Unibis Device Drivers

[FSAC]

    I am presently trying to understand and modify several Unibus device
drivers on a Vax730 running 4.2BDS Unix.  I understand the majority of code
within these drivers but lose track when certain low level calls such as 
physio() and iodone() etc are executed.  Not having the Unix source code
( at the moment, although its on the way ) its also unclear exactly what Unix
does(for you) when a high level operation such as WRITE is called. Does 
anyone out there know of any readily available resource that would help
me with these problems.
				tks,
					Richard Simpson