INT 1Ah AH=00h: read the tick count — 8086 assembly guide

Return the number of timer ticks since midnight in CX:DX, 18.2 per second. AL is nonzero once after midnight has passed.

INT 1Ah AH=00h returns the 32-bit tick count from 0040:006Ch: CX = high word, DX = low word. The timer interrupt adds 1 every 54.9 ms, and the count goes from 0 at midnight to 1 573 039 just before the next one. AL returns the "midnight passed" flag from 0040:0070h, and reading it clears it.

For timing short intervals the low word alone is enough: subtract the starting value and the 16-bit subtraction gives the right answer even across a wrap from FFFFh to 0. The example waits for 18 ticks, about one second, by watching the count:

prints: 18 ticks, about one second (after a pause of about a second)

MOV     AH, 00h       ; BIOS: read the tick count
INT     1Ah           ; CX:DX = ticks since midnight
MOV     BX, DX        ; the low word is enough for short times
WAIT1:
MOV     AH, 00h
INT     1Ah
MOV     AX, DX
SUB     AX, BX        ; ticks gone by (wraps correctly)
CMP     AX, 18        ; 18 ticks: about one second
JB      WAIT1
CALL    PDEC          ; 18
MOV     DX, OFFSET SEC1
MOV     AH, 09h       ; DOS: print the $-string at DS:DX
INT     21h
 
SEC1    DB      " ticks, about one second", 13, 10, "$"
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

Resolution

A tick is 55 ms, so anything measured in ticks is accurate to ±1 tick. For a delay of "at least n ticks" wait for n + 1, because the first tick may come right after you start. For finer timing programs read the timer chip's counter itself (port 40h, after latching it with port 43h), or use INT 15h AH=86h for a delay in microseconds.

Pitfalls

The midnight flag is meant for DOS: DOS reads it to know when to advance the date. A program that calls AH=00h itself around midnight can take the flag first, and on many DOS versions the date then fails to change. Programs that only want the count can read 0040:006Ch directly instead (with CLI around the two words, so a tick cannot fall between them). The count is reset at midnight, so a long measurement that crosses it sees time go backwards. AH=01h sets the count; it changes the system time, so leave it to DOS.