Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uunet!pyrdc!grebyn!ckp
From: ckp@grebyn.com (Checkpoint Technologies)
Newsgroups: comp.sys.amiga.tech
Subject: Re: maximum memory
Message-ID: <14032@grebyn.com>
Date: 14 Nov 89 03:35:42 GMT
References: <3157@ccncsu.ColoState.EDU>
Reply-To: ckp@grebyn.UUCP (Checkpoint Technologies)
Organization: Grebyn Timesharing, Vienna, VA, USA
Lines: 27

In article <3157@ccncsu.ColoState.EDU> conca@handel.cs.colostate.edu (michael vincen conca) writes:
>
>Page 2-1 of Motorola's 68000 Microprocessor User's Manual states that
>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 some other element in the hardware?
>
	The Amiga 1000 design was made a tad more hardware-economical by
breaking the 16 Meg addressing range of the 68000 into 8 2-meg pieces,
and placing one IO device in each. The breakdown was as follows (2 meg
each):

$000000 Chip RAM
$200000 Expansion RAM #1
$400000 Expansion RAM #2
$600000 Expansion RAM #3
$800000 Expansion RAM #4
$A00000	CIAs (both)
$C00000 Custom Chips
$E00000 ROM

	8 times 2 meg equals the 68000's total addressing capability, 16
Meg. Some other devices were added as of the Amiga 500 and 2000 (the
battery clock at $DC0000, 'slow' RAM at $C80000, and IO expansion space
at $E80000), but the main memory map is mostly just as you see there.
There are enormous, gaping, unused holes, but the way the device select
logic is built they are, for now, unuseable.