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There are 3 independent frame stores per set of stores: Luma, Chroma, and "Ext" (usually used for stencil operations). Chroma alternates U and V on even and odd columns, which is why I try to describe it as "16-bit ish" colour depth rather than 16-bit YUV. It has a 2:1 L:C ratio.
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).
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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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