INT 1Ah AH=02h: read the real-time clock time — 8086 assembly guide

Read the time from the battery-backed real-time clock (AT and later): CH hours, CL minutes, DH seconds, in BCD. CF = 1 if the clock has stopped.

INT 1Ah AH=02h reads the time from the CMOS real-time clock, the chip that keeps running from a battery when the PC is off. It returns the hours (0..23) in CH, the minutes in CL and the seconds in DH, each as a BCD byte: two decimal digits in the two nibbles. DL says whether the chip's daylight-saving option is enabled, which is almost never used. The BIOS waits for the chip's update cycle to finish before reading, so the three values belong together.

Since BCD bytes are decimal digits in hex nibbles, printing each byte with a hex routine shows the time directly:

prints the current time from the RTC, e.g. 14:05:32

MOV     AH, 02h       ; BIOS: read the real-time clock
INT     1Ah           ; CH = hours, CL = minutes, DH = seconds, BCD
JC      FAIL1         ; CF = 1: the clock is not running
MOV     AL, CH
CALL    PHEX2         ; BCD prints as decimal through the hex routine
MOV     DL, ":"       ; DL only: DH still holds the seconds
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
MOV     AL, CL
CALL    PHEX2
MOV     DL, ":"
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
MOV     AL, DH
CALL    PHEX2
JMP     DONE1
FAIL1:
MOV     DX, OFFSET STOP1
MOV     AH, 09h       ; DOS: print the $-string at DS:DX
INT     21h
DONE1:
 
STOP1   DB      "the clock is not running$"
PHEX2   PROC    NEAR  ; print AL as two hex digits; keeps every register
PUSH    AX
PUSH    CX
PUSH    DX
MOV     DH, AL        ; keep the byte
MOV     CL, 4
SHR     AL, CL        ; the high nibble
CALL    HEXN1
MOV     AL, DH
AND     AL, 0Fh       ; the low nibble
CALL    HEXN1
POP     DX
POP     CX
POP     AX
RET
HEXN1:
ADD     AL, "0"
CMP     AL, "9"
JBE     HEXN2
ADD     AL, 7         ; past "9": "A".."F"
HEXN2:
MOV     DL, AL
MOV     AH, 02h
INT     21h
RET
PHEX2   ENDP

Pitfalls

The original PC and XT have no RTC; their BIOS has no such function and returns with CF in whatever state, so on those machines check the BIOS date or fall back on DOS. An RTC with a flat battery reports CF = 1 or garbage. The RTC and the DOS time (INT 21h AH=2Ch) can disagree: DOS keeps its own time from the tick count and, before DOS 3.3, INT 21h AH=2Dh did not update the chip. Compare BCD values with BCD constants (CMP CH, 12h for noon), not with binary ones. AH=03h sets the RTC time from the same registers.