Earlier I had found that many but not all TRS-80 Model I and Model III disks begin with the three bytes 00h FEh 11h.

This is the beginning of the boot sector where the CPU interpets it as a NOP followed by a CMP that is then ignored. Its real purpose is to declare which track is the directory track is on, which by default is 11h, track 17.

But just now I've discovered that just about as many disks start the boot sector without the NOP and just with FEh 11h`.

These TRS-80s had many DOSes from different companies which were mostly intercompatible. But sometimes there was a Model I version and a Model III version of the same DOS since the machines were not 100% compatible.

Do we know if certain DOSes used the NOP with the track number then on the third byte and other DOSes put the track number on the second byte with no NOP. Or do we know if this was a difference between Model 1 and 3 versions of the same DOS? Or something else?

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