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For example, the Apple II originally shipped with a base 4K of RAM but could be expanded to 48K, so the maximum was twelve times the base.

Of all the computers ever shipped, which one has had the largest such ratio? (I don't know if there was ever a machine that had no RAM at all in the base configuration, but if so, I'm not counting that.)

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    An original 16k IBM PC could be upgraded to 32MB using a extended (EMS) memory card. Probably never happened but there were extended memory cards that could take up 32MB of RAM and would work in 8-bit ISA slot. – user722 Jul 22 '17 at 21:46
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    I wonder how many IBM PCs might actually have been shipped with 16kb – tofro Jul 22 '17 at 22:10
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    @Ross that’s a similar scale to that possible on modern high-end servers — RAM configurations start in the single-digit gigabyte range and go up to several terabytes... (Not sure about matching ×2048 though!) – Stephen Kitt Jul 22 '17 at 23:18
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    @RossRidge I don't think that should count. IBM PCs could not address more than 1Mb + 64Kb - 16b directly, and there was no virtual memory mechanism yet; therefore that EMS was not RAM in the strict sense. – Leo B. Jul 23 '17 at 5:59
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    I don't see how this question could really be answered because that same 4K base Apple II could be expanded to many megabytes using MMU's, bank switching, etc. With CPLD/FPGA it could, in theory, go to GIGS with clever hacks like using the databus, etc. So what's the upper end? If it's unknown, then how can you calculate the ratio. – cbmeeks Jul 24 '17 at 15:45
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The Altair 8800 shipped with 256 bytes of memory but the 8080 CPU could directly address 64K (with e.g. a Central Data 64K RAM Board), so that's 256×.

In the 1980s, Macrotech released a 2MB RAM board for the S-100 bus which supports 4 such boards, so that's 32,768×.

More recently, hobbyists have built 4MB RAM boards, so that's 65,536× total.

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The Amiga 3000 shipped with 2 MB and is expandable with stock parts up to 1170 MB (4x 256 MB RAM card, 128 MB on CPU card, 18 MB on motherboard) - that's 585x. In theory, it could go up to a full 4 GB (2048x) but no hardware exists to do that.

For on-motherboard expansion, the Power Macintosh 9500 shipped with 32 MB and is expandable up to 1536 MB - that's 48x. My 9500 actually runs 1.5 GB.

For (somewhat) current server systems, 48:1 is rather easy to beat. Our (not quite retro yet) Dell R515 could be ordered with as little as 4 GB and is upgradable to 256 GB. More current systems can go up to several TB and probably start out as low as 8 or 16 GB.

When it comes to super computers, you could probably get six-digit ratios easily...

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  • That is... wow! 11 unused address lines. Thanks for sharing this, and welcome to the site. – wizzwizz4♦ Jul 23 '17 at 9:52
  • Thx! I've been reading for quite a while but hadn't had too much to post. ;-) – Zac67 Jul 23 '17 at 10:28
  • With super computers, I imagine you’d be limited by the base configuration, which tends to be quite large. – Stephen Kitt Jul 23 '17 at 11:12
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    One example of a modern server setup is Dell’s R930 which starts at 8GB and can be configured with 12TB (although the specs say it only supports 3TB). – Stephen Kitt Jul 23 '17 at 11:14
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    Re: the comment about modern servers; maybe the question could have been phrased on a logarithmic scale: approximately "how many spare address lines were there, proportionally", as an approximate measure of how much 'room for extra' was designed in? Then a ZX81 scales from 2^10 to 2^16 bytes of RAM, so that's a logarithmic multiplier of 1.6. The Amiga goes from 2^21 to around 2^30.2, so that's a multiplier of around 1.43. The Dell goes from 2^32 up to 2^38 so that's around 1.19. From 8gb to 12tb would be ~1.32. Etc. – Tommy Jul 25 '17 at 14:33
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The Sinclair ZX-81 came with a single K of memory and could be expanded to 64kBytes (using third-party memory expansions, Memotech, for example).

I think none of the later (home) computers ever managed to exceed that 64-fold expansion option.

Theoretical expandability is maybe beaten by the Cambridge Z88 that came with 32kBytes of main memory and theoretically could have been expanded to 4MBytes (128-fold). Some of this address range would, however, have been used for mass storage on plug-in cards, so it was not really feasible to use all of this range for main memory.

