Question

I am trying to create a sleep/delay procedure in 16bit MASM Assembly x86 that will, say, print a character on the screen every 500ms.
From the research I have done, it seems that there are three methods to achieve this – I would like to use the one that uses CPU clock ticks.

Please note I am running Windows XP through VMWare Fusion on Mac OS X Snow Leopard – I am not sure if that affects anything.

Could someone please point me in the right direction, or provide a working piece of code I can tweak? Thank you!

The code I have found is supposed to print ‘A’ on the screen every second, but does not work (I’d like to use milliseconds anyways).

TOP:
MOV AH,2C
INT 21
MOV BH,DH  ; DH has current second
GETSEC:      ; Loops until the current second is not equal to the last, in BH
MOV AH,2C
INT 21
CMP BH,DH  ; Here is the comparison to exit the loop and print 'A'
JNE PRINTA
JMP GETSEC
PRINTA:
MOV AH,02
MOV DL,41
INT 21
JMP TOP

EDIT: Following GJ’s advice, here’s a working procedure. Just call it

DELAY PROC
 TIMER:
 MOV     AH, 00H
 INT     1AH
 CMP     DX,WAIT_TIME
 JB      TIMER
 ADD     DX,3         ;1-18, where smaller is faster and 18 is close to 1 second
 MOV     WAIT_TIME,DX
 RET
DELAY ENDP

Answer 1

Actually you can use ROM BIOS interrupt 1Ah function 00h, ‘Read Current Clock Count’. Or you can read dword at address $40:$6C but you must ensure atomic read. It is incremented by MS-DOS at about 18.2 Hz.

For more information read: The DOS Clock