Where should the type information be 162

Brian Inglis

PC-DOS and all of its derivatives, which includes Windoze.

Early on Microsoft(R) evaluated the overall speed of IO on typical PC hardware and found that PIO was faster than DMA. So they used PIO. The AT-IDE-ATA hardware has followed that until the development of "Ultra DMA", which is more reasonable. However, using that DMA capability requires special, 80-conductor cables with the 40 extra lines all tied to ground in order to insulate each of the signal lines from the crosstalk. So even on a recent machine you might find a UDMA controller and a UDMA drive connected with a 40-conductor cable and thus falling back to PIO.

Note that PIO exists in at least five flavors PIO-0 through PIO-4.

This insbreastutionalized stupidity wreaks havoc on multi-tasking. PIO is still faster at the bytes-sec level, but it absorbs huge quanbreasties of CPU so overlapping processing with IO just doesn't work well. Microsoft(R) knew this when they went for PIO over DMA, but choose to overoptimize the raw disk speed at the expense of system throughput. IOW it wasn't an accident. It was on purpose.

Even in Windoze v5 (AKA 2000) on a multi-GHz machine a background copy can drag the FG response time out past one second. I don't know if this has been addressed in v6 (AKA 2003), but I doubt it because the situation is an incredible mess.

tj3