UPDATE PROC NEAR LODSB ;get a counter ADD AL,1 ;increment it DAA ;make it BCD CMP AH,AL ;test for modulus JNZ UPDATE1 ;if no rollover MOV AL,0 UPDATE1: MOV [SI-1],AL ;save new count RET UPDATE ENDP