From owner-freebsd-questions Thu Apr 25 16:21:23 1996 Return-Path: owner-questions Received: (from root@localhost) by freefall.freebsd.org (8.7.3/8.7.3) id QAA19174 for questions-outgoing; Thu, 25 Apr 1996 16:21:23 -0700 (PDT) Received: from phaeton.artisoft.com (phaeton.Artisoft.COM [198.17.250.211]) by freefall.freebsd.org (8.7.3/8.7.3) with SMTP id QAA19169 for ; Thu, 25 Apr 1996 16:21:13 -0700 (PDT) Received: (from terry@localhost) by phaeton.artisoft.com (8.6.11/8.6.9) id QAA25808; Thu, 25 Apr 1996 16:16:32 -0700 From: Terry Lambert Message-Id: <199604252316.QAA25808@phaeton.artisoft.com> Subject: Re: bounce buffers To: alk@Think.COM (Tony Kimball) Date: Thu, 25 Apr 1996 16:16:32 -0700 (MST) Cc: terry@lambert.org, questions@freebsd.org In-Reply-To: <199604252300.SAA03545@compound.Think.COM> from "Tony Kimball" at Apr 25, 96 06:00:50 pm X-Mailer: ELM [version 2.4 PL24] MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-questions@freebsd.org X-Loop: FreeBSD.org Precedence: bulk > 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.