Monitors were also an integral feature of most "freezer" cartridges for the C64, like Action Replay and The Final Cartridge I/II/III (you could get monitors you'd load into RAM as well, but the freezer cartridges had lots of extra capabilities and of course the advantage of having the monitor in ROM). For a couple of years, until I got hold of Turbo Assembler (no relation to the Borland PC assembler), I wrote most of my assembler straight into memory using the Action Replay monitor.