I’ve been asked how the “codename” thingy that survives a format works. I didn’t actually try it, but I guess they did. This is how it works (or at least it should):
The whole trick relies in one of the variants of the 1050 diagnostic commands, that let you position the drive head virtually on any place, limited only by the mechanical constraints. And what is more important is that the drive moves the head without updating its internal variables of the current logical track position.
The command was surely implemented for a variety of reasons, like alignment, cleaning, etc. But there is always somebody that finds an unexpected usage
So if you read sector #1 (head will be in track 0) and then move the head to track 1, the drive will still “think” it is in track 0. And if you then try to read any other sector on track 0, it won’t move the head back. It will look for the sector on track 1. Of course, normally it won’t find it there.
Assume now you somehow formatted track 40 (one beyond the last normal one, track 39). And assume you formatted it with sectors headers where the track field is 39 (you could accomplish this with a custom OS like the Happy if you want). You make a normal seek to track 39, for example reading sector 720. Then you use the diagnostic command to move the head to track 40.
At this point the head is on track 40, but the drive “thinks” is in track 39. So if try to read or write for example sector 720, it will read/write it at track 40. And because the track was already formatted (and formatted logically as being track 39) there won’t be any errors. And of course that whatever you write to track 40, it will survive a normal format.
If you however attempt to use this trick to actually format track 40, it won’t work. You can make a logical seek to track 0, and then physically position the head on track 1. And you could think that if you now issue a format disk command the disk will formatted with one track displacement (track 1-40 instead of track 0-39). But before formatting the disk, the drive performs a hardware “restore” by stepping out the head until sensing the track zero detector being active. This is done disregarding the current logical track position.
However, amazing as it sound, the trick does work the other way around, formatting physical tracks –1 to 38. It works as a consequence of two points.
One is the particular way that the zero-track sensor and stepper motor works in the 1050. The 1050 mechanism allows you to position the head even outer than track 0. Something that a PC drive normally doesn’t let you to do.
The other point is exactly how the 1050 ROM performs the hardware restore mentioned above. If you have seen how the head moves during drive power on, you know that it first moves the head inwards, at a middle position, and then outwards until sensing track 0. But the hardware restore at format time doesn’t move the head inwards at all. It just steps out until sensing track 0, which will be already active if the head is at track –1.
Amazing, isn’t it?
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