There was a third-party Machine Code Monitor cartridge. Don't remember who made it. You could write code, as well as view it, or debug it. I used the 6502 manu

There was a third-party Machine Code Monitor cartridge. Don't remember who made it. You could write code, as well as view it, or debug it. I used the 6502 manual. I was taking Machine Code in school at the time (tech school -not "proper" school), and had learned how to trawl the tech literature for guidance.