FAST/128 Software Development System by L. Boere.  Unfortunately I can't remember where I got this and there is no additional documentation on the floppy image.  It appears to be an Assembler and that's about all I can tell you.  If you have further information, please drop me a line.

LADS 123 Assembler is the programming environment used for the book "128 Machine Language Programming for Beginners".  Nothing fancy here, but handy if you're going through the book mentioned.

• Link to book

Merlin 128 is an extremely powerful, comprehensive Macro Assembler system for the Commodore 128 computer. It consists of four main modules and numerous auxiliary and utility programs which comprise one of the most complete assembler systems available for anypersonal computer.

You can find the Merlin 128 manual here.

MONITOR add-on for assembling and disassembling Z80 mnemonics. Use BLOAD and SYS DEC("2D00") to activate. The new MONITOR command "Z" disassembles in Z80 mnemonics (preceded by a dot), and "*" assembles in Z80 mnemonics.