I recently re-watched the original Stargate movie, and I may have missed it...but how did the SG team dial Earth from Abydos? I didn't recall them having found a DHD.
It's glossed over, and never shown. Based on the conversation between O'Neil and Jackson, Jackson was supposed to align the symbols, implying it was a manual dial. Jackson just needed to know the symbols needed, and order of them.
As has been shown in the SG-1 series, all that is needed to activate the Stargate is to manually rotate the ring, and a sufficient power source. SG-1 also explains that
Most Worlds have a Dial Home Device. These are both Season 1 events, so its clear that the ideas were thought out to explain discrepancies between the Movie and the Series.
The novelization explains it in the same manner. The Stargate is completely manually dialed, as the inner ring moves freely. The SGC setup simply moves the inner ring, and there is no direct computer connection. It is all passively sensed.
In the end, the actual means of how it is down does not matter to the plot.
In the movie it was entirely glossed over, so there is no in-universe explanation if you only consider the movie.
The SG-1 pilot very quickly established that Abydos has a DHD, however.