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From: kpneal@eos.ncsu.edu (Kevin P. Neal)
Newsgroups: comp.unix.bsd.freebsd.misc,comp.unix.bsd.netbsd.misc
Subject: Re: Curious about *BSD History
Followup-To: comp.unix.bsd.freebsd.misc,comp.unix.bsd.netbsd.misc
Date: 14 Apr 1996 03:06:26 GMT
Organization: The House of RetroComputing
Lines: 33
Message-ID: <4kpq3i$df6@taco.cc.ncsu.edu>
References: <4k1nue$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>
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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).
The A3000 was the first machine to have 32 bits of address space.
It has onboard motherboard memory. In addition, it was the first
Amiga to have the Zorro-III bus, a 32bit bus. Z-III memory maps to
above the 16MB space, and I believe it is even higher than the
A3000 motherboard memory.
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.
AmigaDOS even provided a flag for allocating memory that made it give
you 24bit DMAable memory.
--
XCOMM Kevin P. Neal, Sophomore, Comp. Sci. \ kpneal@interpath.com
XCOMM Frue, Secret Agent of Smerp (shh!) \ kpneal@eos.ncsu.edu
XCOMM Visit the House of RetroComputing at / Perm. Email:
XCOMM http://www4.ncsu.edu/~kpneal/www/ / kevinneal@bix.com