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Atari 260ST Story, TOS Floppy Boot Process, And How Much RAM?


The Atari 260ST. There’s a lot of misinformation circulating about this model on the internet, including the “fact” that it was only released with 256K of RAM. But what’s the real story behind this machine? Did it ever have 256? Did Atari even plan to release a machine with that little RAM? And what’s that floppy-based TOS boot process all about anyway?

Join me as I find out!

Big thanks to Dan (AKA DJ Slope) from Slope’s Game Room for helping out with this one:


Hey everyone, Rees here, and welcome back to the channel. In this video I’m going to be revisiting this Atari 260ST that I picked up a couple of months back, because I really think we have some unfinished business to attend to.

The Atari 260ST. A machine claimed by many online - including which tends to be the authority on such things - to be a rare early production run built with only 256K of RAM.

So I managed to get my hands on one, and we took a look around it and fired it up, and in that video I found myself repeating this urban legend. But based on some of the comments on that video - all positive and friendly, I should add - from people who actually owned these machines, I’m now not so sure.

For example this one, as I discovered, had 512K fitted at the factory - as well as a further 512K in the form of a third party expansion - and poking around the motherboard it certainly doesn’t seem to me like it was designed to have any less.

So what do we know for sure about the 260ST? Well, we know that it was sold in limited numbers in West Germany and parts of Scandinavia in 1985. We also know that the ROM chips containing Atari’s TOS, the ST’s operating system, weren’t ready in time for launch, meaning that the OS had to ship on floppy disk.

It’s also known that, unlike when running from ROM as per later machines, TOS booted from floppy runs in RAM, taking up around 200K - or so it’s claimed - meaning that, on a theoretical 256K machine, there would only be around 50K left for applications.

So did the model number - 260 - ever actually refer to the amount of physical RAM in line with Atari’s 520 and 1040 numbering scheme, or did it refer to the amount of available RAM (roughly) after booting that early floppy-based TOS on a 512K machine like this one?

Well, I need to finish getting this cleaned up and I really want to return it to its original spec, removing that third party RAM expansion which I think was the cause of some reliability issues. I definitely want to see what the original boot process looked like - and I managed to get hold of some genuine original Atari boot ROMs so we can do just that, and I also want to see whether the surprise floppy drive that came with this when I bought it actually works after I struggled with it before.

Finally, I’m going to dig into some history and investigate the possibility of an ST with only 256K of RAM - or maybe even less - and whether that was something that was ever planned by Atari.

But first, lets get the ST cleaned up.

This machine was pretty filthy when I got it, and I did give it a quick wipe down last time to take the worst of it off. But as it’s on the bench again, I thought I’d give it a thorough scrubbing this time.

Thankfully what’s left of the dirt is just on the surface, so I don’t think a complete stripdown of the keyboard is needed, and an antiseptic kitchen wipe makes short work of the keyboard and those iconic vents, which also happen to be notorious dirt traps.

[Removing RAM expansion]

As we previously determined, this 260ST actually has 1MB of RAM, with 512K soldered to the motherboard, and a third party RAM expansion manufactured by Rocke Computer providing the rest.

I have a couple of STs with 1MB or more of RAM already, and the wiring on this expansion is a bit dubious and possibly the cause of some of the reliability issues I’ve been having - so I’m going to remove that and store it away safely just in case I want to use it later.

Like many RAM upgrades, it plugs into the Shifter socket, with the original Shifter sitting on top. It’s a convenient place to tap into the data bus and a very tidy solution.

If only I could say the same for the other wiring.

Whoever installed the upgrade decided that parcel tape would be a good way to protect the connections to the MMU on the underside of the board, so while I wait for the soldering iron to warm up, I’m also going to use some IPA to clean off this old tape residue.

So with everything back together, it’s time for a quick check to make sure that the machine still boots from the German language 6 chip ROM TOS version 1.0 that’s currently installed - and it does!

The floppy drive didn’t seem to be working last time, so I thought I’d test it out with an old ST Format coverdisk with a nice Quartet demo on it. It’s cool that stuff like this works on even the oldest STs.

With that test successfully completed, I also tried formatting some floppies to use later.

Ah! 360K… OK, so my German is a little rough. As it’s a double sided drive, I’ll try again with the Zweiseitig option… I assume that means double sided, if einseitig is single sided.

Yes, much better.

I’ll also need a copy of SYSINFO to be able to check the available RAM, so I’ll use my DOS PC to write that to a floppy disk using MAKEDISK. I managed to track down a very early version of SYSINFO to keep it as light as possible.

