The Mythical Mac 256K!

[Snial]

Another follow-up post. It might have been a fluke that my modified ROM and Mac 256K configuration ran on System 1.1, so I decided to see how far I could take it.

System 2.0 and 2.1 run as expected, because they also run on a Mac 128K:


I'm puzzled as to why the other inserted disk on System 2.1 came up ejected. Maybe it's timing differences for when the Sony disk emulator on Pce-Mac injects the disks (i.e. at the same time). Anyway, moving on.

System 3.0, 3.2, 3.3 don't run on Infinite Mac's Mac 128K, because they're HFS images. I recreated MFS disk images containing the system software. 3.0 and 3.2 both run on the Mac 128K and Mac 256K configuration, but System 3.3 hung on Mac 128K at "Welcome to Macintosh" (it was OK on Mac 256K).




It's always good to see the 256K appearing on "About this Macintosh"! Moving on.. System 4.0 will boot on both the Mac 128K and Mac 256K, but on Mac 128K you can't run anything, even running the calculator just generates an alert to say there's no memory.


I was a bit puzzled as to why the Finder is version 5.4 for System 4.0, but was 5.5 for System 3.3. Running System 4.1 is OK on the Mac 256K, but hangs again on the Mac 128K. System 4.1 finally presents the (Restart) button when you shut it down!

Finally System 5.0 is where it goes wrong for a Mac 256K. It bombs part of the way through boot up:

It wasn't because I stripped out lots of System files I thought wouldn't be needed, because the same disk will boot on a Mac Plus. It's because it probably needs more than 256kB. System appears to need about 3/5 of 156K and Finder needs about 90% of 156K = 234kB. Maybe eliminating the disk cache (32kB) could fit it in, at the expense of not being able to do anything with it.



Conclusion
Mac 256K can run a couple more System versions than the Mac 128K, but basically the cut-off is about the same, October 1987 vs April 1987 (but effectively June 1986). This means that the Mac 128K only had OS support for 2 years! The Mac 256K despite being very much more usable, would have only had OS support for 2.5 years! A similar fate exists for the Mac 512K, because it came out 6 months later. Meanwhile the Mac Plus could run everything from System 1 to System 7.5.3!

I never had a chance to see these kinds of limitations in early Macs before. I arrived at UEA in autumn 1986, when System 3.2 was already out, but in any case our Macs were already Mac 512s with another lab for Mac Plusses. Yet most of the time we ran System 2.0, because we didn't know any better. In fact we just tended to mix-and-match the contents of System Folders as we saw fit, the concept of an "Installation" was mostly absent. The Mac II lab appeared in autumn 1988 (or maybe it appeared in late 1987 or early 1988, but we didn't have access to it until 1987); so that would have run System 4.1 or even System 5, though I don't remember Multifinder at the time.

 

[David Cook]

Great testing. I admire your completeness.

To me, the biggest improvement is not the ability to run later systems, but instead the size of the documents or applications that can be opened. For example, when I wrote Tiny Transfer, I had to squeeze and trim everywhere to get it to work on a 128K Mac. But, Tiny Transfer had plenty of headroom on a 512K. So, a pico 256K is sweet for new retro-apps.

 

[Snial]

Great testing. I admire your completeness.

Thanks, I got carried away with the versions, but my initial goal was to run System 2.x, because System 1.x really is very naff with all the "Empty Folder" nonsense!

I realise that in my conclusion, I said we mostly ran System 2.0, but also did a mix-and-match with System files. I should have said that in the first couple of years we only seemed to have System 2.x available, and later OSs filtered through to the student community.

It's probably not obvious to readers why we'd end up running System 2 all the way into 1988 and 1989; but there's a basic explanation. Prior to the Mac SE (and Mac II, but we only had about 8 of those), our Macs only had a single floppy drive for storage. We'd go to the Computing building; buy a precious 12cm Floppy disk (400kb) then use someone else's disk to laboriously copy the system + MacWrite + MacPaint + MacPascal to your own disk with lots of disk swapping.

But then we'd just keep the disks; adding documents and our own programs to them until they failed or ran out of space. And if they ran out of space, we'd just copy the OS to the new disk. Because the OS went with us & otherwise stayed in our room, it would never get upgraded. A disk might fail of course, if I accidentally took it to a Gary Glitter concert and then sat or trod on it - even 12cm microfloppies have their limits!

