The dma calls are in isa/dma.c, [...]

Those are only for non-mastering DMA devices, and for setting up
channels for mastering DMA devices.  Most high-performance controllers
like the Adaptec and BusLogic `smart' SCSI card do not use those
functions after boot.

   does netbsd support mapped memory from devices?  

Of course.  This is necessary for several types of devices, including
DEPCA and WD/SMC ethernet cards.

   Programmed IO is simply read/writing a byte at a time, correct?

`Programmed I/O' usually refers specifically to the use of the i386
I/O space, using the pseudo-functions in pio.h.