Messages in this thread |  | | From | "Henning P. Schmiedehausen" <> | Subject | Re: Linux 2.4.13-ac4 | Date | Mon, 29 Oct 2001 17:14:53 +0000 (UTC) |
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Vojtech Pavlik <vojtech@suse.cz> writes:
>On Mon, Oct 29, 2001 at 10:56:35AM +0000, Alan Cox wrote: >> > bytes read from the 8254 get swapped. I've got some indirect evidence >> > that this also could happen with the original i8254. >> >> Im hoping not. That would imply we interrupted someone half way through >> reading the counter which means the locking is screwed up.
>Some old DOS assembly sources say that this sometimes happens without >any interrupts at all - just that the chip is sometimes confused. It'd
Hm. If this is like the 8253 (ugh, way way back in the good old 8085 days I really wired and programmed such a bugger on my CP/M system...), then the problem is, that the 16 bit counter is read in two 8 bit portions. And if you do this:
Hi Lo
00000000 11111111 ----> read low byte: 11111111
Counter increments
00000001 00000000 ----> read high byte: 00000001
=> Counter is 00000001 11111111 which is wrong. :-)
Sames goes for decrement, I have forgotten in which direction this bugger counts.
There was a trick when CPUs had much less MHz:
read low byte -> A read high byte -> B read low byte -> C
If A and C are the same, then the counter didn't change while reading:
byte a, b, c;
a = inb(lowbyte);
for(;;) { b = inb(hibyte); c = inb(lobyte); if(c == a) break; a = c; }
result = b<<8+a;
This assumes that you can't get _two_ changes in the high byte while reading the ports in a row.
>be definitely worth printing the bad and good values when a problem is >detected, so that we know what's happening.
This will happen all the time, so printing out is neither a good idea nor is the read problem described above an error. It is just a quirk in using an 8 bit chip in an 16 bit environment without being able to "latch" the count.
Regards Henning
-- Dipl.-Inf. (Univ.) Henning P. Schmiedehausen -- Geschaeftsfuehrer INTERMETA - Gesellschaft fuer Mehrwertdienste mbH hps@intermeta.de
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