64k ROM and internal 800KB drive

[Dog Cow]

After researching, I'm still not clear on this. Let's say I've got a machine that's been downgraded from a 128K to a 64K ROM, but it still has the stock 800KB DS drive intact. Will this drive function like a 400K MFS drive?

 

[Gorgonops]

With the early versions of the drive, no. It'll crash to a sad Mac on attempting to boot because of a divide-by-zero error. It *may* work with later versions of the drive mechanism.

 

[tjjq44]

I confirm, I use a 512k motherboard with 64k roms in a plus case and a 800k internal drive, it boots 400k MFS (and even 800k MFS) without any problem

The sound the drive makes is just different (lower pitch) than with 128k roms or HD20 init installed

 

[Gorgonops]

As noted, I think it will depend critically on which version of the drive mechanism you have. I have a 512k with the original 400k drive internally, and I have two 800k external drives. One is the "real" Macintosh 800k external, an M0131:

And the other is the later "Snow White"-styled (is that right? whatever the IIgs is) A9M0106. (Reference page for Apple floppy drive models.) The MacOS ROM at powerup checks both the internal and external drive for a bootable disk at power-up so booting from an external drive is a legit thing to do; however, if I attempt to boot from the M0131 I get a divide-by-zero error as detailed by this post on Big Mess O'Wires' site. However, the later A9M0106 works fine. Apparently Apple modified later mechanisms to add a little "slop" to their tach output in order to work around this bug, presumably so they could work as "universal" drives even for Macs that don't have a ROM upgrade.

So, yeah, it will likely depend crucially on the *exact* age and revision of your floppy drive mechanism. Some will work, some won't.

Honestly... I'm not sure what the point of downgrading is. Having the 64k ROM is a novelty, I guess, but I think you can count the number of programs that are actually rendered incompatible by having the 128k one on one hand, and I believe that even in those cases the root cause is usually copy protection schemes that rely on playing evil tricks like varying the disk PWM speed to create freaky non-standard disk formats; anything that tries that will be broken by having the 800k drive mechanism because they self-regulate the speed and ignore PWM, even if you do have the 64k ROM.