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Indeed in the NTSC mode the first 10 lines are strobed by $038 register.
So if the last line of the previous frame was a short line (227 CCKs) then the hpos counter at the end of the 10th line is equal to:
(455 + 5*454 + 5*456) % 512 = 397
455 - last hpos value at the end of the previous frame (assuming a short line)
5 - number of short and long lines during the first 10 display lines
454 - length of a short line (227 CCKs)
456 - length of a long line (228 CCKs)
If the previous frame ends with long line (228 CCKs) then this value is 399.
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