I'm trying to run the Main Stack Watermarking procedure as described in Application Note 316, page 7. (https://www.keil.com/appnotes/files/apnt_316.pdf)
Pre-compile there's a message "*** Using Compiler 'V5.06 update 1 (build 61)', folder: 'C:\Keil_v5\ARM\ARMCC\Bin'", which is probably important to know. I am a newbie at this so please let me know if there's any other information you'd need.
This seems like it might be it!
Unfortunately the "Assembler Option" isn't visible on my system, but after some searching around I found I need to have the Arm Compiler Version 6, which requires a Keil update for me. I'm doing that now and if everything works out I'll accept your answer.