[wallacoloo]

The combination of a mask rom and reprogrammable flash on the same product surprised me. Why would you want a customizable mask rom if you’ve already got 8x or more of flash? Is it a power consideration: are mask roms lower-power to read from?

[dmitrygr]

NOR flash is also slow. That is why a lot of microcontrollers that run at hundreds of megahertz have very strange flash accelerators that read slowly in parallel (STM uses a 128-bit-wide bus for example), and then dispense data/code to the CPU quickly in 16 or 32-bit pieces. Most ROMs can be read very quickly.

[pjc50]

Not unusual - the mask rom can't be bricked. So it's not unusual to put a first-stage boot loader there.

[irq]

One possible explanation could be the desire to have a guaranteed-never-modified bootloader that is rock solid, which allows for firmware updates on the flash (in addition to booting the system in normal use). This is how Apple's iOS devices are designed, for example.