Used by the routine at DRAW_WALLS.

The screen is taken as sixteen columns 16 pixels wide, x / 16, a bit each in COLUMNS_DRAWN (bit 7 of x picking the byte). Every tile column in a turn lies at the same x within its 16 pixels, since the cells' corners are 128 pixels apart, so a column of wall and a screen column always coincide. The column's bit is patched into the BIT at TEST_COLUMN and the SET at CLAIM_COLUMN. DRAW_CELLS starts each turn with the first column (left of the buffer) and the last three (right of it) already claimed, so walls are clipped at the sides here.

An unclaimed column is claimed, then drawn: the lower tile with its bottom row at DRAW_Y (PUT_TILE), its colour up to the top (COLOUR_WALL), and the upper tile 64 lines above it. A column off the screen's 0-255 range in x or y is skipped, and IY moved past its two tiles either way.