Date: Thu, 25 Apr 1996 16:16:32 -0700 (MST) From: Terry Lambert <terry@lambert.org> To: alk@Think.COM (Tony Kimball) Cc: terry@lambert.org, questions@freebsd.org Subject: Re: bounce buffers Message-ID: <199604252316.QAA25808@phaeton.artisoft.com> In-Reply-To: <199604252300.SAA03545@compound.Think.COM> from "Tony Kimball" at Apr 25, 96 06:00:50 pm
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> Bounce buffers are required for 24 bit (ISA) bus master DMA > to addresses over 16M. > > Only bus-mastering DMA, then? I'm quite vague on PC hardware, > so correct my errors, please, but is there not a slave mode of > DMA operation as well? Are bounce buffers required for slave mode? No. The on-board DMA controller is on the local memory bus and therefore does not suffer the same access limitations as if it were on the I/O bus. Typically, the only device which may use this is the built-in FDC on most systems, and that's useless because the FDC chips in Intel hardware wither have no FIFO whatsoever, or an 8 or 16 character FIFO, but the fact is undetectable without crashing the hardware without the FIFO. In any case, "hostmaster" DMA will not have the problem because it will only access device memory from system space instead of system memory from device space. Device memory, by definition, is mapped to locations accessable to the devices. 8-). Terry Lambert terry@lambert.org --- Any opinions in this posting are my own and not those of my present or previous employers.
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