An interesting feature of the Apple II was that it had three rows of sockets for RAM chips, each of which could take either 4k or 16k chips. That meant the minimum configuration was 4K (cheap) but it could be expanded in increments all the way up to 48K (enough to get useful work done).

Were any computers ever released that did the same trick with 16k vs 64k chips? I could imagine that being useful in the late seventies to early eighties, though it might be technically trickier due to some of the pins being used differently, as described in the answers to Can a 16K computer be upgraded to 64K?

To be clear, I'm talking about sockets on the main board, that could take either kind of RAM chip. There were certainly computers like the IBM PC that could take either kind by virtue of being able to take memory in expansion cards; that's a different thing.

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    For what it's worth, the Commodore 1700, 1764 and 1750 RAM expansions for the C-64 / C-128 all use the same PCB, populated with 16 x 64 kBit, 8 x 256 kBit and 16 x 256 kBit, respectively. The configuration is selected by means of a jumper (64 / 256 kBit) and an optional resistor (populated only for 16 ICs). – Michael Graf Dec 12 '20 at 20:21
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    Not fitting here, but I found quite cool at the time: A friend did expand his Apple II to 192 KiB (well, 208 counting the language card) by putting 4164 into the sockets, bending 1,8,9 and adding a paging logic, so either of the 4 pages of each bank could be swapped in or out. I always wanted to go the same way, but somehow never did. – Raffzahn Dec 12 '20 at 20:54
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    As far as I can tell, Acorn explicitly did not do this. BBC model B uses 16K RAMs through several board revisions, including a major re-layout associated with export models. Model B+ uses 64K RAMs, Master uses 256K (64Kx4b) RAMs. – Chromatix Dec 12 '20 at 20:56
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    I expanded my Atari 800XL in a similar way from 64Kx1 bit to 256Kx1 bit RAMs. The pinouts of the 4164 and 41256 DRAMs were compatible, but more importantly, Atari already wired the extra (multiplexed) address line on the PCB. The only thing that was needed were a few TTL ICs for address decoding and page/bank switching (I did this with a single GAL just because I could). This mod was even compatible with the later 130XE. – StarCat Dec 13 '20 at 7:30

The Tandy Color Computer 2 could be upgraded from 16k to 64k by simply swapping chips and adding a jumper.

Early revisions used eight 16k x 1 bit or 64k x 1 bit DRAMs. The 16k chips were type MCM4517 or 2118, which only require a +5V power supply (unlike the 4116 which needs +12V, +5V and -5V).

Later revisions used two 16k x 4 bit or 64k x 4 bit DRAMs. Here is my CoCo2 upgraded from 16k to 64k. The two 64k x 4 DRAMs are in the upper right area (between the 2 white sockets intended to take a 64k daughter board), and the 64k 'RAM SIZE' jumper is in the lower left corner.

enter image description here


Siemens PC-D, a 186 based DOS machine (*1) has 32 sockets to be either populated by 64 Ki chips or 256 Ki, allowing memory configurations from 128 KiB up to 1 MiB:

  • Two rows of 4164 (8 each) -> 128 KiB
  • Four rows of 4164 -> 256 KiB (*2)
  • Two rows of 41256 -> 512 KiB
  • Four rows of 41256 -> 1024 KiB

Selection was done by a single DIP switch, that did feed several PAL generating/routing the signals.

Since all chip position were socketed, a field upgrade could be done without the need to (un)solder, much the same way as with the Apple II.

*1 - Originally developed as a Unix workstation, but soon, with the MMU removed, successful marketed as DOS computer.

*2 - There was also an upgrade version with a 256 KiB daughter board, using 4164, plugged into the MMU connectors to bring a 256 KiB setup up to 512 KiB without pulling the existign chips - after all, at that time (1984), 4164 still held considerable value.

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    "Four rows of 4164 -> 1024 KiB" did you mean 41256 here? – ecm Dec 12 '20 at 22:17
  • @ecm Ups, yes.. – Raffzahn Dec 12 '20 at 22:22
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    Saw the question in the Hot Questions list, came here to say Siemens PC-D and you already wrote my answer. We had a bunch of these at work back in the day. I upgraded several of them from 4x4164 to 4x41256. The left over 4164 chips went in those daughter boards you mentioned to upgrade several other machines. (We bought the daughter boards empty, because Siemens RAM prices were inflated and we were buying the RAM in bulk directly from Texas Instruments anyway to put in our products.) – Tonny Dec 13 '20 at 11:24

The main issue is that 16kbits DRAM chips like 4116 used to require 3 power supply voltages : +5V, -5V, +12V, while more recent DRAM chips such as 4164 only need a +5V power supply.


        4116    4164           41256
 Pin 1   -5V    no connect     Address bit 8
 Pin 8  +12V    +5V            +5V
 Pin 9  +5V     Adress bit 7   Address bit 7

Not only several power supplies and signals would have to be strapped, but any wiring error would have fried the memory chips and maybe other chips around.

Some later models of 16kbits DRAM chips (as Bruce Abbott explained above) had pinout compatible with 64kbits chips, but they weren't available when computers like the Apple ][ were designed.


The Vortex memory upgrade board for the Amstrad CPC 464/664 would kind-of qualify as answer, as it is (like the PC-D) capable of taking x1 memory chips of different size, but also (like the PC-D) it is about 64KiBits vs. 256KiBits and not about 16KiBits vs 64KiBits. This is an upgrade board, and not the mainboard itself, but it hooks into the computer into a way that it logically becomes part of the mainboard. The same PCB can be equipped with

  • a single bank of 4164 (64KB)
  • two banks of 4164 (128KB)
  • a single bank of 41256 (256KB)
  • a bank of 41256 and a second bank of 4164 (320KB)
  • both banks filled with 41256 (512KB)

Each bank has its own socketed data latch (a 74HCT373), so if you bought the card with just one bank installed, you don't just need to buy memory chips for the second bank, but you need to add the missing second '373 as well.

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