From: adrie@ica.philips.nl (Adrie Koolen) Subject: Re: Query: Drivers for AHA >1522< ? Date: Fri, 31 Jul 1992 06:42:43 GMT
In article <1992Jul29.204834.20882@adaptx1.UUCP> neese@adaptx1.UUCP (Roy Neese) writes:
>In article <1992Jul23.232138.20172@colorado.edu> drew@ophelia.cs.colorado.edu (Drew Eckhardt) writes:
>> If you're talking about sustained transfer rate, it's going to be
>> faster to do polled I/O because you don't have the couple of clock
>> cycles associated with each bus on period - you just crank data
>> across the AT bus topspeed, 16 bits at a time.
>
>Nope. Polled I/O can never be faster than bus master DMA as it takes
>twicw as many cycles to move each word doing polled I/O as it does for a
>bus master to move the same word.
>The overhead you speak of is for acquiring the bus, wchi takes 2 cycles
>and then of course it takes another 2 cylces to release the bus. But to
>show a more subjective view. Let's make some assumptions, good
>assumptions. Let's say 1 cycle per word for bus master DMA and 2 cycles
>per word for polled I/O (you have to do a read and write for polled I/O,
>whereas bus masters do either a read or a write). Now let's move 512
>bytes of data. Let's see,... for polled that would be 1024 cycles and
>for a bus master 512 + 2 + 2 = 516. Nuff' said.
Unless I misunderstand the conditions of your example transfer, I have to
disagree.
I assume that you mean "AT-bus cycle" when you talk about cycles. Clearly,
you can't transfer one word over the AT-bus in only one cycle. I was under
the impression that it takes some 6 or 8 cycles.
That would seem to be a bigger advantage for bus mastering controllers, but
you're wrong in the assumption that the write into (or read from) memory
is done over the AT-bus. Normally, this is done over the local CPU bus
(33 MHz?), which is much faster than the AT-bus (8 MHz?).
Furthermore, it would be unwise to take the AT-bus for 512 cycles. Under
normal 1542b operation, the bus freed after some 7 cycles, so the overhead
of getting and releasing the bus is bigger.
I agree that bus mastering I/O is faster than polled I/O over the AT-bus,
but not with the factor you indicate.
Adrie Koolen (adrie@ica.philips.nl)
Philips Consumer Electronics, Eindhoven, the Netherlands