Re: DMA at 2.8 MHz?

[toddpw@cco.caltech.edu (Todd P. Whitesel)]

There is some confusion here.

What Drew was talking about was the speed at which the peripheral bus runs;
this is 1 mhz and can't go any faster for compatibility with older //e
peripheral cards like he said. This limits the maximum transfer rate between
the RF and the IIgs to 1 megabyte/second, and in practice you get less
because of O/S and driver overhead. I have yet to ever measure anything
approaching the 1 mb/sec limit in actual operation because of this. I
would love to be proven wrong though.

What Jim posted from the hardware reference were descriptions of what speed
the 65816 could be running at while DMA is occurring. This is independent
of the data transfer speed and only affects whether or not the CPU can do
anything while DMA is going on in the system. Existing DMA drivers all seem
to drop the CPU down to 1 mhz and wait for the transfer to finish, and there
are some good reasons for doing it that way.

Todd Whitesel
toddpw @ cco.caltech.edu