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From: Terry Lambert <terry@lambert.org>
Newsgroups: comp.unix.bsd.freebsd.misc,comp.unix.bsd.netbsd.misc
Subject: Re: Curious about *BSD History
Date: Sun, 14 Apr 1996 13:58:37 -0700
Organization: Me
Lines: 43
Message-ID: <317166FD.4F08B4E2@lambert.org>
References: <4k1nue$lm8@orb.direct.ca> <3165791B.52BFA1D7@FreeBSD.org> <4kh916$1pa@cynic.portal.ca> <4kl863$rno@news.rhrz.uni-bonn.de> <1996Apr12.210743.28292@wavehh.hanse.de> <317006C4.77C0450E@lambert.org> <4kpq3i$df6@taco.cc.ncsu.edu>
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Kevin P. Neal wrote:
]
] Terry Lambert (terry@lambert.org) wrote:
] : Like Amiga needs bounce buffers for "device accessable" vs.
] : "device inaccessable" ("fast") RAM.
]
] Ummm. The Amiga has 512kb-2mb of "chip" ram, accessable to ZorroII
] DMA capable devices. This RAM is located at $00000000. The Z-II
] bus is the one located on the 68000 machines, and as such has only 24
] bits of address.
]
] The Z-II "RAM" address space, where Z-II RAM boards map to, is located
] right above the "chip" ram. This RAM is actually "fast" RAM, because
] the processor does not have to go through the "Agnus" co-processor
] to gain access to the bus to the memory. Instead, it goes to the Z-II
] bus to the ram (which the co-processors can't get to).
[ ... ]
] A Z-II device should be able to DMA into memory below 16mb, where some
] of this memory is "fast". The Z-III devices should be able to DMA into
] any RAM. A Z-II DMA board _would_ need bounce buffers to go into
] Z-III/A3000 RAM. That is the only case that I can think of.
This should be made to work, too, of course.
I was thinking about things like blitter lists, or sprites for
mouse pointers, which would need "bouncing" to low memory for
the hardware to get at them.
DMA is a two way street, so so is bounce buffering. You need to
bounce ISA DMA on SCSI *writes* as well as reads.
-- Terry "Bet my A1000 serial number is lower than your" Lambert
8-)
Terry Lambert
terry@lambert.org
---
Any opinions in this posting are my own and not those of my present
or previous employers.