INT 1Ah: the time-of-day BIOS — 8086 assembly guide

BIOS time services: the tick count since midnight (18.2 per second) and, on an AT or later, the battery-backed real-time clock's time and date in BCD.

INT 1Ah gives access to two different clocks. The first is the tick count, a 32-bit number at 0040:006Ch that the timer interrupt (INT 08h) increments 18.2 times a second and that starts from zero at midnight. It exists on every PC, costs nothing to read, and is the usual way to time things and to seed random numbers. The second is the real-time clock of the AT: a chip with its own battery (the Motorola MC146818 and its successors, in the CMOS) that keeps the time and date while the PC is off.

AH	Service
00h	read the tick count (CX:DX) and the midnight flag (AL)
01h	set the tick count
02h	read the RTC time: hours, minutes, seconds in BCD
03h	set the RTC time
04h	read the RTC date: century, year, month, day in BCD
05h	set the RTC date
06h / 07h	set / cancel the RTC alarm (INT 4Ah)

Two clocks, one time

At boot the BIOS reads the RTC and sets the tick count to match, and DOS takes its time and date from there; after that the two run independently. DOS's INT 21h AH=2Ch works from the tick count, so setting the time with DOS changes the ticks but, on DOS before 3.3, not the RTC. A day has 1 573 040 (1800B0h) ticks: the timer runs at 1 193 182 Hz / 65 536 = 18.2065 Hz.

The example turns the tick count into hours and minutes. Dividing by 1092 (ticks per minute, rounded) is close enough for a clock display:

prints the time of day from the tick count, hours and minutes, e.g. 14:05. Dividing by 1092 instead of 1092.4 makes it run a little fast, by about 20 seconds in the afternoon.

MOV     AH, 00h       ; BIOS: ticks since midnight
INT     1Ah           ; CX:DX = ticks
MOV     AX, DX
MOV     DX, CX        ; DX:AX = ticks
MOV     BX, 1092      ; ticks per minute: 18.2065 x 60
DIV     BX            ; AX = minutes since midnight (at most 1440)
MOV     BL, 60
DIV     BL            ; AL = hours, AH = minutes
MOV     BH, AH
XOR     AH, AH
CALL    PDEC          ; the hours
MOV     DL, ":"
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
MOV     AL, BH
XOR     AH, AH
CMP     AL, 10        ; a leading zero for the minutes
JAE     TWO1
MOV     DL, "0"
PUSH    AX
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
POP     AX
TWO1:
CALL    PDEC          ; the minutes
 
PDEC    PROC    NEAR  ; print AX in decimal; keeps every register
PUSH    AX
PUSH    BX
PUSH    CX
PUSH    DX
MOV     BX, 10
XOR     CX, CX        ; digits on the stack
DECD1:
XOR     DX, DX
DIV     BX            ; AX = AX / 10, DX = the last digit
PUSH    DX
INC     CX
TEST    AX, AX
JNZ     DECD1
DECD2:
POP     DX            ; the digits come back first to last
ADD     DL, "0"
MOV     AH, 02h
INT     21h
LOOP    DECD2
POP     DX
POP     CX
POP     BX
POP     AX
RET
PDEC    ENDP

BCD

The RTC functions return binary-coded decimal: each nibble is one decimal digit, so 14 hours is 14h, 59 minutes 59h. That is convenient for display, since printing the byte as two hex digits shows the decimal number, and awkward for arithmetic. To turn a BCD byte into binary, take the high nibble times 10 plus the low nibble; AAM and AAD help.

MOV     AL, CL   ; a BCD byte, e.g. 59h
MOV     AH, AL
AND     AL, 0Fh  ; low digit: 9
MOV     CL, 4
SHR     AH, CL   ; high digit: 5
AAD              ; AL = AH x 10 + AL = 59 (3Bh)

In DOSBox-X

DOSBox-X takes the time from the host at start, so both clocks show the host's local time (on a server, often UTC). Its RTC is emulated in the CMOS at ports 70h and 71h, so programs that read the chip directly work too.