Technical Q&A's

HW 14 - DMA Not Working with PCI Bus (15-July-95)

Q I am writing a driver for a PCI board for the Power Mac, and I'm having trouble keeping the DMA working. In my initial imlementation, I want to send 2400 bands of data, each band consisting of 4800 bytes. I can send the data via programmed I/O as 1200 longword writes in I/O space via the Expansion Bus Manager, but when I attempt to use DMA (instead of programmed I/O) the Mac locks up (it doesn't even allow access to MacsBug). Are there any known problems with using DMA with a PCI card?

A There are no known problems using DMA with a PCI card. The only thing programmed I/O is supposed to do is handle any part of the band that isn't naturally aligned with pages (the DMA is supposed to handle the part that is aligned). Since programmed I/O works, your problem may be hardware-related to your card's DMA (bus mastering).

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