That debugger was not in the original "64K" ROM BTW. It was the "128K" ROM included in the Mac Plus that introduced it.
Re:Mac System 1.0 (1998)
by userbinator on Sunday February 14, 2016 @01:54PM
(#11099441)
Having the UI code in ROM doesn't preclude (and might actually benefit) shipping a simple but versatile sort of debugging tool with the OS. But Apple decided against doing so, making the system more appliance-like overall.
DOS's DEBUG isn't just a debugger; it also functions as a disk editor, hex editor, memory inspector, and general "system inspecting/tinkering" tool.
There was a debugger BTW, the very limited programmer's key (which most users only used to kill the current app if it crashed). It could be replaced with the more powerful Macsbug.