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From: jms@tardis.Tymnet.COM (Joe Smith)
Newsgroups: comp.sys.amiga.tech
Subject: Re: maximum memory
Summary: ROM and custom chips use up some of the address space
Message-ID: <792@tardis.Tymnet.COM>
Date: 14 Nov 89 08:45:37 GMT
References: <3157@ccncsu.ColoState.EDU>
Reply-To: jms@tardis.Tymnet.COM (Joe Smith)
Organization: McDonnell Douglas Field Service Co, San Jose CA
Lines: 27

In article <3157@ccncsu.ColoState.EDU> conca@handel.cs.colostate.edu (michael vincen conca) writes:
>the 68000 can 'directly access 16 megabytes' of memory. What limits the
>Amiga to a max of 9 megs of memory? Is it the custom chips or ...

Glossing over some of the details, here's how the 16 one-megabyte
ranges are allocated:

	000000-0FFFFF = "chip" RAM, accessable by the custom chips
	100000-1FFFFF = reserved for future expansion of chip RAM
	2, 3, 4, 5, 6, 7, 8, 9 = 8 megabytes of expansion (fast) RAM
	A00000-AFFFFF = reserved
	B00000-BFFFFF = includes addresses for 8520 interface chips
	C00000-CFFFFF = includes 2nd 512K for A500/A2000 with old Agnus
	D00000-DFFFFF = Agnus/Denise/Paula registers start at DFF000
	E00000-EFFFFF = 2nd half is reserved for expansion slots
	F00000-FFFFFF = 2nd half decodes as ROM addresses

For an A2000 with Fat Agnus, the 9 megabytes are 1/2 meg at 0, 8 meg
at 2 through 9, and 1/2 meg at C.  With the new 1-meg Agnus, it is
1 meg at 0, 8 meg at 2 through 9, and nothing at C.  I think someone
has made a card that can be used as C00000-CFFFFF which would bring the
total up to 10 megabytes.
-- 
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