(c) Copyright 1990 Commodore-Amiga, Inc. All rights reserved. The information contained herein is subject to change without notice, and is provided "as is" without warranty of any kind, either express or implied. The entire risk as to the use of this information is assumed by the user. Expanding Fast RAM on the Amiga 3000 by Rob Wyesham The Amiga has two types of RAM memory: fast RAM and chip RAM. Fast RAM on the Amiga is memory which can only be accessed by the processor. Chip RAM on the other hand is shared by the processor and the Amiga's custom chips. When a custom chip and the processor both want access to chip RAM, the custom chip always wins, and the processor must wait. But with fast RAM the processor has exclusive access and so always runs at full speed. The A3000 comes with 1 megabyte of fast RAM. It can, however, be expanded to as many as 16 megabytes. This article describes how to install and test extra fast RAM on the A3000 motherboard. The chips that you'll need for the expansion are known as ZIP chips (for zigzag-inline- package). ZIPs can be organized in one of two ways, either as 256K x 4 bits, for 1-megabit parts, or as 1M x 4 bits, for 4-megabit parts. Each ZIP chip also comes in one of two modes, either page mode or static column mode. The latter are also known as SCRAM chips. So, there are four kinds of chips you can use. These are shown below. Four Kinds of ZIP Chips for Expanding A3000 Fast RAM o 1-megabit (256K x 4 bits) page mode o 1-megabit (256K x 4 bits) static column mode (SCRAM) o 4-megabit (1 M x 4 bits) page mode o 4-megabit (1 M x 4 bits) static column mode (SCRAM) There is an advantage to using SCRAM chips to expand fast memory on the A3000. If the RAM controller detects SCRAM chips in the fast RAM array, it will enable the burst mode of the 68030, thereby greatly reducing RAM access times. Note however that if the controller detects even one ordinary, page mode DRAM then the 68030<2>'<0>s burst mode will be disabled. So if you're going to spend extra money expanding with SCRAMs, then use only SCRAMs. Don't mix SCRAM chips with page mode chips. Whether you use SCRAM or page mode chips, you will need to get at least eight of them since all eight sockets in a bank of fast RAM must be filled for the A3000 to work properly. This means you can upgrade fast RAM in increments of either 1 or 4 megabytes. Whichever chip type you select, make sure that the RAM speed is rated 80 nanoseconds or better. The part numbers of 80ns RAM chips usually end with -8 or -80. Inside the A3000 To open the A3000 and gain access to the fast RAM array, you need a #2 Phillips screwdriver. Five screws hold down the cover, two on each side, and one on the back. Once you have the cover off, you'll need to remove the disk drive chassis as well. There are six screws to remove here, three in front, two on the back panel, and a hidden one. The hidden screw lies behind the hard disk drive underneath the drive's ribbon cable. Once you have the screws out, detach the drive cables from the motherboard, remove the drive chassis, and set it aside. Now you can see the fast RAM array. It's on the lower right part of the motherboard, and has two parts. The first part is a row of eight DIP sockets numbered backwards from U857D through U850D, and populated with DRAMs. This is the 1MB of fast RAM that the 3000 is shipped with. Behind the row of DIPs lie two columns of ZIP sockets, numbered from U850 through U881, but not in numerical order (see the diagram below). Fast RAM ------------------- ZIP Sockets Expansion slot | | --------------- || | Fat Gary | | U881 U879 | || | | | U873 U871 | || ------------------- | U865 U863 | || | U857 U855 | || | U880 U878 | || --------- | U872 U870 | || | 68030 | | U864 U862 | || | | | U856 U854 | --------------------- || --------- | U877 U875 | U271 U272 U273 U274 | || | U869 U867 | | || | U861 U859 | U267 U268 U269 U270 | || | U853 U851 | --------------------- || | U876 U874 | Socketed Chip RAM || | U868 U866 | || | U860 U858 | --------------------- || | U852 U850 | U263 U264 U265 U266 | || --------------- | || -------------------------------------------------- U259 U260 U261 U262 | || | U857D U856D U855D U854D U853D U852D U851D U850D| --------------------- || -------------------------------------------------- Soldered Chip RAM || Fast RAM DIP Sockets || (Front of the A3000 