The Midnight Assembly System for C-128 (MAS-128) is a 1987/1988 symbolic assembler by Matthew Montchalin/Mountain Wizardry Software for Commodore 128 assembly-language programming. The manual explains how to load, customize, and use MAS-128, including its RAM 0/RAM 1 bank-aware object-code generation, full 80-column screen editor, Quick Keys, disk and RAMDOS support, immediate commands, expression evaluation, pseudo-opcodes, file conversion tools, and included sample/source utilities such as Breaker-128, S-128, Swirling Sprites, and Music Demo. Overall, it is both a user guide and technical reference for serious C-128 assembly programmers who want an integrated editing, assembling, disk-management, and debugging workflow.

Here is a link to the MAS-128 software.