Several disks in this collection contain source code. If you do a catalog, you see the files MXFLS and ASM.CODE. Taken together, these are the LISA assembler. In the assembler, source code is located starting at $1800. The object code is at $800-$17FF. Thus, modules are limited to 1000 hex long. In BEX, most modules are stored from $7200-$81FF, which is 1000 hex long.
Boot a source code disk, at the DOS prompt, type BRUN MXFLS. The Lisa assembler is run. It has an exclamation point (!) prompt. At the prompt, type LOAD (file name). You can assemble the file by entering A followed by Enter. To save the object code, type (control-D)BSAVE QQQ,A$800,L$3FF (Enter). Of course, change the file name and the length as appropriate. In BEX, the file name of the major assembly language modules was MOD followed by a number.
In the BEX BASIC programs, the variable U contains the start of the configuration bytes. If you see PEEK(U+69), take the 69 and convert decimal to hex, and you get $45. As you see below, $1E45 contained the slot of the VersaBraille. Of course, POKE means to place a new value in a memory location. Now you can see how to read and write from parts of the BEX configuration from BASIC.
ST -- the location $8200, the start of the page buffer (where text page is loaded)
CD -- the location $7200, the start of the current code module (notice that modules 1,2 and 3 are stored in auxillary memory to speed up disk access)
DK -- the number of disk drives in the configuration
L2 -- 0 if at the Learner level, 1 if at the User or Master Level
L3 -- 0 if at the Learner or User Level, 1 if at the Master Level
So how does BEX work? It has large amounts of assembly language. At anyone time, there is only one BASIC program loaded at a time. For the Main disk, there are four BASIC programs: MAIN, SECOND, PAGEMENU and TAPE-VB. These are the three menus of the Main Side of BEX.
Remember, most of these notes and the source code were written about 25 years ago. A few details will be hard to come by. Good luck.