The 65ISR is a processor that I designed. People have said that it is similar in design to the P1 in that programming it would be similar.
It is like a 65c02. The 65ISR-chico version only has ISRs --- there is no main program. The 65ISR-abu version does have a main program.
In both cases, the ISRs or the main-program can't be interrupted --- they have to explicitly release control.
The ISRs release control when they finish. The main-program releases control after setting up a vector to allow it to restart again when it gets control back.
I invented this scheme --- nothing like this has been done before --- it might be a bad idea.
The 65ISR isn't directly related to the P1 though, so this may be too off-topic for the Parallax Forum. If so, let me know, and I won't post any further messages like this that aren't directly related to Parallax products.