7B74-$7B7B: a mask for each of the eight half-rows of the keyboard, ORed with the reading so that those keys are never seen as pressed. The masks hide CAPS SHIFT ($01 in the first half-row) and SYMBOL SHIFT ($02 in the last), which are read separately as shift keys, and the unused keys 1, 3, 4 ($0D in the 1-5 half-row) and 9 ($02 in the 0-6 half-row).
7B7C-$7B7D: the newly pressed key found by the last scan: the bits in 7B7C and the half-row in $7B7D, or 0 for none.
7B7E-$7B85: the previous reading of each half-row, used to detect keys that have just gone down.