Understanding the persistent bitmap cache

I'm trying to understand the inner workings of the persistent bitmap
cache.  When looking at what seems to be stored, it seems like the
bitmap width and height are always set to 0x40 (64), even if the
bitmap length is much less than 0x1000 (4096 = 64*64).  Where are the
missing pixels?

Or, if those values are just dummy values: where are the real width
and height stored?

Finally: What is the sequence for tiling the screen into 64x64 tiles?
(eg Bottom-up, left-right) Or is it nonstandard?

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