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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.
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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For a Fast Wipe Framestore function code, the selected frame store or store(s) will be wiped to the relevant value in the Brush Store colour latches, starting at CXPOS,CYPOS set on the Store Address card, and running for the inverse of the values set in BXLEN,BYLEN.
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That's one of the things what had me a bit stymied: I was assuming BXLEN and BYLEN were positive length values, but in reality they're the inverse of the length, loaded into a pair of cascading 4-bit counter ICs which terminate the command once they roll over to 12-bit 0.
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