Thanks. Yes it looks like you can't overwrite ROM, which for a real cartridge or hardware I'd expect, but not in an emulator ;) It's odd because I can happily load a file in to any area of ROM without issue. I'm using the Mame Debugger to communicate between a running program and my development tool...

I'm using the Mame Debugger to communicate between a running program and my development tools. I send strings to the debugger from the tools to alter parts of memory for 'realtime' update. This works well. I can FILL parts of RAM, and I can load in files to ROM areas. However when I try to FILL rom ...