In some older versions of the NOISE.SYS random driver I experimented with calling the BIOS Wait function which uses the CMOS timer to pause, using the "drift" between timing differences. There appears to be some variation here, but I don't have enough documentation (and have yet to hack with the BIOS myself) to figure out what goes on exactly when one calls Int 15h/AH=86h, so I don't know if this is "real" clock drift of if the variation is caused by somehting else unsuitable for an RNG. Still, it seems interesting. Does anyone have decently detailed tech specs for this function? Source is enclosed below for reference. No copyrights on it. ---Rob <WlkngOwl@unix.asb.com> ----- Attachment begins here ----- {$F-} const timer0 = $40; timercntl = $43; WaitInterval = 977; function SampleTimerWord: Word; assembler; asm mov al, 0c2h out timercntl, al { Latch status and count for timer 0 } in al, timer0 { Get status word } test al, 2 { Remember mode 2 v. mode 3 for later } mov ch, al in al, timer0 { Get count low byte } mov ah, al in al, timer0 { Get count high byte } xchg ah, al jz @GotSample { If mode 2, skip this last bit...} add ch, ch { Top bit of status byte into CF} rcr ax, 1 { Shift data down and accumulate} @GotSample: end; function Sample: Integer; assembler; asm { From Ralph Brown's Interrupt List: --------B-1586------------------------------- INT 15 - BIOS - WAIT (AT,PS) AH = 86h CX:DX = interval in microseconds Return: CF clear if successful (wait interval elapsed) CF set on error or AH=83h wait already in progress AH = status (see #0390) Note: the resolution of the wait period is 977 microseconds on most systems because most BIOSes use the 1/1024 second fast interrupt from the AT real-time clock chip which is available on INT 70 SeeAlso: AH=41h,AH=83h,INT 1A/AX=FF01h,INT 70 } call SampleTimerWord push ax xor cx, cx mov dx, WaitInterval mov ah, 86h int 15h jnc @NoError { does this affect timings much? } xor ax, ax jmp @Abort @NoError: call SampleTimerWord pop bx sub ax, bx @Abort: end; begin { Note: repeated/rapid calls to Sample() crashes the system or causes BOUND interrupts (which triggers the Print Screen function on PCs). } WriteLn(Sample:6); end. Rob. --- Send a blank message with the subject "send pgp-key" (not in quotes) to <WlkngOwl@unix.asb.com> for a copy of my PGP key.