I did something similar on the C64 (6502). Programmatically user code could mask interrupts, but as long as it didn’t it worked fine. It saved all the registers

I did something similar on the C64 (6502). Programmatically user code could mask interrupts, but as long as it didn’t it worked fine. It saved all the registers and restored them prior to switching execution threads. The unified memory space required that each thread ask for an absolute memory offset to be allocated and then voluntarily stay in that space.