The Hard Disc OS for the SD IDE Interface.
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 plus there were others such as those on Sam Supplement 51.
Tragedy struck for HDOS when the source was lost in a drive crash on one of Nev’s machines:
Quote from Format Vol 12 No 2 (Jan 1999) from Bob Brenchley:
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.
Sławomir Grodkowski also wrote a copy utility on Sam Supplement 53, Geoff Bridges a mouse drive and front end in issue 54.