🚀 go-pugleaf

In article <7891@cbmvax.UUCP> daveh@cbmvax.UUCP (Dave Haynie) writes:
>You apparently have to be real clever to get fast disk I/O on a PC, though 
>I'm not at all sure just why.  Obviously an AT bus is limited to 2 MB/sec
>transfers or thereabouts if you use the AT's DMA chip (and if it runs full
>bus speeds).
[...]
> It [the AT bus] apparently doesn't permit
>DMA masters, however, so all the "DMA" that happens on the bus is via the
>AT's DMA controller chip.  Since that chip has to make two bus crossings to
>transfer one datum, that tells me that the effective DMA speed is 2 MB/s,
>and also that DMA isn't necessarily an advantage over programmed I/O, at
>least as long as your DMA and programmed I/O are the full 16 bits wide (I
>don't know much else on the AT DMA chip).

The situation is actually worse.  The DMA chip is clocked at 4MHz in an
AT and it takes 7 cycles for a bus xfer: 1.75 microseconds per transfer.
It does a "fly-by" operation to I/O though, so it does NOT take two of
these for a full I/O to/from memory operation, just one.

No-one I know of uses the AT's DMA for hard disk transfers; they all use
programmed I/O.  The block-move and block-io instructions allow full 16 bit
xfers at the full bus rate.

>Does it really have to go 8 bits at a time, even on a 16 bit bus.  That would 
>tend to put a heavy damper on things.  Ick.
>Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests"

No, there is provision for 3 8-bit DMA channels and 4 16-bit channels (I
think: it could be the other way 'round).

In the average AT only the floppies use the DMA channels, just one 8 bitter.
Really ironic I think: for years the PC and XT add-on makers cried because
there were too few DMA channels to go around.  When the AT came out, it
had oodles of crippled channels that are now ignored!
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