Unlike later Macs that stored firmware on a discrete Flash ROM chip, the MCPX chip had (hardwired at the factory, non-updatable).
That instruction set was the MCPX Boot ROM. mcpx boot rom
To put that in perspective, the icon you just clicked to open this blog post is likely larger than the entire codebase that boots a game console. In an era where our operating systems are gigabytes in size, fitting a secure boot process into half a kilobyte is a feat of engineering that borders on art. Unlike later Macs that stored firmware on a