ARM uses a mix patents, copyright and trade secret protections for their ISA. The give you the RTL/vhdl/verilog for the core when you license it and they forbid you from changing the core in the license agreement to use it.
You can clean room implement the trade secret part, but the patents would be an issue and ARM could still sue and drag things out.
You also could never legally call it ARM because it's trademarked. This makes it harder for semiconductor vendors to sell chips.
See the Qualcomm lawsuit shitshow which is now causing Qualcomm to invest big in RISCV.
Yeah, for hard layouts that the vendor has already done, that is true. Either way, it's transferred encrypted and protected so you can't screw with it.
You can clean room implement the trade secret part, but the patents would be an issue and ARM could still sue and drag things out.
You also could never legally call it ARM because it's trademarked. This makes it harder for semiconductor vendors to sell chips.
See the Qualcomm lawsuit shitshow which is now causing Qualcomm to invest big in RISCV.