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I forget what it was, like banks 0x80 and above or something like that will be accessed at 3.58mhz if a certain register bit is set.
Yes, any
ROM (*) access with A23 set (addr&0x800000!=0) will be FastROM if $420d.d0 is set.
(*) $[80-bf]:[8000-ffff] + $[c0-ff]:[0000-ffff]
FastROM means one access takes six cycles at 21.47MHz, SlowROM takes eight.
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I remember hearing also some people suggested that because the system has 64kb of RAM, you could upload/copy code to RAM so that it would run faster than off a 200ns SlowROM chip.
First, the SNES has 128kb WRAM; and second, it is always accessed at the same speed as SlowROM, regardless of $420d.d0 FastROM setting.
Kind of unfortunate that Nintendo was too cheap to use faster RAM memory, given that the CPU was
the bottleneck of the SNES.