System P-7.1 Bootable 800K Floppy Flop!

[Snial]

This is a weird one. I've made passing comments on it a few times in a couple of other threads, but I think I have proof of it now.

I can boot up my Mac Plus under P-7.1 on my 40MB HD. I have a Zip drive attached too and an external FD. I have an 800kB, bootable disk with System 3.x and another 800kB disk (which was a MacWrite II installation disk).

Booting from the Sys3 disk was fine and the HD mounted too.

I could copy its System Folder to the MWII disk OK and then the MWII disk would boot OK.​

I could format the MWII disk, then copy the Sys3 System Folder to the MWII disk and then the MWII disk would boot OK.​

Booting from the HD was fine and it could read the Sys3 disk.

I could copy Sys3 System folder to the MWII disk formatted by Sys3 and on reboot the Mac Plus would display the [X] folder icon and eject the disk.​

I could format the MWII disk using System P-7.1, then copy the Sys3 System Folder to the MWII disk and on reboot the Mac Plus would display the [X] folder icon and eject the disk.​

Has anyone seen this behaviour before and is there a decent explanation for it?

 

[Mk.558]

Probably a difference in the boot blocks. Compare them back to back with something like BootConfigure or Fedit. Some or both of those applications may not work very well past System 6. Instructions for editing the boot blocks with Fedit is in the More Info section below.

 

[LaPorta]

I would venture that running System 3 and copying it allows for something to be set that System 7 does not do (or does something else) to the boot blocks that will not allow System 3 to start up. I’d bet if you copied the System 3 System Folder under System 7, but then started from a System 6 disk and used the “Set Startup…” command on the System 3 disk, it would then work.

 

[Snial]

Probably a difference in the boot blocks. Compare them back to back with something like BootConfigure or Fedit. Some or both of those applications may not work very well past System 6. Instructions for editing the boot blocks with Fedit is in the More Info section below.

OK, I always thought the boot blocks never changed, but of course, they might have done by System 7.

I would venture that running System 3 and copying it allows for something to be set that System 7 does not do (or does something else) to the boot blocks that will not allow System 3 to start up. I’d bet if you copied the System 3 System Folder under System 7, but then started from a System 6 disk and used the “Set Startup…” command on the System 3 disk, it would then work.

I'll try and do that too. I guess that if it's a boot block issue, it might be worthwhile writing a BootBlock update that works under System 7.

 

[Snial]

I managed to download FEdit from Macintosh Garden. From there I found that indeed, if you use System 7.1 and:

• Erase a 800kB disk (here, DiskA)
• Copy System 3 from another 800kB disk to DiskA.
• Run FEdit and check the boot blocks, the standard boot blocks are missing.

Whereas if you:

• Boot from the System 3 disk.
• Erase 800kB diskA.
• Copy the System Folder to Disk A
• Run FEdit, the standard boot blocks are there.

And in the first case, if you update the disk with the standard boot blocks it does, indeed boot.

FEdit isn't supposed to be System 7 compatible though. Once I'd written the standard boot blocks to DiskA; I found that I couldn't then reboot into that original System 7.1: sometimes I had an Address Error and later the Mac Plus stopped seeing the drive. I ended up booting into a System 7.1 copy on another Zip disk and inspecting the HD; then I found I could boot up from it.

I think I might try to boot from 7.0.x and see if the same thing happens? And if it does, regress back to System 6.

 

[adespoton]

I'd be curious what happens if you install all systems on the same drive in different folders, too. Do the boot blocks change when you bless different folders?

 

[Mk.558]

Only one of them will be "blessed". The others will not and cannot be all blessed as well. Using System Picker is/was a common tool to swap between different boot folders.

 

[Snial]

I'd be curious what happens if you install all systems on the same drive in different folders, too. Do the boot blocks change when you bless different folders?

Only one of them will be "blessed". The others will not and cannot be all blessed as well. Using System Picker is/was a common tool to swap between different boot folders.

I think @adespoton is wondering if the contents of the boot blocks change when you switch between different OSs. You can do this manually too:

1. Rename System Folder to something else, e.g. the System version, such as Sys6Folder.
2. Rename the System Folder of the System you're interested in (e.g. Sys3Folder) to System Folder. It won't be blessed yet.
3. Move its System file (or suitcase) out of its System Folder and then move it back in. This should bless the new System Folder.
4. Reboot into the intended System.

I suspect that if the Boot blocks do change, then the contents depend on the System you were running when you blessed the new System Folder. I also have a sort-of-theory for why System 7.1 won't bless the 800kB disk: it's because you can't fit even a minimal System 7.1 on an 800kB disk. Even System 6 barely fits. The System Suitcase for 7.0.1 takes 1.7MB and System 7.0.1 Finder takes 355kB. Since it probably can't really check the memory requirements of the System File/Suitcase it simply disables booting.

I believe though that System 7.0 and 7.1 could be installed from 800kB disks, Apple did that by booting up from a minimal version of System 6.0.8 and then running the installer (perhaps the installer runs instead of the Finder). And the installer worked on System 6.0.8.

There's probably also madder ways of getting System 7.0 (maybe 7.1) to boot from 800kB disks: compress the System folder onto a couple of 800kB disks; then install a fake System containing a dearchiver, decompressor and bootable RAM disk. On a 4MB Mac Plus with a 2MB compressed RAM disk, this would give you ≈4MB of storage for the OS + 800kB for applications. That would be enough for a ZIP disk driver/CD extension/Drive Setup and nearly 1MB RAM left for running applications. Enough to perform a setup and copying data to a new target. It's so obvious though, I guess that's already been done.

 

[joevt]

The blessed System Folder is a 4 byte ID in the Master Directory Block. The bless --info in Mac OS X will show the ID and what folder that ID belongs to. My dumpvols.sh script for Mac OS X can dump partition map and volume boot info.
https://gist.github.com/joevt/a99e3af71343d8242e0078ab4af39b6c

 

[adespoton]

So since the MDB is part of the HFS partition, it likely wouldn't have any effect on the boot partition; changing the blessed folder therefore wouldn't cause a boot partition update. So formatting the disk or using HD SC Util is the only way to go, and what that does will vary from OS version to OS version....

 

[Snial]

The blessed System Folder is a 4 byte ID in the Master Directory Block. The bless --info in Mac OS X will show the ID and what folder that ID belongs to. My dumpvols.sh script for Mac OS X can dump partition map and volume boot info.
https://gist.github.com/joevt/a99e3af71343d8242e0078ab4af39b6c

So since the MDB is part of the HFS partition, it likely wouldn't have any effect on the boot partition; changing the blessed folder therefore wouldn't cause a boot partition update. So formatting the disk or using HD SC Util is the only way to go, and what that does will vary from OS version to OS version....

OK, the technique I described earlier doesn't really work, but I did manage to do it by a variation of the same, manual technique.

I started with System Folder containing System 7.0; a folder called System6 containing System 6 including the System suitcase:

1. I created a folder called System7.
2. √ I moved all the files apart from the System Suitcase in System Folder to the System7 folder.
3. √ I moved all the files apart from the System Suitcase in System6 to System Folder.
4. √ I moved System Folder:System to the root level of the HD.
5. √I moved System6:System to System Folder. Now, System Folder still appears blessed.
6. √I moved HD40:System suitcase to the System7 folder.
7. √ I rebooted and I was in System 6 (but In Finder. I had to run Special:Set Startup.. to switch to MultiFinder).

The '√'s are me just going through all the steps again to prove it. Now I need to switch back to System 7. There's one caveat. Going back to System 7 requires me to first switch back to using Finder in System 6.