🚀 go-pugleaf

In article <17184@louie.udel.EDU> Darren asks:
>A while back, somebody asked about the memory map of the Amiga and
>was answered by "ExecBase is a $4". Although correct, this is not
>what I'm asking. I'm just curious about the actual memory mapping
>in the Amiga. Where IS chip ram?
The physical hardware address for chip ram is designated as $000000 to $1FFFFF,
a 2 meg physical space.  The Amiga 1000 has 256K of this implemented on board
($000000 to $03FFFF), with an additional 256K implementable by a commonly
available add-on module, resulting in 512K of chip ram ($000000 to $07FFFF).
The Amiga 2000 was manufactured initially with 512K of chip ram on-board, as
that is as large an address space as can be addressed by either the original
A1000 custom chip blitter (19-bit internal and on-board blitter bus).  The
initial A2000 custom chips, while reorganized to incorporate some of the
A1000's surrounding glue login within the blitter itself still used the same
internal 19-bit bus.  However, the A2000 was built with a 20-bit external
bus in anticipation of the doubling of chip ram space that has now evidently
arrived with the new production A2000 mother boards (above Rev 6) which also
come with the "VGA-mode" 4-color blitter - presto, 1 meg chip ram located
from $000000 to $0FFFFF.  I do not know if the new mother boards have perhaps
a 21 bit bus, but I suspect that they do not, and such is being reserved for
the "Amiga 3000 class 32-bit main bus machines."  The details of such machines
are not yet determined, though, as the cost/performance analysis will have to
come into line with the protracted market.  For instance, an Amiga-style
alternate-clock-edge bus pair was a good move to give the Amiga an advantage
in certain types of graphics programs, but it failed to out-perform the
Atari-ST in straightforward number-crunching applications where the Atari's
8MHz CPU clock was absolutely faster than the Amiga's NTSC video-compatible
7.1591MHz CPU clock.

>What is $c00000 ram?
The $C00000 ram, also known as the "Ranger Memory" was a solution to the
internal memory expansion problem for the Amiga 1000 beyond 512K (non-chip)
without complicating the A1000 external bus address space specification.
If you have a copy of the original Byte article (August 1985) about the
Amiga 1000, you can look on page 88 at Figure 2, "The Amiga memory map"
and see that the space $C00000 to $DFF000 is marked "0.966 MEGABYTES-
RESERVED".  As well, the chip space from $07FFFF/$080000 to $1FFFFF/$200000
is marked "1.5 MEGABYTES-RESERVED".  As far as I know, the Byte Magazine
Figure 2 has never been updated in a publicly available forum.  The Byte
article does specify that the fast memory bus has an eight megabyte span
from $200000 to $9FFFFF, and all memory and devices with addresses $A00000
and higher will be decoded as resident on the blitter bus.  Thus $C00000
memory, while not addressable by the 19-bit blitter is resident on the
blitter bus, and is thus known as "Slow Fast Ram" among the meticulous.
Interestingly enough, most A2000s with less-than-Rev 6 motherboards have
512K of $C00000 memory implemented on the mother board.  I do not know if
this is still the situation with the new A2000s.

>Where is FAST ram?
Fast ram is designated as $200000 to $9FFFFF.

>where do devices configure?
Add-on boards which have autoconfig hardware will be added to the bus
starting at $200000 unless an A2620 MC68020 card or MC68030 card is present,
in which case all of the A2620's on-board 32-bit memory will be configured
first, starting at the $200000 boundary and working upwards until the $A00000
boundary has been reached.  Any boards on the expansion bus which have not
been configured by that point in the proceedings will thereafter be ignored.
So why have them on the expansion bus, then? you ask...  Well so that when
you chord the mouse to get your MC68000 processor on reboot you can get the
up to four meg you wouldn't otherwise have been able to have in 68000 mode,
that's why...  Do keep in mind, however, that the autoconfig firmware will
place the $C00000 memory ahead of the $200000 memory at config time - thus
the reason for the need to run "FastMemFirst" if you want true fast memory
to be used to load programs listed in your Startup-Sequence.  I might add,
however, that running FastMemFirst has a weird effect on the final location
of your recoverable ram disk, if you use one.  I advise setting up the rrd
before calling FastMemFirst, otherwise the rrd (depending on which one you
use) may end up getting located in slow fast memory.  The CBM Amiga CATS
group does provide detailed information about autoconfig firmware, circuitry,
life, the universe, and EVERYTHING in an inexpensive format.  The address and
available publications, costs, etc. have been posted at frequent intervals in
this newsgroup for the past several light years, so take an interest in local
affairs, ask a friend for a copy of such a posting and order the applicable
publications from CATS...

>Where is the ROM/WCS//re-live-boot-rom-on A1000s?
There is 192K of ROM/WCS space designated at $FD0000 to $FFFFFF.
I don't know if this will-be/has-been expanded, and if so, how so.

>And so on...     --- Darren
Well, perhaps the thing to do is reproduce the Byte 1985 table here so all
can check it out and so that the knowlegeable on the net can correct it to
the Amiga's current glory.  I mean, while a 68000 may be limited to a 16 meg
address range ($000000 to $FFFFFF), a 68020 or 030 isn't.  At least one 020
manufacturer puts his 32-bit memory above 16-meg (much to CBM's horror and
disgust).  I am inclined to think that is not exactly what CBM has in mind -
but as Jim Mackraz has so aptly pointed out, the value of my speculative
notions is not tied to any honorable currency.  Mackraz.  Mackraz would know.
Mackraz, where are you when we really need you?

Here is the table, generated from information found in Byte:
------------------------
Byte Magazine, August 1985 Vol. 10 No. 8, Page 88, Figure 2:

ADDRESS
(HEXADECIMAL)

000000						< 256K OF CHIP RAM
						  (RAM FOR STANDARD MACHINE)
040000						  256K OF CHIP RAM
						  OPTIONAL PLUG-IN MEMORY
080000
		1.5 MEGABYTES- RESERVED
200000
		8 MEGABYTES RESERVED
		FOR FUTURE USE AS PROCESSOR 
		RAM AND PERIPHERALS
		(LOCATED IN EXPANSION BOX)
A00000
		1.988 MEGABYTES- RESERVED
BFD000						< 12K RESERVED AS ADDRESS SPACE
						  FOR TWO 8520 SERIAL I/O CHIPS
C00000						  (ADDRESSED AT BFD0FF AND
						  BFE0FE HEXADECIMAL)
		0.966 MEGABYTE-  RESERVED

DFF000						< CUSTOM CHIPS ARE ADDRESSED
						  IN THIS 4K SPACE
E00000
		512K- RESERVED
E80000
		512K CONTROL AREA FOR
		CONFIGURATION EXPANSION AREA
F00000
		832K- RESERVED
FD0000						< 192K OF SYSTEM ROM
						< FFFFFF IS FINAL ADDRESS-
						  TOTAL WORKSPACE IS
						  16 MEGABYTES
------------------------
That's all, folks.
					Howard Hull
					hull@hao.ucar.edu