Correct, the 4k bit is ignored by the F18A and also by the 9938 and later (which is why the 80 column hack ignores it -- 80-column 9938 software often leaves that bit at 0.)   When you're in 4k mode, it's not just masking off the address bits. The 9918A generates not a linear address to the DRAM, but a row address and a column address -- both of these are affected by the 4k mode setting.   If there really was only 4k attached, then yes, you could just mask the address and it would be right,
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