To me, the biggest improvement is not the ability to run later systems, but instead the size of the documents or applications that can be opened. For example, when I wrote Tiny Transfer, I had to squeeze and trim everywhere to get it to work on a 128K Mac. But, Tiny Transfer had plenty of headroom on a 512K.

I figure that segmentation can typically squeeze 1.5x to 2x the code you'd otherwise be able to get in an application before it gets really sluggish or clunky. Similar to the ratio of VM memory to physical memory on System 7. But I suppose Tiny Transfer wouldn't want to hang around loading parts of its app while also trying to download over serial at the same time?

So, a pico 256K is sweet for new retro-apps.

Do you have a PICO-mac yet? I don't think I'll get one until I've finished MØBius.

 

[nathall]

Finally System 5.0 is where it goes wrong for a Mac 256K. It bombs part of the way through boot up:
View attachment 80511
It wasn't because I stripped out lots of System files I thought wouldn't be needed, because the same disk will boot on a Mac Plus. It's because it probably needs more than 256kB. System appears to need about 3/5 of 156K and Finder needs about 90% of 156K = 234kB. Maybe eliminating the disk cache (32kB) could fit it in, at the expense of not being able to do anything with it.

It’s actually because System 5 and above remove patches to the 64K ROM necessary to use the newer systems on the old ROM. Same thing happens on a stock 512k. Getting System 5 to run on such a configuration is as simple as copying PTCH resource 105 from System file 4.1 to the System file you’re trying to use.

I’m willing to bet your 256k Mac would boot System 5 by doing the above, although like you said you probably couldn’t do much with it.

400k disk space saving tip: you can delete all PTCH resources above 105 from the System file if you only ever plan on using said disk to boot a machine with the 64K ROM. They are for later ROMs.

 

[Snial]

It’s actually because System 5 and above remove patches to the 64K ROM necessary to use the newer systems on the old ROM. Same thing happens on a stock 512k. Getting System 5 to run on such a configuration is as simple as copying PTCH resource 105 from System file 4.1 to the System file you’re trying to use.

OK.

I’m willing to bet your 256k Mac would boot System 5 by doing the above, although like you said you probably couldn’t do much with it.

It's an interesting task to try.

400k disk space saving tip: you can delete all PTCH resources above 105 from the System file if you only ever plan on using said disk to boot a machine with the 64K ROM. They are for later ROMs.

Because I'm still using an emulator (Pce-Mac), it uses a Sony floppy disk ROM extension (mapped to 0xfe0000 I think) which supports 800kB disks - and possibly bigger disks too if they can be supported under MFS. I think it may be possible to do that, because MFS uses a sort-of- 12-bit FAT format, which I think means you can support up to 2MB or 4MB.

My next steps probably are to retry getting it to work with miniVMac + the changes I made, but reintroducing the memory test. A PICO-MAC 256K will be a subsequent goal. It should be possible with my MØBius 68K emulator. It would also be good to track down any of the original Mac engineers to see if any Mac 256Ks existed and if they did, whether they built them the way I think they did. That is, they used a stock ROM that supported only 128K and 512K, but had 256K of RAM. They had a physical switch or jumper that enabled the upper 128K of RAM. They did a power on in 128K mode, then when they saw the [?] disk icon, they engaged the switch (or jumper) and inserted the FD. The reason they did this was to give themselves extra space for debugging Mac applications, and then flip back to 128K to test again on a stock Mac 128K.

Mind you, I've just realised my Mac 256K, 64K ROM would also work with this debugging technique and a physical switch. You could power on with the switch engaged and it'd boot as a Mac 256K when you inserted the FD. You could power on with the switch remaining disengaged and it'd boot as a Mac 128K when you inserted the FD. You could power on with the switch disengaged, then engage it and insert the FD to complete booting as a Mac 256K.

Ultimately, but this is a big ask for any Mac 128K owner, whether a physical Mac 256K could ever be constructed, by adding another bank of 512K RAM with my Mac 256K, 64K Rom! A Mac 256K could also be constructed from a Mac 512K by forcing A18 low, then the upper 256K mirrors the bottom 256K.

My hardware hack does assume that there are no address conflicts on 256K boundaries on a 64K ROM Mac. That will be true for RAM and ROM, not sure about I/O.