After a cold boot, we can see that this machine does indeed have 512K onboard, with 127 used by TOS and SYSINFO, leaving a healthy 385K free for applications.

After the issues last time, it’s also really cool to see that floppy drive working properly. Perhaps they were related to the general instability caused by those dodgy connections on the RAM expansion.

[Swap TOS chips]

So now onto that original boot process I was talking about. TOS, the Atari ST’s operating system, was always intended to be run from ROM, but the ROM chips weren’t ready in time for the computer’s launch. So Atari’s engineers developed much simpler ROM chips that could bootstrap TOS from floppy disk, and it just so happens that I’ve managed to get my hands on some.

Of course this was a pretty impractical setup, so the early STs were recalled and upgraded to a proper full version of TOS in ROM once it became available.

So downgrading this machine is just a case of removing the 6 TOS ROMs and replacing them with the two floppy bootstrap ROMs.

The boot process is actually a really cool thing and not something that many ST owners will have even seen in action, so lets take a look.

Booting the machine without a disk in the drive gives us this awesome bootloader environment, which is styled to look like the ST’s standard GEM desktop - but of course it isn’t - and none of the menu options actually do anything.

Now I just need to write a boot floppy using my trusty DOS PC again - and in this case, it seems appropriate to go with the same German TOS 1.0 that we were running before - but I could in fact use any version.

As you can see, checking the desktop info just shows that copyright 1985 notice with no version number as before.

So I’ll just fire up SYSINFO again so we can check out the RAM situation, and now we can see that we only have 209K free this time, which is 176K less than we had when booting from ROM - space that’s taken up by TOS itself.

The cool thing is that that Quartet demo from that old ST Format coverdisk - with the sampled sound - still works perfectly fine.

I also decided to check out a commercial game. The bootstrap ROMs can actually boot anything, not just TOS, and Xenon 2 fires up and runs just fine. The good thing about this setup is that, because TOS isn’t being loaded of course, games have the full 512K of RAM available to use.

But of course none of this answers the question - was there ever an ST with 256K of RAM? Well, lets take a trip back to the winter Consumer Electronics Show - or CES - from 1985 where the ST was demonstrated to the public for the very first time.

So - straight from the horse’s mouth. There were two models planned - the 130ST, with 131000 bytes of RAM, and the 520ST, with 524000 - and I can only assume that they used bytes because they sounded like big impressive numbers.

There are also references to the 130ST in The First ST Book by First Publishing, which as far as I can see, was indeed the very first book published about the Atari ST, way back in 1985. I found a scan on AtariAge but I haven’t managed to track down a physical copy just yet, although I’d love to add one to the collection.

It also has screenshots of that early version of GEM - the Graphic Environment Manager which essentially makes up the STs desktop - that was demoed at CES, so I can only assume that the authors of the book were working with that same prototype hardware. Incidentally, GEM was later reworked slightly at the request of Apple to reduce its similarity to the Mac’s operating system - the most obvious change being the desktop icons.

Also in this book we find a clue as to why that lower end version of the ST was dropped at such a late stage. There’s a whole paragraph about Digital Research’s CP/M 68k, which was the original operating system underpinning the GEM desktop, incidentally also written by Digital Research, and what was running on those prototype machines at that Consumer Electronics Show.

If you know your CP/M history you’ll know that it was available for a wide range of machines and was pretty much the standard in the late 70s and early 80s before MS-DOS came along, but that’s a story for another video.

Working with Digital Research, Atari’s engineers decided to change the STs underlying OS right at the last minute to CP/M’s upcoming replacement GEMDOS, which was a much more capable but heavier OS, and this combined with falling RAM prices lead Atari to drop those lower end models.

And I think this is also a pretty big clue as to why those TOS ROM chips were delayed - because they were still being reworked to accommodate the new operating system even as production was ramping up and STs were leaving the factory.

So hopefully that’s shed some light once and for all on the 260ST and what it’s all about.

Now, what I’m not sure about is whether the final version of the MMU or Memory Management Unit that went into these machines actually supports 256K or even 128K of RAM - I’m actually inclined to think that yes, it could well be possible by swapping the RAM chips out, although I seriously doubt that anything would actually be able to boot.

Perhaps that would be an interesting experiment for another time.

So thank you very much as always for joining me, and I hope to see you again next time.

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Relevant Links:
My 260ST pickup video:
Atari Museum 260ST page:
videoholicULTIMATE winter CES 1985 part 3:
First Atari ST Book:
GEMDOS Reference Manual:

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