The IDE drive appeared as Drive 7 (d7:) so could be accessed using a patched SamDOS to call the HDOS code as required for LOAD/SAVE etc. but not DIR.
The additional command set was not integrated into Sam BASIC so bespoke CALL commands and variables had to be in place e.g. CALL base+3,"DIR 0 /" where base was where the code was located in memory.
MasterBasic's hidden procedure function could be used to provide commands in a more firendly fashion.
Distribution also included some HD utilities by Ian Spencer.
Tragedy struck for HDOS when the source was lost in a drive crash on one of Nev’s machines:
Nev Young wrote HDOS using a C compiler on the PC which was designed to produce Z80 object code as its output. This was by far the most efficient way to write the operating system, I know because Nev and I talked it over on the telephone and in person for many (many) hours. However, due to a faulty hard drive on the PC, the C source for HDOS no longer exists.
A 'DOS Shell' program for HDOS was published on Fred 80.