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There are three main types of memory for the A3000, CHIP, FAST and
FAST external memory.  Of them, the A3000 supports 2MB of CHIP and up to 16
MB of FAST on the motherboard.  External FAST memory usually resides on a
Zorro expansion card or a FAST slot accelerator.

For CHIP RAM, every A3000 desktop ships with 1MB soldered on the
motherboard.  There are 8 sockets for another 1MB.  Many A3000
configurations shipped with 1MB of FAST memory installed, which were
intended to be moved to these CHIP sockets later on.  

Fast RAM on the A3000 desktop is added via 32 sockets for ZIP
package DRAM.  These chips are pretty hard to find, and most of the time,
other A3000 owners sell them into the used market. Other sources of ZIP RAM
include desoldering them from old A2620/30 accelerators and the Commodore CDTV,
both of which use the lower density 256Kx4 chips.  Of course, you'd have
to scavenge four CDTV's to get a usable bank of 4MB, assuming you didn't
burn out any chips from the desoldering process.  Two variations of
these ZIPs exist: Page Mode and Static Column.  SC ZIPs have the advantage
of being 10% faster, but Page Mode ZIPs are more compatible with the A3640
040 CPU board with 2.04 ROMs (as opposed to 3.1 ROMs which use SC fine), 
and also fit into the A2386 Bridgeboard and run faster in that.  Page 
Modes are also slightly cheaper.

In general, you must fill out the entire bank with chips, and the
lower the speed the better.  The given speeds are minimum guidelines
specified by Commodore.  Anything past 70 ns is needlessly fast though.

Chip RAM for Amiga 3000 Desktop
Via the U267-U274 DIP package sockets...

Density         Name    Package         Speed   Pcs/Bnk Banks/Total
256Kbit x 4     44256   20 pin DIP      120ns   8       1/1MByte

These can be either Static Column or Page Mode access type DRAMs.
It does not matter which type, there is no speed degredation.  These are 
very common chips, and you can find them on older VGA boards for PCs.
They're also the same kind used on the A2091 RAM and SCSI board.  Chip RAM
autoconfigures without jumpers.

Fast RAM for Amiga 3000
Via the U850D-U857D DIP package Bank 0 Sockets...

256Kbit x 4             20 pin DIP      80ns    8       1/1MByte
1Mbit   x 4             20 pin DIP      80ns    8       1/4MBytes

Via the U850-U881 ZIP package Bank 0-3 Sockets...

256Kbit x 4             20 pin ZIP      80ns    8       4/4MBytes
1Mbit   x 4             20 pin ZIP      80ns    8       4/16MBytes

These should all either be Static Column or Page Mode.

If a mixture of both Static Column and Page Mode fast memory is fitted,
this can cause a software failure during boot, when CPU caches are enabled.

As noted above, some A3000's shipped with 1MByte of FAST memory in
DIP package, waiting to be transplanted into the sockets for CHIP RAM, once
more FAST RAM was installed.  Those FAST memory sockets are the U850D-U857D
20 pin DIP places, Bank 0.  This bank is electrically mapped to Bank 0
U850-U857, 20 pin ZIP sockets.  As a result, one can make a three megabyte
A3000 configuration by putting 256Kbit x 4 chips into the FAST RAM DIP
sockets without buying ZIPs.  Or a six meg setup, using 1MBit x 4 chips.
The DIP FAST sockets cannot be used with the ZIP sockets all together, as
the two Bank 0 areas are basically pinned to each other (as noted by the
same component numbers, one set with a D for DIP).  One could have 4MBytes
of DIPs for FAST, then another 12MBytes of FAST in ZIPs placed into Banks
1-3, but not 4MBytes in the DIP area and 16MBytes in all four ZIP banks.  

256Kbit and 1Mbit banks cannot be mixed.  One cannot have 4MBytes of 
1Mbit chips, and then 1MByte via another bank of 256Kbit chips.  Of
course, one can't mix individual chips either!  All four banks are either
low or high density banks.  

Jumper J852 selects the size of the RAM, and 1-2 designates 1Mbit
sizing.  2-3, further from the arrowhead, denotes 256Kbit.

Tower A3000
For the A3000T, CHIP RAM is in ZIP package, but they're 256Kbit x 4.
The FAST RAM is in similar configuration at the same locations.

SIMM Adapters
A number of sources for ZIP to SIMM converter boards have popped up
and are often discussed on Usenet and elsewhere, on web pages.  These
little boards have machine legs which fit into the ZIP sockets and give 72
pin SIMM sockets for adding memory.  Be aware that most, if not all, SIMM
RAM is Page Mode.

FAST External Memory
As mentioned above, FAST External Memory resides on Zorro expansion
cards, or FAST slot expansion cards.  16 bit RAM boards made for the
A2000 in general will work fine in an Amiga 3000 system, but as the main
68030 CPU is 32 bit, anything stored in this 16 bit segment will be slowly
accessed, compared to the 32bit motherboard memory.

32 bit RAM boards will most likely utilize the Zorro III bus.  An
example would be the DKB3128, which has four SIMM sockets, and also the
PP&S ProRAM 3000, which is no longer made.