A2091 and Zorro-II DMA memory

[SpeedGeek]

This has been a well known design limitation of the A2091 for a long time now. It's also been a topic for discussion on many Amiga forums (including Amiga.org).

The A2091s DMAC was designed to support the 68000 bus architecture and likewise is limited to transfers within the 24 bit (16MB) address space. The C= driver (scsi.device) defaults to PIO mode if it can not perform DMA transfers. This results in much slower transfer rates.

Performance is significantly improved by using a software patch (i.e. vbak2091) or a Guru ROM to perform buffered DMA transfers above 24 bit address space.

[SpeedGeek]

Zorro2 cards can't do DMA above 24 bit address space. But they can do DMA to a buffer in memory below 24 bit address space. Then a 32 bit CPU can copy this buffer to memory above 24 bit address space. That's why it's called buffered DMA.