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    In terms of actual RAM you could touch though, the ZX81 surely topped out at 56kb? Though you could hit 60kb on a ZX80. In both cases I'm thinking of the ROM size. – Tommy Jul 22 '17 at 21:46
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    @Tommy The original as it came from the shop was advertised as 64k, but only had 56k usable. You could, however, modify this and copy ROM to RAM with some modifications. – tofro Jul 22 '17 at 22:20
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    Even the basic 1K ZX81 with the factory 16K expansion pack is 16:1 which beats the 12:1 of the Apple II in the question. – JeremyP Jul 24 '17 at 8:09
  • @JeremyP Well, the Apple II also got a factory 16k pack, aka language card, so the ratio here is also 16:1 :)) I think the question needs a better definition what does count, 'cause otherwise one could put seven 2 MiB RAM Cards in, totaling in a 3600:1 ratio :)) – Raffzahn Sep 14 '17 at 21:12
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An additional 64x option: the Enterprise 64 came with the 64kb the name implies, and the base hardware implements a paging scheme of up to 256 separate 16kb segments — a 64 multiplier since that's 256*16/64, and 4mb total.

Technical documentation seems to be sparse, but see e.g. this page and R16 to R21. The top two bits of the Z80's inherent 16-bit address bus are replaced with 8 by table lookup. So it's a 22-bit address bus, right there in the base machine.

You'll likely find other machines for which similar paging schemes provide similar RAM limits, but I think this is a rare example of the scheme being fully implemented in the base hardware.

I've already been outgunned, but another high multiple is the Mac SE/30, which shipped with 1mb in its lowest configuration but can be upgraded to 128mb. Apple's official documentation gives a limit of 32mb, but that's just because of the size of SIMMs available at launch. No hardware modifications are required to support 128mb of RAM.

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    I was thinking about mentioning the Enterprise (as I have one in my collection and like it very much) - There was, however, afaik no commercial offering of memory expansion that would actually have used the possible limit. I happen to have some EP128 advertisements of Enterprise Germany where the maximum available was an expansion to 576kBytes. – tofro Jul 23 '17 at 11:15
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    @tofro yeah, I wasn't sure where to draw the line on that. I figured that the base hardware having 22 address lines, that you could point at, its designers having intending that the user be able to fit that much memory, was sufficient. So I guess I went with: address lines usable without modification or additional hardware versus installed memory. Which is not to discount contemporaneous commercial modifications, just another option. – Tommy Jul 23 '17 at 13:20
  • I had my Enterprise upgraded to 2MB for a short time - I didn't have many programs that could actually make use of that much memory, and the initial memory test taking so long annoyed me so much that I have now reduced it to 256k ;) – tofro Jul 23 '17 at 20:09
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This answer doesn't win the contest at all, but I include it for interest's sake and the hope to inspire future readers to play with the hardware.

The Commodore VIC-20 shipped with 5 kB of RAM and officially expanded to 32 kB, but very easily, and even using hardware available in the first year or so of its existence, it could be expanded to 40 kB (3 kB expander or SuperExpander [the latter of which had BASIC language enhancements], 16 kB expander, and 2 x 8 kB expanders [with their DIP switches configured appropriately so that the RAM doesn't overlap). That's an 8x multiplier over the stock RAM. Unlike some of the other competitors, all VIC-20s shipped with 5 kB so this would be a very normal multiplier.

Today, it's even easier to expand a VIC-20 as there are many, many RAM expanders available. 32 kB expanders are very easy to find, and adding a 3 kB expander and a cartridge slot expander will attain the same result. (There may even be single-cartridge ways of adding the entire 35 kB additional RAM required, but if there are, I'm not aware of them.)

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If you allow addition of bank-switched RAM, the ratio can get very silly. For example, the 16KB Sinclair ZX Spectrum could have been expanded using an upgrade whose schematics were published in the Your Spectrum magazine (and which was also available as an assembled module, IIRC) up to 4MB, is by a factor of 256. I believe the same system could have been used with little or modification on a ZX81 (originally sold with just 1KB RAM) for a factor of 4096 expansion.

Custom built hardware could of course have taken it even further. But I'm not sure there would have been much point. Fully populating that 4MB expansion in 1986 would have cost somewhere between £500 and £1000, and the RAM chips (512 of them) would have dissipated a somewhat cozy 10W at idle and closer to 100W during a refresh cycle. I don't know whether anyone actually did it at the time.

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