This is the companion assembler to Moder-80 and is certainly packed with facilities.
First, and very important, it is compatible with, and saveable to Microdrives and can access the Spectrum network. Code sections can be saved and loaded to/from either of these.
Second, it is compatible with any printer interface, because, not only can you relocate it in memory, but also determine the location and extent of workspaces for source and object code, leaving room for driver software. It can be co-resident with a monitor/disassembler. Full marks for versatility.
Unlike the normal Spectrum editor, Gener-80 has a 40 column, full-screen version, allowing alteration anywhere on the screen, but syntax is checked on entry. Unlike some assemblers where mistakes are only found at assembly time. The usual features of move, find and delete are here, with string find. Assembly is fast.
I struggled with Moder-80 as a result of inexperience, and an unfriendly manual. A more experienced friend recommends it as useful. Gener-80's manual is first class, full of hints and examples. An excellent machine code development program, but you'll still need a book!