conca@handel.cs.colostate.edu (michael vincen conca) (11/13/89)
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?
-=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=-
Mike Conca, Computer Science Dept. * conca@handel.cs.colostate.edu
Colorado State University * conca@129.82.102.32
'Back off man - I understand computers!'ckp@grebyn.com (Checkpoint Technologies) (11/14/89)
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.
jms@tardis.Tymnet.COM (Joe Smith) (11/14/89)
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. -- Joe Smith (408)922-6220 | SMTP: JMS@F74.TYMNET.COM or jms@gemini.tymnet.com McDonnell Douglas FSCO | UUCP: ...!{ames,pyramid}!oliveb!tymix!tardis!jms PO Box 49019, MS-D21 | PDP-10 support: My car's license plate is "POPJ P," San Jose, CA 95161-9019 | narrator.device: "I didn't say that, my Amiga did!"
FelineGrace@cup.portal.com (Dana B Bourgeois) (11/14/89)
[line eater food] Commodore/Amiga chopped up the 16 Meg space by allocating some for chip ram, some for ROM(WCS), some for hardware expansion(registers and ROM), and some for expansion RAM(fast RAM) and some space for later use(reserved). Turns out that the bottom 1 Meg is chip, there is a meg reserved, the WCS and hardware take up another couple of megs or so and half of the 16 Meg address space is fast ram. Coming from an 8 bit machine, or an MS-DOS machine or even a small (medium-sized?) minicomputer, 9 Megabytes of memory must have seemed like plenty of address space. In 1984 that would've been 16x8x9 DRAMs at $5 each or 1152 chips costing about $5500!! By 1985 you could get 256K DRAMs so the number of chips is down to about 250 but the cost is still over $5000. It isn't until 1986 that the price of 256K DRAMs would lower the price of 9 Megabytes to say $3000. I'd say that the designers were far seeing indeed to have left over half of the available address space when they could've said, "Who'll ever need more than 2 or 3 megabytes?" The 32 bit machines will probably result in more fast ram beyond the 16 megabyte limit of 24 address bits. THere is lots of room in the map for hardware expansion and my guess is that the architecture will be redesigned before the memory map becomes outdated. One area where the mistakes of MS-DOS weren't repeated. Dana @ cup.portal.com
billsey@agora.UUCP (Bill Seymour) (11/15/89)
From article <3157@ccncsu.ColoState.EDU:, by conca@handel.cs.colostate.edu (michael vincen conca):
:
: Page 2-1 of Motorola's 68000 Microprocessor User's Manual states that
: the 68000 can 'directly access 16 megabytes' of memory. What
: limits the Amiga to a max of 9 megs of memory? Is it the custom
: chips or some other element in the hardware?
There are several areas in the memory map that can't be used for
expansion memory, or at least not now... The one meg right abouve chip
mem is reserved for future chip ram extensions, the area between A00000
and C00000 is set aside for I/O devices, as is D00000 and E00000. F00000
is mostly ROM space.
: -=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=--=*=-
: Mike Conca, Computer Science Dept. * conca@handel.cs.colostate.edu
: Colorado State University * conca@129.82.102.32
: 'Back off man - I understand computers!'
--
-Bill Seymour ...tektronix!reed!percival!agora!billsey
...tektronix!sequent.UUCP!calvin!billsey
Bejed, Inc. NES, Inc. Northwest Amiga Group At Home Sometimes
(503) 691-2552 (503) 246-9311 (503) 656-7393 BBS (503) 640-0842steve@ntmtka.mn.org (Steve Wahl) (11/16/89)
O.K., I understand with the 16 Meg address space of the 68000 processor,
9 megs (or possibly 10 w/$C00000) of address space was reserved for RAM
in the Amiga.
However, the 68020 and 68030 have 32 bit address busses, not 24 as in the
68000. That's 4 Gig, I believe. I'm sure most of you know these facts,
I'm just stating them for those who don't.
Are there any restrictions in the Amiga OS for using the rest of this space
(well, probably just *some* of it :-) ) for RAM? What address locations
are currently occupied by the 32-bit ram on the current 68020/030 processor
cards?
I guess I would have to call this academic questioning for now; my B2000 only
has the 1 Meg it came with at this point! But I would like to know.
--> Steve
--
Steve Wahl
Northern Telecom, Inc. (612) 932-8079
S-100, 9701 Data Park steve@ntmtka.mn.org
Minnetonka, MN 55343 {rosevax,bungia}!ntmtka!steve