INT 1Ah AH=04h: read the real-time clock date — 8086 assembly guide

Read the date from the real-time clock: CH century, CL year, DH month, DL day, all in BCD. CF = 1 if the clock has stopped.

INT 1Ah AH=04h reads the date kept by the CMOS clock: the century in CH (19h or 20h), the year within it in CL, the month (01h..12h) in DH and the day of the month in DL, all BCD. The weekday the chip also keeps is not returned; DOS computes its own (INT 21h AH=2Ah).

prints the current date from the RTC, year-month-day, e.g. 2026-10-23

MOV     AH, 04h       ; BIOS: read the RTC date
INT     1Ah           ; CH = century, CL = year, DH = month, DL = day
JC      FAIL1
MOV     BX, DX        ; keep month and day: printing uses DL
MOV     AL, CH
CALL    PHEX2         ; "20"
MOV     AL, CL
CALL    PHEX2         ; "26": the year in two BCD bytes
MOV     DL, "-"
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
MOV     AL, BH
CALL    PHEX2         ; the month
MOV     DL, "-"
MOV     AH, 02h       ; DOS: print the character in DL
INT     21h
MOV     AL, BL
CALL    PHEX2         ; the day
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

The century

The MC146818 itself only counts years 00..99. The century byte is a convention of the IBM AT's CMOS layout (address 32h), maintained by the BIOS, and some old BIOSes did not roll it over at the end of 1999: they reported 1900 for 2000. That is the "Y2K" BIOS problem, and why careful programs treat a year below 80 with century 19h as 20xx. DOS does the same internally, since its own date starts at 1980.

Pitfalls

As with the time, there is no RTC before the AT. The date only advances while the machine runs or the battery lasts; DOS's date, advanced at midnight from the tick count, can drift away from it if a program swallows the midnight flag of INT 1Ah AH=00h. AH=05h sets the date from the same registers.