In the backlit Portable, the top 1MB is simply never decoded and so doesn't get used. I guess that something else is occupying memory at that address.

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Yes, I think there MUST have been a ROM revision coinciding with the backlit portable, the only way to know for sure, notwithstanding expertise within the forum, is to compare an original Portable ROM with that of a backlit one. It would make sense then that the PB 100 uses the same ROM as the backlit Portable. Looking at the backlight upgrade procedure for the Portable, the backlight card plugs into the ROM slot and more than likely it is the new video data (backlight state) that uses that last 9MB address.