OK, this is very dusty material in my memory, so I'm relying a bit on Google/Wikipedia here. I may be able to find some of my printed documentation, but it will take a while.
The granule was the allocation unit for the filesystem. I don't remember the granule size on single-density drives, but Wikipedia says for double-density drives it was 6 sectors per ...
The best resource is a book called TRS80 Disk and other mysteries.
I found a copy online at:
My real copy is old and under a bunch of dust, but it does have the complete disk mapping ...
/CMD files are a series of tagged records. Some record types have meaning only for certain features of some of the TRS-80 DOSes. Only two seem important for loading and executing them.
byte: size* in bytes
*size 0 → 256 bytes
*size for type 1 only, size 1 → 257 bytes, size 2 → 258 bytes
type 1: object code (...
After Googling this for an hour or two and not finding much, I have found something solid just minutes after posting my question!
From page 40 of TRS8BIT volume 09 issue 02 from June 2015 there is an article on a "FreHD Expansion Interface" which includes this paragraph:
If a WDC1771 Disc Controller is found then the LII ROM programs the
chip to ...