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Each Frame Store card has 640kbytes of RAM, arranged internally as 640x1024 bytes, or more accurately, ten "stripes" of 64x1024 bytes. There are six in a standard system. The top 128 lines of each Frame Store are reserved for the palette box (I) or the menu box (II).
There's a set of PROMs and such that remap the 640x1024 store into an 800x768 usable area, but with the top 128 lines reserved, that gives an effective usable area of 800x600 - and there's our hallowed 4:3.
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Writes to the System Bus with CSR=8 will, on alternating columns, push either a UY or VY pair (8 bits each) into the latches on the Brush Store card. I was first confused why the Y value for the UY pair was always 0, until I realised that Y always gets overwritten by the VY pair.
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Depending on the current 4-bit function code, this value can be written into the Brush Store (a set of three 256x256 8-bit planes), written into the Frame Store, or simply left in the latches to be used by a subsequent command with the Fixed Colour Select bit set.
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