The most significant byte of the attribute file address starts off as 88 for the top third of the screen. In the middle third it becomes 89 and in the lower third it reaches 90. The most significant byte of the display file address (for the top pixel row of each character row) is 64 in the top third, 72 in the middle third and 80 in the lower third. Generally speaking, therefore, the most significant byte in the display file address increases by eight for every increase of one in the attribute address most significant byte, so multiplying the latter by eight (giving 192, 200 or 208, values roll over 255-0 boundary with excess truncated) would put it on the same scale as the former (64, 72 or 80).

Performing an AND 91 removes the most significant bit, converting the possible values to 64, 72 or 80, so the most significant byte of the attribute file address has been transformed into the most significant byte of the corresponding display file address. However, after the multiplication by eight, there is no way that the lowest two bits could be set, so there is no need to mask these with the AND command, so AND 88 would be just as effective (and probably more correct) than an AND 91.

See also code at 54170-54176.

Used by the routine at 47682.