Fenestra6502 is a 6502-based system with an extra circuit to improve the performance of procedure/function calls.
The circuit realizes "zero-page windows", which is similar to "register windows" adopted in several RISC processors. The circuit detects the instruction fetch of JSR/RTS and changes the mapping of zero-page on physical memory.
Since 6502 has no pointer registers, the operations of stack frames are expensive compared to other modern processors. The "zero-page windows" dramatically simplifies the operations and improve the performance of procedure/function calls.