On my last post I was complaining on how slow the development process was low because of the IO emulation routine cost. I decided to stop developing it.
But the next morning, I could not stop reflecting on the speed problem, and I found out a method. Replacing PPUADDR and PPUWR call system by an array of routines depending on the address. Array[PPUADDR >> 2] = routine@. This array is big but it fits in WRAM.
It allowed to remove all the PPUADDR incrementing code who changed the IO routine depending on the address. It made a 20 rendering lines gain by removing the PPUADDR write IO emulation from Bank zero where the emulation code is, and replacing it with a short routine set in ram.
I believed that the gain in cycles was not enough because of sound emulation, but it looks like that sound emulation in the SPC700 needs only to be updated once per frame. In Super Mario Bros, it can be done between line 80 and line 240 where the game does nothing. In fact plenty of cycles are available for sound emulation update.
All in all, it IS possible to run Super Mario Brothers on the Snes with automatic conversion.