Title: The Source
Publisher: Logical Systems Inc.
Publication date: 1982
Number of volumes: 3
The Source was a three volume set containing the complete source code of TRSDOS 6.2 (which was the same as LS‑DOS 6.2). Each individual book cost $99, so total cost for a complete set was $297. Later on, these prices were heavily discounted.
Volume one of The Source contains the source code for the core operating system (which included BOOT/SYS, SYS0/SYS through SYS6/SYS, and SYS9/SYS through SYS13/SYS). Volume two contains the source code for the library commands (the ones found in SYS6/SYS, SYS7/SYS, and SYS8/SYS). Volume three contains the source code for most of the utilities included with TRSDOS 6 (not including BASIC).
One curious detail about The Source is that the source code is blanked out wherever the code makes access to the Model 4 hardware (although the comments remain). This was required as part of an agreement to protect proprietary Radio Shack information. Of course, the opcodes are still visible and it is easy to disassemble the instructions yourself.
These books obviously were aimed at a very narrow audience. They were important references for anyone writing Model 4 programs and fascinating reading for anyone who just wanted to understand how the system worked. At the time, it was also impressive to be able to see the full source code of a commercially available operating system. I personally found The Source along with The Programmer’s Guide to be my most valuable references for Model 4 programming.
If you’re interested but don’t have your own copy of The Source, there are still places where you can find the reconstructed source code of LS‑DOS. One place is Tim Mann’s Misosys Software and Documents Page. Another is Frank Durda’s LS‑DOS 6.3.1 Source Code Restoration Project