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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 interprets it as a NOP followed by a CMP that is then ignored. Its real purpose is to declare which track 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 DOS's from different companies which were mostly compatible. 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 DOS's used the NOP with the track number then on the third byte and other DOS's 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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  • Since you already found them, mind to add on what products did so? In addition, this looks quite like a duplicate to your own question (and findings)
    – Raffzahn
    May 24, 2022 at 14:26
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    @Raffzahn While very general questions that start from little knowledge are simply the nature of asking questions, these tend to generate smaller, more specific questions. It's perfectly reasonable (and even a good idea) to extract those generated questions into separate RCSE questions that can focus on the specific point. Once an appropriate answer to this question appears, the original more general question and/or its answers can be edited to reference this one.
    – cjs
    May 24, 2022 at 15:00

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