Motherboard) Expanding RAM Let's assume a simple upgrade to start: you want to expand fast RAM by 1MB (2MB fast RAM altogether). This means that you're using eight 1-megabit RAM chips. One way to proceed is to install the eight ZIPs in bank 1, that is, in ZIP sockets U858-U865. The reason we begin with bank 1 rather than with bank 0 is that the bank 0 ZIPs are mapped to the same address space as the row of DIP sockets, U850D-U857D. The row of DIP's is said to mirror bank 0. So there's no point in filling the bank 0 ZIP sockets in this case. If you want to expand fast RAM by 1MB and chip RAM by 1MB, you can do it by installing 16 1-megabit ZIP chips. The first step is to move the RAM from the fast RAM DIP sockets on the right side of the motherboard (U850D-U857D) to the chip RAM DIP sockets on the left side of the motherboard (U267-U274). This gives you 2MB of chip RAM but no fast RAM. Now install the 16 ZIP chips in the bank 0 and 1 ZIP sockets (U850-U865) to get 2MB of fast RAM. Yet another expansion option is to install 4MB worth of 1-megabit ZIPs. This upgrade requires filling all 32 ZIP sockets, and emptying the row of DIP sockets (U850D- U857D). Remember, you can reuse the DIPs that you remove to fill out the chip RAM array (U267-U274) which gives you 2MB of chip RAM. Bigger upgrades use 4-megabit ZIPs. If you intend to use these parts, you have no choice about removing the chips in U850D-U857D. The RAM controller can handle either 1-megabit parts or 4-megabit parts, but not both, which is why the DIPs must be removed. Fortunately, you can reuse these to expand your chip RAM. Start your upgrade with bank 0, and populate the banks in order. Bank two starts at U866, and bank 3 starts at U874. The addresses that the banks correspond to are shown in the table below. Notice that the memory starts high and builds down, unlike most memory systems, which tend to build upward. RAM Bank 1 Megabit Parts 4 Megabit Parts Board Component --------------------------------------------------------------------- 0 $07f00000-$07ffffff $07c00000-$07ffffff U850 - U857* 1 $07e00000-$07efffff $07800000-$07bfffff U858 - U865 2 $07d00000-$07dfffff $07400000-$077fffff U866 - U873 3 $07c00000-$07cfffff $07000000-$073fffff U874 - U881 * Bank 0 has the same address space space as DIP sockets U850D-U857D Actual installation of the ZIP chips is easy. Pin 1 of a ZIP socket lies on the left side of the socket, the side with the little capacitors. The capacitors have numbers next to them (like C880). On the ZIP chips themselves, pin 1 is on the same side as the bevelled corner. To install a chip, line up pin 1 of the chip with pin one of the socket, and push down until the chip is seated. You may have to move a jumper, J852, as well. J852 tells the RAM controller whether 1-megabit or 4-megabit parts are installed. The jumper is surrounded on the motherboard by a silktop figure. The pointed end on this figure indicates pin one on the jumper. If you are installing 1-megabit RAM chips, make sure that the jumper covers pins 2 and 3. This is the default factory setting, so J852 should already be set this way. If you are installing 4-megabit chips, move the jumper so that it covers pins 1 and 2. Use the DOS Avail function to test your installation. If the amount that Avail reports is substantially less than you expected, make sure that each chip is aligned in its socket and is fully seated. Finish the installation by replacing the disk drive chassis, reattaching the drive cables to the motherboard, and replacing the cover. The chart below shows chips which may be used in expanding your 3000's fast RAM. We've tested the 1-megabit parts, but not the 4-megabit parts. Static column RAM can be used in the chip RAM array, but the Amiga won't take advantage of the SCRAMs extra speed (so save your SCRAM chips for expanding fast RAM instead of chip RAM). 256K X 4 Page Mode256K X 4 Static Column Mode Samsung KM44C256AZ TOSHIBA TC514258 NEC uPD424256 NEC uPD424258 MOTOROLA MCM514258 1M X 4 Page Mode 1M X 4 Static Column Mode TOSHIBA TC514410 TOSHIBA TC514402 HITACHI HM514400 OKI MSM514402 NEC uPD424400 OKI MSM514400 The DIP sockets in the fast RAM array will accept the next generation of 4-megabit DRAMs which are to be packaged as DIPs. Remember that modifying your Amiga voids your warranty. Contact your Commodore Authorized Service Center for more information.